realtek-pc-beep.rst

  1===============================
  2Realtek PC Beep Hidden Register
  3===============================
  4
  5This file documents the "PC Beep Hidden Register", which is present in certain
  6Realtek HDA codecs and controls a muxer and pair of passthrough mixers that can
  7route audio between pins but aren't themselves exposed as HDA widgets. As far
  8as I can tell, these hidden routes are designed to allow flexible PC Beep output
  9for codecs that don't have mixer widgets in their output paths. Why it's easier
 10to hide a mixer behind an undocumented vendor register than to just expose it
 11as a widget, I have no idea.
 12
 13Register Description
 14====================
 15
 16The register is accessed via processing coefficient 0x36 on NID 20h. Bits not
 17identified below have no discernible effect on my machine, a Dell XPS 13 9350::
 18
 19  MSB                           LSB
 20  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 21  | |h|S|L|         | B |R|       | Known bits
 22  +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 23  |0|0|1|1|  0x7  |0|0x0|1|  0x7  | Reset value
 24  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 25
 261Ah input select (B): 2 bits
 27  When zero, expose the PC Beep line (from the internal beep generator, when
 28  enabled with the Set Beep Generation verb on NID 01h, or else from the
 29  external PCBEEP pin) on the 1Ah pin node. When nonzero, expose the headphone
 30  jack (or possibly Line In on some machines) input instead. If PC Beep is
 31  selected, the 1Ah boost control has no effect.
 32
 33Amplify 1Ah loopback, left (L): 1 bit
 34  Amplify the left channel of 1Ah before mixing it into outputs as specified
 35  by h and S bits. Does not affect the level of 1Ah exposed to other widgets.
 36
 37Amplify 1Ah loopback, right (R): 1 bit
 38  Amplify the right channel of 1Ah before mixing it into outputs as specified
 39  by h and S bits. Does not affect the level of 1Ah exposed to other widgets.
 40
 41Loopback 1Ah to 21h [active low] (h): 1 bit
 42  When zero, mix 1Ah (possibly with amplification, depending on L and R bits)
 43  into 21h (headphone jack on my machine). Mixed signal respects the mute
 44  setting on 21h.
 45
 46Loopback 1Ah to 14h (S): 1 bit
 47  When one, mix 1Ah (possibly with amplification, depending on L and R bits)
 48  into 14h (internal speaker on my machine). Mixed signal **ignores** the mute
 49  setting on 14h and is present whenever 14h is configured as an output.
 50
 51Path diagrams
 52=============
 53
 541Ah input selection (DIV is the PC Beep divider set on NID 01h)::
 55
 56  <Beep generator>   <PCBEEP pin>    <Headphone jack>
 57          |                |                |
 58          +--DIV--+--!DIV--+       {1Ah boost control}
 59                  |                         |
 60                  +--(b == 0)--+--(b != 0)--+
 61                               |
 62               >1Ah (Beep/Headphone Mic/Line In)<
 63
 64Loopback of 1Ah to 21h/14h::
 65
 66               <1Ah (Beep/Headphone Mic/Line In)>
 67                               |
 68                        {amplify if L/R}
 69                               |
 70                  +-----!h-----+-----S-----+
 71                  |                        |
 72          {21h mute control}               |
 73                  |                        |
 74          >21h (Headphone)<     >14h (Internal Speaker)<
 75
 76Background
 77==========
 78
 79All Realtek HDA codecs have a vendor-defined widget with node ID 20h which
 80provides access to a bank of registers that control various codec functions.
 81Registers are read and written via the standard HDA processing coefficient
 82verbs (Set/Get Coefficient Index, Set/Get Processing Coefficient). The node is
 83named "Realtek Vendor Registers" in public datasheets' verb listings and,
 84apart from that, is entirely undocumented.
 85
 86This particular register, exposed at coefficient 0x36 and named in commits from
 87Realtek, is of note: unlike most registers, which seem to control detailed
 88amplifier parameters not in scope of the HDA specification, it controls audio
 89routing which could just as easily have been defined using standard HDA mixer
 90and selector widgets.
 91
 92Specifically, it selects between two sources for the input pin widget with Node
 93ID (NID) 1Ah: the widget's signal can come either from an audio jack (on my
 94laptop, a Dell XPS 13 9350, it's the headphone jack, but comments in Realtek
 95commits indicate that it might be a Line In on some machines) or from the PC
 96Beep line (which is itself multiplexed between the codec's internal beep
 97generator and external PCBEEP pin, depending on if the beep generator is
 98enabled via verbs on NID 01h). Additionally, it can mix (with optional
 99amplification) that signal onto the 21h and/or 14h output pins.
100
101The register's reset value is 0x3717, corresponding to PC Beep on 1Ah that is
102then amplified and mixed into both the headphones and the speakers. Not only
103does this violate the HDA specification, which says that "[a vendor defined
104beep input pin] connection may be maintained *only* while the Link reset
105(**RST#**) is asserted", it means that we cannot ignore the register if we care
106about the input that 1Ah would otherwise expose or if the PCBEEP trace is
107poorly shielded and picks up chassis noise (both of which are the case on my
108machine).
109
110Unfortunately, there are lots of ways to get this register configuration wrong.
111Linux, it seems, has gone through most of them. For one, the register resets
112after S3 suspend: judging by existing code, this isn't the case for all vendor
113registers, and it's led to some fixes that improve behavior on cold boot but
114don't last after suspend. Other fixes have successfully switched the 1Ah input
115away from PC Beep but have failed to disable both loopback paths. On my
116machine, this means that the headphone input is amplified and looped back to
117the headphone output, which uses the exact same pins! As you might expect, this
118causes terrible headphone noise, the character of which is controlled by the
1191Ah boost control. (If you've seen instructions online to fix XPS 13 headphone
120noise by changing "Headphone Mic Boost" in ALSA, now you know why.)
121
122The information here has been obtained through black-box reverse engineering of
123the ALC256 codec's behavior and is not guaranteed to be correct. It likely
124also applies for the ALC255, ALC257, ALC235, and ALC236, since those codecs
125seem to be close relatives of the ALC256. (They all share one initialization
126function.) Additionally, other codecs like the ALC225 and ALC285 also have this
127register, judging by existing fixups in ``patch_realtek.c``, but specific
128data (e.g. node IDs, bit positions, pin mappings) for those codecs may differ
129from what I've described here.