cdrom-standard.rst

   1=======================
   2A Linux CD-ROM standard
   3=======================
   4
   5:Author: David van Leeuwen <david@ElseWare.cistron.nl>
   6:Date: 12 March 1999
   7:Updated by: Erik Andersen (andersee@debian.org)
   8:Updated by: Jens Axboe (axboe@image.dk)
   9
  10
  11Introduction
  12============
  13
  14Linux is probably the Unix-like operating system that supports
  15the widest variety of hardware devices. The reasons for this are
  16presumably
  17
  18- The large list of hardware devices available for the many platforms
  19  that Linux now supports (i.e., i386-PCs, Sparc Suns, etc.)
  20- The open design of the operating system, such that anybody can write a
  21  driver for Linux.
  22- There is plenty of source code around as examples of how to write a driver.
  23
  24The openness of Linux, and the many different types of available
  25hardware has allowed Linux to support many different hardware devices.
  26Unfortunately, the very openness that has allowed Linux to support
  27all these different devices has also allowed the behavior of each
  28device driver to differ significantly from one device to another.
  29This divergence of behavior has been very significant for CD-ROM
  30devices; the way a particular drive reacts to a `standard` *ioctl()*
  31call varies greatly from one device driver to another. To avoid making
  32their drivers totally inconsistent, the writers of Linux CD-ROM
  33drivers generally created new device drivers by understanding, copying,
  34and then changing an existing one. Unfortunately, this practice did not
  35maintain uniform behavior across all the Linux CD-ROM drivers.
  36
  37This document describes an effort to establish Uniform behavior across
  38all the different CD-ROM device drivers for Linux. This document also
  39defines the various *ioctl()'s*, and how the low-level CD-ROM device
  40drivers should implement them. Currently (as of the Linux 2.1.\ *x*
  41development kernels) several low-level CD-ROM device drivers, including
  42both IDE/ATAPI and SCSI, now use this Uniform interface.
  43
  44When the CD-ROM was developed, the interface between the CD-ROM drive
  45and the computer was not specified in the standards. As a result, many
  46different CD-ROM interfaces were developed. Some of them had their
  47own proprietary design (Sony, Mitsumi, Panasonic, Philips), other
  48manufacturers adopted an existing electrical interface and changed
  49the functionality (CreativeLabs/SoundBlaster, Teac, Funai) or simply
  50adapted their drives to one or more of the already existing electrical
  51interfaces (Aztech, Sanyo, Funai, Vertos, Longshine, Optics Storage and
  52most of the `NoName` manufacturers). In cases where a new drive really
  53brought its own interface or used its own command set and flow control
  54scheme, either a separate driver had to be written, or an existing
  55driver had to be enhanced. History has delivered us CD-ROM support for
  56many of these different interfaces. Nowadays, almost all new CD-ROM
  57drives are either IDE/ATAPI or SCSI, and it is very unlikely that any
  58manufacturer will create a new interface. Even finding drives for the
  59old proprietary interfaces is getting difficult.
  60
  61When (in the 1.3.70's) I looked at the existing software interface,
  62which was expressed through `cdrom.h`, it appeared to be a rather wild
  63set of commands and data formats [#f1]_. It seemed that many
  64features of the software interface had been added to accommodate the
  65capabilities of a particular drive, in an *ad hoc* manner. More
  66importantly, it appeared that the behavior of the `standard` commands
  67was different for most of the different drivers: e. g., some drivers
  68close the tray if an *open()* call occurs when the tray is open, while
  69others do not. Some drivers lock the door upon opening the device, to
  70prevent an incoherent file system, but others don't, to allow software
  71ejection. Undoubtedly, the capabilities of the different drives vary,
  72but even when two drives have the same capability their drivers'
  73behavior was usually different.
  74
  75.. [#f1]
  76   I cannot recollect what kernel version I looked at, then,
  77   presumably 1.2.13 and 1.3.34 --- the latest kernel that I was
  78   indirectly involved in.
  79
  80I decided to start a discussion on how to make all the Linux CD-ROM
  81drivers behave more uniformly. I began by contacting the developers of
  82the many CD-ROM drivers found in the Linux kernel. Their reactions
  83encouraged me to write the Uniform CD-ROM Driver which this document is
  84intended to describe. The implementation of the Uniform CD-ROM Driver is
  85in the file `cdrom.c`. This driver is intended to be an additional software
  86layer that sits on top of the low-level device drivers for each CD-ROM drive.
  87By adding this additional layer, it is possible to have all the different
  88CD-ROM devices behave **exactly** the same (insofar as the underlying
  89hardware will allow).
  90
  91The goal of the Uniform CD-ROM Driver is **not** to alienate driver developers
  92whohave not yet taken steps to support this effort. The goal of Uniform CD-ROM
  93Driver is simply to give people writing application programs for CD-ROM drives
  94**one** Linux CD-ROM interface with consistent behavior for all
  95CD-ROM devices. In addition, this also provides a consistent interface
  96between the low-level device driver code and the Linux kernel. Care
  97is taken that 100% compatibility exists with the data structures and
  98programmer's interface defined in `cdrom.h`. This guide was written to
  99help CD-ROM driver developers adapt their code to use the Uniform CD-ROM
 100Driver code defined in `cdrom.c`.
 101
 102Personally, I think that the most important hardware interfaces are
 103the IDE/ATAPI drives and, of course, the SCSI drives, but as prices
 104of hardware drop continuously, it is also likely that people may have
 105more than one CD-ROM drive, possibly of mixed types. It is important
 106that these drives behave in the same way. In December 1994, one of the
 107cheapest CD-ROM drives was a Philips cm206, a double-speed proprietary
 108drive. In the months that I was busy writing a Linux driver for it,
 109proprietary drives became obsolete and IDE/ATAPI drives became the
 110standard. At the time of the last update to this document (November
 1111997) it is becoming difficult to even **find** anything less than a
 11216 speed CD-ROM drive, and 24 speed drives are common.
 113
 114.. _cdrom_api:
 115
 116Standardizing through another software level
 117============================================
 118
 119At the time this document was conceived, all drivers directly
 120implemented the CD-ROM *ioctl()* calls through their own routines. This
 121led to the danger of different drivers forgetting to do important things
 122like checking that the user was giving the driver valid data. More
 123importantly, this led to the divergence of behavior, which has already
 124been discussed.
 125
 126For this reason, the Uniform CD-ROM Driver was created to enforce consistent
 127CD-ROM drive behavior, and to provide a common set of services to the various
 128low-level CD-ROM device drivers. The Uniform CD-ROM Driver now provides another
 129software-level, that separates the *ioctl()* and *open()* implementation
 130from the actual hardware implementation. Note that this effort has
 131made few changes which will affect a user's application programs. The
 132greatest change involved moving the contents of the various low-level
 133CD-ROM drivers\' header files to the kernel's cdrom directory. This was
 134done to help ensure that the user is only presented with only one cdrom
 135interface, the interface defined in `cdrom.h`.
 136
 137CD-ROM drives are specific enough (i. e., different from other
 138block-devices such as floppy or hard disc drives), to define a set
 139of common **CD-ROM device operations**, *<cdrom-device>_dops*.
 140These operations are different from the classical block-device file
 141operations, *<block-device>_fops*.
 142
 143The routines for the Uniform CD-ROM Driver interface level are implemented
 144in the file `cdrom.c`. In this file, the Uniform CD-ROM Driver interfaces
 145with the kernel as a block device by registering the following general
 146*struct file_operations*::
 147
 148	struct file_operations cdrom_fops = {
 149		NULL,			/* lseek */
 150		block _read ,		/* read--general block-dev read */
 151		block _write,		/* write--general block-dev write */
 152		NULL,			/* readdir */
 153		NULL,			/* select */
 154		cdrom_ioctl,		/* ioctl */
 155		NULL,			/* mmap */
 156		cdrom_open,		/* open */
 157		cdrom_release,		/* release */
 158		NULL,			/* fsync */
 159		NULL,			/* fasync */
 160		NULL			/* revalidate */
 161	};
 162
 163Every active CD-ROM device shares this *struct*. The routines
 164declared above are all implemented in `cdrom.c`, since this file is the
 165place where the behavior of all CD-ROM-devices is defined and
 166standardized. The actual interface to the various types of CD-ROM
 167hardware is still performed by various low-level CD-ROM-device
 168drivers. These routines simply implement certain **capabilities**
 169that are common to all CD-ROM (and really, all removable-media
 170devices).
 171
 172Registration of a low-level CD-ROM device driver is now done through
 173the general routines in `cdrom.c`, not through the Virtual File System
 174(VFS) any more. The interface implemented in `cdrom.c` is carried out
 175through two general structures that contain information about the
 176capabilities of the driver, and the specific drives on which the
 177driver operates. The structures are:
 178
 179cdrom_device_ops
 180  This structure contains information about the low-level driver for a
 181  CD-ROM device. This structure is conceptually connected to the major
 182  number of the device (although some drivers may have different
 183  major numbers, as is the case for the IDE driver).
 184
 185cdrom_device_info
 186  This structure contains information about a particular CD-ROM drive,
 187  such as its device name, speed, etc. This structure is conceptually
 188  connected to the minor number of the device.
 189
 190Registering a particular CD-ROM drive with the Uniform CD-ROM Driver
 191is done by the low-level device driver though a call to::
 192
 193	register_cdrom(struct cdrom_device_info * <device>_info)
 194
 195The device information structure, *<device>_info*, contains all the
 196information needed for the kernel to interface with the low-level
 197CD-ROM device driver. One of the most important entries in this
 198structure is a pointer to the *cdrom_device_ops* structure of the
 199low-level driver.
 200
 201The device operations structure, *cdrom_device_ops*, contains a list
 202of pointers to the functions which are implemented in the low-level
 203device driver. When `cdrom.c` accesses a CD-ROM device, it does it
 204through the functions in this structure. It is impossible to know all
 205the capabilities of future CD-ROM drives, so it is expected that this
 206list may need to be expanded from time to time as new technologies are
 207developed. For example, CD-R and CD-R/W drives are beginning to become
 208popular, and support will soon need to be added for them. For now, the
 209current *struct* is::
 210
 211	struct cdrom_device_ops {
 212		int (*open)(struct cdrom_device_info *, int)
 213		void (*release)(struct cdrom_device_info *);
 214		int (*drive_status)(struct cdrom_device_info *, int);
 215		unsigned int (*check_events)(struct cdrom_device_info *,
 216					     unsigned int, int);
 217		int (*media_changed)(struct cdrom_device_info *, int);
 218		int (*tray_move)(struct cdrom_device_info *, int);
 219		int (*lock_door)(struct cdrom_device_info *, int);
 220		int (*select_speed)(struct cdrom_device_info *, unsigned long);
 221		int (*get_last_session) (struct cdrom_device_info *,
 222					 struct cdrom_multisession *);
 223		int (*get_mcn)(struct cdrom_device_info *, struct cdrom_mcn *);
 224		int (*reset)(struct cdrom_device_info *);
 225		int (*audio_ioctl)(struct cdrom_device_info *,
 226				   unsigned int, void *);
 227		const int capability;		/* capability flags */
 228		int (*generic_packet)(struct cdrom_device_info *,
 229				      struct packet_command *);
 230	};
 231
 232When a low-level device driver implements one of these capabilities,
 233it should add a function pointer to this *struct*. When a particular
 234function is not implemented, however, this *struct* should contain a
 235NULL instead. The *capability* flags specify the capabilities of the
 236CD-ROM hardware and/or low-level CD-ROM driver when a CD-ROM drive
 237is registered with the Uniform CD-ROM Driver.
 238
 239Note that most functions have fewer parameters than their
 240*blkdev_fops* counterparts. This is because very little of the
 241information in the structures *inode* and *file* is used. For most
 242drivers, the main parameter is the *struct* *cdrom_device_info*, from
 243which the major and minor number can be extracted. (Most low-level
 244CD-ROM drivers don't even look at the major and minor number though,
 245since many of them only support one device.) This will be available
 246through *dev* in *cdrom_device_info* described below.
 247
 248The drive-specific, minor-like information that is registered with
 249`cdrom.c`, currently contains the following fields::
 250
 251  struct cdrom_device_info {
 252	const struct cdrom_device_ops * ops;	/* device operations for this major */
 253	struct list_head list;			/* linked list of all device_info */
 254	struct gendisk * disk;			/* matching block layer disk */
 255	void *  handle;				/* driver-dependent data */
 256
 257	int mask;				/* mask of capability: disables them */
 258	int speed;				/* maximum speed for reading data */
 259	int capacity;				/* number of discs in a jukebox */
 260
 261	unsigned int options:30;		/* options flags */
 262	unsigned mc_flags:2;			/*  media-change buffer flags */
 263	unsigned int vfs_events;		/*  cached events for vfs path */
 264	unsigned int ioctl_events;		/*  cached events for ioctl path */
 265	int use_count;				/*  number of times device is opened */
 266	char name[20];				/*  name of the device type */
 267
 268	__u8 sanyo_slot : 2;			/*  Sanyo 3-CD changer support */
 269	__u8 keeplocked : 1;			/*  CDROM_LOCKDOOR status */
 270	__u8 reserved : 5;			/*  not used yet */
 271	int cdda_method;			/*  see CDDA_* flags */
 272	__u8 last_sense;			/*  saves last sense key */
 273	__u8 media_written;			/*  dirty flag, DVD+RW bookkeeping */
 274	unsigned short mmc3_profile;		/*  current MMC3 profile */
 275	int for_data;				/*  unknown:TBD */
 276	int mrw_mode_page;			/*  which MRW mode page is in use */
 277  };
 278
 279Using this *struct*, a linked list of the registered minor devices is
 280built, using the *next* field. The device number, the device operations
 281struct and specifications of properties of the drive are stored in this
 282structure.
 283
 284The *mask* flags can be used to mask out some of the capabilities listed
 285in *ops->capability*, if a specific drive doesn't support a feature
 286of the driver. The value *speed* specifies the maximum head-rate of the
 287drive, measured in units of normal audio speed (176kB/sec raw data or
 288150kB/sec file system data). The parameters are declared *const*
 289because they describe properties of the drive, which don't change after
 290registration.
 291
 292A few registers contain variables local to the CD-ROM drive. The
 293flags *options* are used to specify how the general CD-ROM routines
 294should behave. These various flags registers should provide enough
 295flexibility to adapt to the different users' wishes (and **not** the
 296`arbitrary` wishes of the author of the low-level device driver, as is
 297the case in the old scheme). The register *mc_flags* is used to buffer
 298the information from *media_changed()* to two separate queues. Other
 299data that is specific to a minor drive, can be accessed through *handle*,
 300which can point to a data structure specific to the low-level driver.
 301The fields *use_count*, *next*, *options* and *mc_flags* need not be
 302initialized.
 303
 304The intermediate software layer that `cdrom.c` forms will perform some
 305additional bookkeeping. The use count of the device (the number of
 306processes that have the device opened) is registered in *use_count*. The
 307function *cdrom_ioctl()* will verify the appropriate user-memory regions
 308for read and write, and in case a location on the CD is transferred,
 309it will `sanitize` the format by making requests to the low-level
 310drivers in a standard format, and translating all formats between the
 311user-software and low level drivers. This relieves much of the drivers'
 312memory checking and format checking and translation. Also, the necessary
 313structures will be declared on the program stack.
 314
 315The implementation of the functions should be as defined in the
 316following sections. Two functions **must** be implemented, namely
 317*open()* and *release()*. Other functions may be omitted, their
 318corresponding capability flags will be cleared upon registration.
 319Generally, a function returns zero on success and negative on error. A
 320function call should return only after the command has completed, but of
 321course waiting for the device should not use processor time.
 322
 323::
 324
 325	int open(struct cdrom_device_info *cdi, int purpose)
 326
 327*Open()* should try to open the device for a specific *purpose*, which
 328can be either:
 329
 330- Open for reading data, as done by `mount()` (2), or the
 331  user commands `dd` or `cat`.
 332- Open for *ioctl* commands, as done by audio-CD playing programs.
 333
 334Notice that any strategic code (closing tray upon *open()*, etc.) is
 335done by the calling routine in `cdrom.c`, so the low-level routine
 336should only be concerned with proper initialization, such as spinning
 337up the disc, etc.
 338
 339::
 340
 341	void release(struct cdrom_device_info *cdi)
 342
 343Device-specific actions should be taken such as spinning down the device.
 344However, strategic actions such as ejection of the tray, or unlocking
 345the door, should be left over to the general routine *cdrom_release()*.
 346This is the only function returning type *void*.
 347
 348.. _cdrom_drive_status:
 349
 350::
 351
 352	int drive_status(struct cdrom_device_info *cdi, int slot_nr)
 353
 354The function *drive_status*, if implemented, should provide
 355information on the status of the drive (not the status of the disc,
 356which may or may not be in the drive). If the drive is not a changer,
 357*slot_nr* should be ignored. In `cdrom.h` the possibilities are listed::
 358
 359
 360	CDS_NO_INFO		/* no information available */
 361	CDS_NO_DISC		/* no disc is inserted, tray is closed */
 362	CDS_TRAY_OPEN		/* tray is opened */
 363	CDS_DRIVE_NOT_READY	/* something is wrong, tray is moving? */
 364	CDS_DISC_OK		/* a disc is loaded and everything is fine */
 365
 366::
 367
 368	int tray_move(struct cdrom_device_info *cdi, int position)
 369
 370This function, if implemented, should control the tray movement. (No
 371other function should control this.) The parameter *position* controls
 372the desired direction of movement:
 373
 374- 0 Close tray
 375- 1 Open tray
 376
 377This function returns 0 upon success, and a non-zero value upon
 378error. Note that if the tray is already in the desired position, no
 379action need be taken, and the return value should be 0.
 380
 381::
 382
 383	int lock_door(struct cdrom_device_info *cdi, int lock)
 384
 385This function (and no other code) controls locking of the door, if the
 386drive allows this. The value of *lock* controls the desired locking
 387state:
 388
 389- 0 Unlock door, manual opening is allowed
 390- 1 Lock door, tray cannot be ejected manually
 391
 392This function returns 0 upon success, and a non-zero value upon
 393error. Note that if the door is already in the requested state, no
 394action need be taken, and the return value should be 0.
 395
 396::
 397
 398	int select_speed(struct cdrom_device_info *cdi, unsigned long speed)
 399
 400Some CD-ROM drives are capable of changing their head-speed. There
 401are several reasons for changing the speed of a CD-ROM drive. Badly
 402pressed CD-ROM s may benefit from less-than-maximum head rate. Modern
 403CD-ROM drives can obtain very high head rates (up to *24x* is
 404common). It has been reported that these drives can make reading
 405errors at these high speeds, reducing the speed can prevent data loss
 406in these circumstances. Finally, some of these drives can
 407make an annoyingly loud noise, which a lower speed may reduce.
 408
 409This function specifies the speed at which data is read or audio is
 410played back. The value of *speed* specifies the head-speed of the
 411drive, measured in units of standard cdrom speed (176kB/sec raw data
 412or 150kB/sec file system data). So to request that a CD-ROM drive
 413operate at 300kB/sec you would call the CDROM_SELECT_SPEED *ioctl*
 414with *speed=2*. The special value `0` means `auto-selection`, i. e.,
 415maximum data-rate or real-time audio rate. If the drive doesn't have
 416this `auto-selection` capability, the decision should be made on the
 417current disc loaded and the return value should be positive. A negative
 418return value indicates an error.
 419
 420::
 421
 422	int get_last_session(struct cdrom_device_info *cdi,
 423			     struct cdrom_multisession *ms_info)
 424
 425This function should implement the old corresponding *ioctl()*. For
 426device *cdi->dev*, the start of the last session of the current disc
 427should be returned in the pointer argument *ms_info*. Note that
 428routines in `cdrom.c` have sanitized this argument: its requested
 429format will **always** be of the type *CDROM_LBA* (linear block
 430addressing mode), whatever the calling software requested. But
 431sanitization goes even further: the low-level implementation may
 432return the requested information in *CDROM_MSF* format if it wishes so
 433(setting the *ms_info->addr_format* field appropriately, of
 434course) and the routines in `cdrom.c` will make the transformation if
 435necessary. The return value is 0 upon success.
 436
 437::
 438
 439	int get_mcn(struct cdrom_device_info *cdi,
 440		    struct cdrom_mcn *mcn)
 441
 442Some discs carry a `Media Catalog Number` (MCN), also called
 443`Universal Product Code` (UPC). This number should reflect the number
 444that is generally found in the bar-code on the product. Unfortunately,
 445the few discs that carry such a number on the disc don't even use the
 446same format. The return argument to this function is a pointer to a
 447pre-declared memory region of type *struct cdrom_mcn*. The MCN is
 448expected as a 13-character string, terminated by a null-character.
 449
 450::
 451
 452	int reset(struct cdrom_device_info *cdi)
 453
 454This call should perform a hard-reset on the drive (although in
 455circumstances that a hard-reset is necessary, a drive may very well not
 456listen to commands anymore). Preferably, control is returned to the
 457caller only after the drive has finished resetting. If the drive is no
 458longer listening, it may be wise for the underlying low-level cdrom
 459driver to time out.
 460
 461::
 462
 463	int audio_ioctl(struct cdrom_device_info *cdi,
 464			unsigned int cmd, void *arg)
 465
 466Some of the CD-ROM-\ *ioctl()*\ 's defined in `cdrom.h` can be
 467implemented by the routines described above, and hence the function
 468*cdrom_ioctl* will use those. However, most *ioctl()*\ 's deal with
 469audio-control. We have decided to leave these to be accessed through a
 470single function, repeating the arguments *cmd* and *arg*. Note that
 471the latter is of type *void*, rather than *unsigned long int*.
 472The routine *cdrom_ioctl()* does do some useful things,
 473though. It sanitizes the address format type to *CDROM_MSF* (Minutes,
 474Seconds, Frames) for all audio calls. It also verifies the memory
 475location of *arg*, and reserves stack-memory for the argument. This
 476makes implementation of the *audio_ioctl()* much simpler than in the
 477old driver scheme. For example, you may look up the function
 478*cm206_audio_ioctl()* `cm206.c` that should be updated with
 479this documentation.
 480
 481An unimplemented ioctl should return *-ENOSYS*, but a harmless request
 482(e. g., *CDROMSTART*) may be ignored by returning 0 (success). Other
 483errors should be according to the standards, whatever they are. When
 484an error is returned by the low-level driver, the Uniform CD-ROM Driver
 485tries whenever possible to return the error code to the calling program.
 486(We may decide to sanitize the return value in *cdrom_ioctl()* though, in
 487order to guarantee a uniform interface to the audio-player software.)
 488
 489::
 490
 491	int dev_ioctl(struct cdrom_device_info *cdi,
 492		      unsigned int cmd, unsigned long arg)
 493
 494Some *ioctl()'s* seem to be specific to certain CD-ROM drives. That is,
 495they are introduced to service some capabilities of certain drives. In
 496fact, there are 6 different *ioctl()'s* for reading data, either in some
 497particular kind of format, or audio data. Not many drives support
 498reading audio tracks as data, I believe this is because of protection
 499of copyrights of artists. Moreover, I think that if audio-tracks are
 500supported, it should be done through the VFS and not via *ioctl()'s*. A
 501problem here could be the fact that audio-frames are 2352 bytes long,
 502so either the audio-file-system should ask for 75264 bytes at once
 503(the least common multiple of 512 and 2352), or the drivers should
 504bend their backs to cope with this incoherence (to which I would be
 505opposed). Furthermore, it is very difficult for the hardware to find
 506the exact frame boundaries, since there are no synchronization headers
 507in audio frames. Once these issues are resolved, this code should be
 508standardized in `cdrom.c`.
 509
 510Because there are so many *ioctl()'s* that seem to be introduced to
 511satisfy certain drivers [#f2]_, any non-standard *ioctl()*\ s
 512are routed through the call *dev_ioctl()*. In principle, `private`
 513*ioctl()*\ 's should be numbered after the device's major number, and not
 514the general CD-ROM *ioctl* number, `0x53`. Currently the
 515non-supported *ioctl()'s* are:
 516
 517	CDROMREADMODE1, CDROMREADMODE2, CDROMREADAUDIO, CDROMREADRAW,
 518	CDROMREADCOOKED, CDROMSEEK, CDROMPLAY-BLK and CDROM-READALL
 519
 520.. [#f2]
 521
 522   Is there software around that actually uses these? I'd be interested!
 523
 524.. _cdrom_capabilities:
 525
 526CD-ROM capabilities
 527-------------------
 528
 529Instead of just implementing some *ioctl* calls, the interface in
 530`cdrom.c` supplies the possibility to indicate the **capabilities**
 531of a CD-ROM drive. This can be done by ORing any number of
 532capability-constants that are defined in `cdrom.h` at the registration
 533phase. Currently, the capabilities are any of::
 534
 535	CDC_CLOSE_TRAY		/* can close tray by software control */
 536	CDC_OPEN_TRAY		/* can open tray */
 537	CDC_LOCK		/* can lock and unlock the door */
 538	CDC_SELECT_SPEED	/* can select speed, in units of * sim*150 ,kB/s */
 539	CDC_SELECT_DISC		/* drive is juke-box */
 540	CDC_MULTI_SESSION	/* can read sessions *> rm1* */
 541	CDC_MCN			/* can read Media Catalog Number */
 542	CDC_MEDIA_CHANGED	/* can report if disc has changed */
 543	CDC_PLAY_AUDIO		/* can perform audio-functions (play, pause, etc) */
 544	CDC_RESET		/* hard reset device */
 545	CDC_IOCTLS		/* driver has non-standard ioctls */
 546	CDC_DRIVE_STATUS	/* driver implements drive status */
 547
 548The capability flag is declared *const*, to prevent drivers from
 549accidentally tampering with the contents. The capability flags actually
 550inform `cdrom.c` of what the driver can do. If the drive found
 551by the driver does not have the capability, is can be masked out by
 552the *cdrom_device_info* variable *mask*. For instance, the SCSI CD-ROM
 553driver has implemented the code for loading and ejecting CD-ROM's, and
 554hence its corresponding flags in *capability* will be set. But a SCSI
 555CD-ROM drive might be a caddy system, which can't load the tray, and
 556hence for this drive the *cdrom_device_info* struct will have set
 557the *CDC_CLOSE_TRAY* bit in *mask*.
 558
 559In the file `cdrom.c` you will encounter many constructions of the type::
 560
 561	if (cdo->capability & ~cdi->mask & CDC _<capability>) ...
 562
 563There is no *ioctl* to set the mask... The reason is that
 564I think it is better to control the **behavior** rather than the
 565**capabilities**.
 566
 567Options
 568-------
 569
 570A final flag register controls the **behavior** of the CD-ROM
 571drives, in order to satisfy different users' wishes, hopefully
 572independently of the ideas of the respective author who happened to
 573have made the drive's support available to the Linux community. The
 574current behavior options are::
 575
 576	CDO_AUTO_CLOSE	/* try to close tray upon device open() */
 577	CDO_AUTO_EJECT	/* try to open tray on last device close() */
 578	CDO_USE_FFLAGS	/* use file_pointer->f_flags to indicate purpose for open() */
 579	CDO_LOCK	/* try to lock door if device is opened */
 580	CDO_CHECK_TYPE	/* ensure disc type is data if opened for data */
 581
 582The initial value of this register is
 583`CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK`, reflecting my own view on user
 584interface and software standards. Before you protest, there are two
 585new *ioctl()'s* implemented in `cdrom.c`, that allow you to control the
 586behavior by software. These are::
 587
 588	CDROM_SET_OPTIONS	/* set options specified in (int)arg */
 589	CDROM_CLEAR_OPTIONS	/* clear options specified in (int)arg */
 590
 591One option needs some more explanation: *CDO_USE_FFLAGS*. In the next
 592newsection we explain what the need for this option is.
 593
 594A software package `setcd`, available from the Debian distribution
 595and `sunsite.unc.edu`, allows user level control of these flags.
 596
 597
 598The need to know the purpose of opening the CD-ROM device
 599=========================================================
 600
 601Traditionally, Unix devices can be used in two different `modes`,
 602either by reading/writing to the device file, or by issuing
 603controlling commands to the device, by the device's *ioctl()*
 604call. The problem with CD-ROM drives, is that they can be used for
 605two entirely different purposes. One is to mount removable
 606file systems, CD-ROM's, the other is to play audio CD's. Audio commands
 607are implemented entirely through *ioctl()\'s*, presumably because the
 608first implementation (SUN?) has been such. In principle there is
 609nothing wrong with this, but a good control of the `CD player` demands
 610that the device can **always** be opened in order to give the
 611*ioctl* commands, regardless of the state the drive is in.
 612
 613On the other hand, when used as a removable-media disc drive (what the
 614original purpose of CD-ROM s is) we would like to make sure that the
 615disc drive is ready for operation upon opening the device. In the old
 616scheme, some CD-ROM drivers don't do any integrity checking, resulting
 617in a number of i/o errors reported by the VFS to the kernel when an
 618attempt for mounting a CD-ROM on an empty drive occurs. This is not a
 619particularly elegant way to find out that there is no CD-ROM inserted;
 620it more-or-less looks like the old IBM-PC trying to read an empty floppy
 621drive for a couple of seconds, after which the system complains it
 622can't read from it. Nowadays we can **sense** the existence of a
 623removable medium in a drive, and we believe we should exploit that
 624fact. An integrity check on opening of the device, that verifies the
 625availability of a CD-ROM and its correct type (data), would be
 626desirable.
 627
 628These two ways of using a CD-ROM drive, principally for data and
 629secondarily for playing audio discs, have different demands for the
 630behavior of the *open()* call. Audio use simply wants to open the
 631device in order to get a file handle which is needed for issuing
 632*ioctl* commands, while data use wants to open for correct and
 633reliable data transfer. The only way user programs can indicate what
 634their *purpose* of opening the device is, is through the *flags*
 635parameter (see `open(2)`). For CD-ROM devices, these flags aren't
 636implemented (some drivers implement checking for write-related flags,
 637but this is not strictly necessary if the device file has correct
 638permission flags). Most option flags simply don't make sense to
 639CD-ROM devices: *O_CREAT*, *O_NOCTTY*, *O_TRUNC*, *O_APPEND*, and
 640*O_SYNC* have no meaning to a CD-ROM.
 641
 642We therefore propose to use the flag *O_NONBLOCK* to indicate
 643that the device is opened just for issuing *ioctl*
 644commands. Strictly, the meaning of *O_NONBLOCK* is that opening and
 645subsequent calls to the device don't cause the calling process to
 646wait. We could interpret this as don't wait until someone has
 647inserted some valid data-CD-ROM. Thus, our proposal of the
 648implementation for the *open()* call for CD-ROM s is:
 649
 650- If no other flags are set than *O_RDONLY*, the device is opened
 651  for data transfer, and the return value will be 0 only upon successful
 652  initialization of the transfer. The call may even induce some actions
 653  on the CD-ROM, such as closing the tray.
 654- If the option flag *O_NONBLOCK* is set, opening will always be
 655  successful, unless the whole device doesn't exist. The drive will take
 656  no actions whatsoever.
 657
 658And what about standards?
 659-------------------------
 660
 661You might hesitate to accept this proposal as it comes from the
 662Linux community, and not from some standardizing institute. What
 663about SUN, SGI, HP and all those other Unix and hardware vendors?
 664Well, these companies are in the lucky position that they generally
 665control both the hardware and software of their supported products,
 666and are large enough to set their own standard. They do not have to
 667deal with a dozen or more different, competing hardware
 668configurations\ [#f3]_.
 669
 670.. [#f3]
 671
 672   Incidentally, I think that SUN's approach to mounting CD-ROM s is very
 673   good in origin: under Solaris a volume-daemon automatically mounts a
 674   newly inserted CD-ROM under `/cdrom/*<volume-name>*`.
 675
 676   In my opinion they should have pushed this
 677   further and have **every** CD-ROM on the local area network be
 678   mounted at the similar location, i. e., no matter in which particular
 679   machine you insert a CD-ROM, it will always appear at the same
 680   position in the directory tree, on every system. When I wanted to
 681   implement such a user-program for Linux, I came across the
 682   differences in behavior of the various drivers, and the need for an
 683   *ioctl* informing about media changes.
 684
 685We believe that using *O_NONBLOCK* to indicate that a device is being opened
 686for *ioctl* commands only can be easily introduced in the Linux
 687community. All the CD-player authors will have to be informed, we can
 688even send in our own patches to the programs. The use of *O_NONBLOCK*
 689has most likely no influence on the behavior of the CD-players on
 690other operating systems than Linux. Finally, a user can always revert
 691to old behavior by a call to
 692*ioctl(file_descriptor, CDROM_CLEAR_OPTIONS, CDO_USE_FFLAGS)*.
 693
 694The preferred strategy of *open()*
 695----------------------------------
 696
 697The routines in `cdrom.c` are designed in such a way that run-time
 698configuration of the behavior of CD-ROM devices (of **any** type)
 699can be carried out, by the *CDROM_SET/CLEAR_OPTIONS* *ioctls*. Thus, various
 700modes of operation can be set:
 701
 702`CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK`
 703   This is the default setting. (With *CDO_CHECK_TYPE* it will be better, in
 704   the future.) If the device is not yet opened by any other process, and if
 705   the device is being opened for data (*O_NONBLOCK* is not set) and the
 706   tray is found to be open, an attempt to close the tray is made. Then,
 707   it is verified that a disc is in the drive and, if *CDO_CHECK_TYPE* is
 708   set, that it contains tracks of type `data mode 1`. Only if all tests
 709   are passed is the return value zero. The door is locked to prevent file
 710   system corruption. If the drive is opened for audio (*O_NONBLOCK* is
 711   set), no actions are taken and a value of 0 will be returned.
 712
 713`CDO_AUTO_CLOSE | CDO_AUTO_EJECT | CDO_LOCK`
 714   This mimics the behavior of the current sbpcd-driver. The option flags are
 715   ignored, the tray is closed on the first open, if necessary. Similarly,
 716   the tray is opened on the last release, i. e., if a CD-ROM is unmounted,
 717   it is automatically ejected, such that the user can replace it.
 718
 719We hope that these option can convince everybody (both driver
 720maintainers and user program developers) to adopt the new CD-ROM
 721driver scheme and option flag interpretation.
 722
 723Description of routines in `cdrom.c`
 724====================================
 725
 726Only a few routines in `cdrom.c` are exported to the drivers. In this
 727new section we will discuss these, as well as the functions that `take
 728over` the CD-ROM interface to the kernel. The header file belonging
 729to `cdrom.c` is called `cdrom.h`. Formerly, some of the contents of this
 730file were placed in the file `ucdrom.h`, but this file has now been
 731merged back into `cdrom.h`.
 732
 733::
 734
 735	struct file_operations cdrom_fops
 736
 737The contents of this structure were described in cdrom_api_.
 738A pointer to this structure is assigned to the *fops* field
 739of the *struct gendisk*.
 740
 741::
 742
 743	int register_cdrom(struct cdrom_device_info *cdi)
 744
 745This function is used in about the same way one registers *cdrom_fops*
 746with the kernel, the device operations and information structures,
 747as described in cdrom_api_, should be registered with the
 748Uniform CD-ROM Driver::
 749
 750	register_cdrom(&<device>_info);
 751
 752
 753This function returns zero upon success, and non-zero upon
 754failure. The structure *<device>_info* should have a pointer to the
 755driver's *<device>_dops*, as in::
 756
 757	struct cdrom_device_info <device>_info = {
 758		<device>_dops;
 759		...
 760	}
 761
 762Note that a driver must have one static structure, *<device>_dops*, while
 763it may have as many structures *<device>_info* as there are minor devices
 764active. *Register_cdrom()* builds a linked list from these.
 765
 766
 767::
 768
 769	void unregister_cdrom(struct cdrom_device_info *cdi)
 770
 771Unregistering device *cdi* with minor number *MINOR(cdi->dev)* removes
 772the minor device from the list. If it was the last registered minor for
 773the low-level driver, this disconnects the registered device-operation
 774routines from the CD-ROM interface. This function returns zero upon
 775success, and non-zero upon failure.
 776
 777::
 778
 779	int cdrom_open(struct inode * ip, struct file * fp)
 780
 781This function is not called directly by the low-level drivers, it is
 782listed in the standard *cdrom_fops*. If the VFS opens a file, this
 783function becomes active. A strategy is implemented in this routine,
 784taking care of all capabilities and options that are set in the
 785*cdrom_device_ops* connected to the device. Then, the program flow is
 786transferred to the device_dependent *open()* call.
 787
 788::
 789
 790	void cdrom_release(struct inode *ip, struct file *fp)
 791
 792This function implements the reverse-logic of *cdrom_open()*, and then
 793calls the device-dependent *release()* routine. When the use-count has
 794reached 0, the allocated buffers are flushed by calls to *sync_dev(dev)*
 795and *invalidate_buffers(dev)*.
 796
 797
 798.. _cdrom_ioctl:
 799
 800::
 801
 802	int cdrom_ioctl(struct inode *ip, struct file *fp,
 803			unsigned int cmd, unsigned long arg)
 804
 805This function handles all the standard *ioctl* requests for CD-ROM
 806devices in a uniform way. The different calls fall into three
 807categories: *ioctl()'s* that can be directly implemented by device
 808operations, ones that are routed through the call *audio_ioctl()*, and
 809the remaining ones, that are presumable device-dependent. Generally, a
 810negative return value indicates an error.
 811
 812Directly implemented *ioctl()'s*
 813--------------------------------
 814
 815The following `old` CD-ROM *ioctl()*\ 's are implemented by directly
 816calling device-operations in *cdrom_device_ops*, if implemented and
 817not masked:
 818
 819`CDROMMULTISESSION`
 820	Requests the last session on a CD-ROM.
 821`CDROMEJECT`
 822	Open tray.
 823`CDROMCLOSETRAY`
 824	Close tray.
 825`CDROMEJECT_SW`
 826	If *arg\not=0*, set behavior to auto-close (close
 827	tray on first open) and auto-eject (eject on last release), otherwise
 828	set behavior to non-moving on *open()* and *release()* calls.
 829`CDROM_GET_MCN`
 830	Get the Media Catalog Number from a CD.
 831
 832*Ioctl*s routed through *audio_ioctl()*
 833---------------------------------------
 834
 835The following set of *ioctl()'s* are all implemented through a call to
 836the *cdrom_fops* function *audio_ioctl()*. Memory checks and
 837allocation are performed in *cdrom_ioctl()*, and also sanitization of
 838address format (*CDROM_LBA*/*CDROM_MSF*) is done.
 839
 840`CDROMSUBCHNL`
 841	Get sub-channel data in argument *arg* of type
 842	`struct cdrom_subchnl *`.
 843`CDROMREADTOCHDR`
 844	Read Table of Contents header, in *arg* of type
 845	`struct cdrom_tochdr *`.
 846`CDROMREADTOCENTRY`
 847	Read a Table of Contents entry in *arg* and specified by *arg*
 848	of type `struct cdrom_tocentry *`.
 849`CDROMPLAYMSF`
 850	Play audio fragment specified in Minute, Second, Frame format,
 851	delimited by *arg* of type `struct cdrom_msf *`.
 852`CDROMPLAYTRKIND`
 853	Play audio fragment in track-index format delimited by *arg*
 854	of type `struct cdrom_ti *`.
 855`CDROMVOLCTRL`
 856	Set volume specified by *arg* of type `struct cdrom_volctrl *`.
 857`CDROMVOLREAD`
 858	Read volume into by *arg* of type `struct cdrom_volctrl *`.
 859`CDROMSTART`
 860	Spin up disc.
 861`CDROMSTOP`
 862	Stop playback of audio fragment.
 863`CDROMPAUSE`
 864	Pause playback of audio fragment.
 865`CDROMRESUME`
 866	Resume playing.
 867
 868New *ioctl()'s* in `cdrom.c`
 869----------------------------
 870
 871The following *ioctl()'s* have been introduced to allow user programs to
 872control the behavior of individual CD-ROM devices. New *ioctl*
 873commands can be identified by the underscores in their names.
 874
 875`CDROM_SET_OPTIONS`
 876	Set options specified by *arg*. Returns the option flag register
 877	after modification. Use *arg = \rm0* for reading the current flags.
 878`CDROM_CLEAR_OPTIONS`
 879	Clear options specified by *arg*. Returns the option flag register
 880	after modification.
 881`CDROM_SELECT_SPEED`
 882	Select head-rate speed of disc specified as by *arg* in units
 883	of standard cdrom speed (176\,kB/sec raw data or
 884	150kB/sec file system data). The value 0 means `auto-select`,
 885	i. e., play audio discs at real time and data discs at maximum speed.
 886	The value *arg* is checked against the maximum head rate of the
 887	drive found in the *cdrom_dops*.
 888`CDROM_SELECT_DISC`
 889	Select disc numbered *arg* from a juke-box.
 890
 891	First disc is numbered 0. The number *arg* is checked against the
 892	maximum number of discs in the juke-box found in the *cdrom_dops*.
 893`CDROM_MEDIA_CHANGED`
 894	Returns 1 if a disc has been changed since the last call.
 895	For juke-boxes, an extra argument *arg*
 896	specifies the slot for which the information is given. The special
 897	value *CDSL_CURRENT* requests that information about the currently
 898	selected slot be returned.
 899`CDROM_TIMED_MEDIA_CHANGE`
 900	Checks whether the disc has been changed since a user supplied time
 901	and returns the time of the last disc change.
 902
 903	*arg* is a pointer to a *cdrom_timed_media_change_info* struct.
 904	*arg->last_media_change* may be set by calling code to signal
 905	the timestamp of the last known media change (by the caller).
 906	Upon successful return, this ioctl call will set
 907	*arg->last_media_change* to the latest media change timestamp (in ms)
 908	known by the kernel/driver and set *arg->has_changed* to 1 if
 909	that timestamp is more recent than the timestamp set by the caller.
 910`CDROM_DRIVE_STATUS`
 911	Returns the status of the drive by a call to
 912	*drive_status()*. Return values are defined in cdrom_drive_status_.
 913	Note that this call doesn't return information on the
 914	current playing activity of the drive; this can be polled through
 915	an *ioctl* call to *CDROMSUBCHNL*. For juke-boxes, an extra argument
 916	*arg* specifies the slot for which (possibly limited) information is
 917	given. The special value *CDSL_CURRENT* requests that information
 918	about the currently selected slot be returned.
 919`CDROM_DISC_STATUS`
 920	Returns the type of the disc currently in the drive.
 921	It should be viewed as a complement to *CDROM_DRIVE_STATUS*.
 922	This *ioctl* can provide *some* information about the current
 923	disc that is inserted in the drive. This functionality used to be
 924	implemented in the low level drivers, but is now carried out
 925	entirely in Uniform CD-ROM Driver.
 926
 927	The history of development of the CD's use as a carrier medium for
 928	various digital information has lead to many different disc types.
 929	This *ioctl* is useful only in the case that CDs have \emph {only
 930	one} type of data on them. While this is often the case, it is
 931	also very common for CDs to have some tracks with data, and some
 932	tracks with audio. Because this is an existing interface, rather
 933	than fixing this interface by changing the assumptions it was made
 934	under, thereby breaking all user applications that use this
 935	function, the Uniform CD-ROM Driver implements this *ioctl* as
 936	follows: If the CD in question has audio tracks on it, and it has
 937	absolutely no CD-I, XA, or data tracks on it, it will be reported
 938	as *CDS_AUDIO*. If it has both audio and data tracks, it will
 939	return *CDS_MIXED*. If there are no audio tracks on the disc, and
 940	if the CD in question has any CD-I tracks on it, it will be
 941	reported as *CDS_XA_2_2*. Failing that, if the CD in question
 942	has any XA tracks on it, it will be reported as *CDS_XA_2_1*.
 943	Finally, if the CD in question has any data tracks on it,
 944	it will be reported as a data CD (*CDS_DATA_1*).
 945
 946	This *ioctl* can return::
 947
 948		CDS_NO_INFO	/* no information available */
 949		CDS_NO_DISC	/* no disc is inserted, or tray is opened */
 950		CDS_AUDIO	/* Audio disc (2352 audio bytes/frame) */
 951		CDS_DATA_1	/* data disc, mode 1 (2048 user bytes/frame) */
 952		CDS_XA_2_1	/* mixed data (XA), mode 2, form 1 (2048 user bytes) */
 953		CDS_XA_2_2	/* mixed data (XA), mode 2, form 1 (2324 user bytes) */
 954		CDS_MIXED	/* mixed audio/data disc */
 955
 956	For some information concerning frame layout of the various disc
 957	types, see a recent version of `cdrom.h`.
 958
 959`CDROM_CHANGER_NSLOTS`
 960	Returns the number of slots in a juke-box.
 961`CDROMRESET`
 962	Reset the drive.
 963`CDROM_GET_CAPABILITY`
 964	Returns the *capability* flags for the drive. Refer to section
 965	cdrom_capabilities_ for more information on these flags.
 966`CDROM_LOCKDOOR`
 967	 Locks the door of the drive. `arg == 0` unlocks the door,
 968	 any other value locks it.
 969`CDROM_DEBUG`
 970	 Turns on debugging info. Only root is allowed to do this.
 971	 Same semantics as CDROM_LOCKDOOR.
 972
 973
 974Device dependent *ioctl()'s*
 975----------------------------
 976
 977Finally, all other *ioctl()'s* are passed to the function *dev_ioctl()*,
 978if implemented. No memory allocation or verification is carried out.
 979
 980How to update your driver
 981=========================
 982
 983- Make a backup of your current driver.
 984- Get hold of the files `cdrom.c` and `cdrom.h`, they should be in
 985  the directory tree that came with this documentation.
 986- Make sure you include `cdrom.h`.
 987- Change the 3rd argument of *register_blkdev* from `&<your-drive>_fops`
 988  to `&cdrom_fops`.
 989- Just after that line, add the following to register with the Uniform
 990  CD-ROM Driver::
 991
 992	register_cdrom(&<your-drive>_info);*
 993
 994  Similarly, add a call to *unregister_cdrom()* at the appropriate place.
 995- Copy an example of the device-operations *struct* to your
 996  source, e. g., from `cm206.c` *cm206_dops*, and change all
 997  entries to names corresponding to your driver, or names you just
 998  happen to like. If your driver doesn't support a certain function,
 999  make the entry *NULL*. At the entry *capability* you should list all
1000  capabilities your driver currently supports. If your driver
1001  has a capability that is not listed, please send me a message.
1002- Copy the *cdrom_device_info* declaration from the same example
1003  driver, and modify the entries according to your needs. If your
1004  driver dynamically determines the capabilities of the hardware, this
1005  structure should also be declared dynamically.
1006- Implement all functions in your `<device>_dops` structure,
1007  according to prototypes listed in  `cdrom.h`, and specifications given
1008  in cdrom_api_. Most likely you have already implemented
1009  the code in a large part, and you will almost certainly need to adapt the
1010  prototype and return values.
1011- Rename your `<device>_ioctl()` function to *audio_ioctl* and
1012  change the prototype a little. Remove entries listed in the first
1013  part in cdrom_ioctl_, if your code was OK, these are
1014  just calls to the routines you adapted in the previous step.
1015- You may remove all remaining memory checking code in the
1016  *audio_ioctl()* function that deals with audio commands (these are
1017  listed in the second part of cdrom_ioctl_. There is no
1018  need for memory allocation either, so most *case*s in the *switch*
1019  statement look similar to::
1020
1021	case CDROMREADTOCENTRY:
1022		get_toc_entry\bigl((struct cdrom_tocentry *) arg);
1023
1024- All remaining *ioctl* cases must be moved to a separate
1025  function, *<device>_ioctl*, the device-dependent *ioctl()'s*. Note that
1026  memory checking and allocation must be kept in this code!
1027- Change the prototypes of *<device>_open()* and
1028  *<device>_release()*, and remove any strategic code (i. e., tray
1029  movement, door locking, etc.).
1030- Try to recompile the drivers. We advise you to use modules, both
1031  for `cdrom.o` and your driver, as debugging is much easier this
1032  way.
1033
1034Thanks
1035======
1036
1037Thanks to all the people involved. First, Erik Andersen, who has
1038taken over the torch in maintaining `cdrom.c` and integrating much
1039CD-ROM-related code in the 2.1-kernel. Thanks to Scott Snyder and
1040Gerd Knorr, who were the first to implement this interface for SCSI
1041and IDE-CD drivers and added many ideas for extension of the data
1042structures relative to kernel~2.0. Further thanks to Heiko Eißfeldt,
1043Thomas Quinot, Jon Tombs, Ken Pizzini, Eberhard Mönkeberg and Andrew Kroll,
1044the Linux CD-ROM device driver developers who were kind
1045enough to give suggestions and criticisms during the writing. Finally
1046of course, I want to thank Linus Torvalds for making this possible in
1047the first place.