omap_pm.rst

  1=====================
  2The OMAP PM interface
  3=====================
  4
  5This document describes the temporary OMAP PM interface.  Driver
  6authors use these functions to communicate minimum latency or
  7throughput constraints to the kernel power management code.
  8Over time, the intention is to merge features from the OMAP PM
  9interface into the Linux PM QoS code.
 10
 11Drivers need to express PM parameters which:
 12
 13- support the range of power management parameters present in the TI SRF;
 14
 15- separate the drivers from the underlying PM parameter
 16  implementation, whether it is the TI SRF or Linux PM QoS or Linux
 17  latency framework or something else;
 18
 19- specify PM parameters in terms of fundamental units, such as
 20  latency and throughput, rather than units which are specific to OMAP
 21  or to particular OMAP variants;
 22
 23- allow drivers which are shared with other architectures (e.g.,
 24  DaVinci) to add these constraints in a way which won't affect non-OMAP
 25  systems,
 26
 27- can be implemented immediately with minimal disruption of other
 28  architectures.
 29
 30
 31This document proposes the OMAP PM interface, including the following
 32five power management functions for driver code:
 33
 341. Set the maximum MPU wakeup latency::
 35
 36   (*pdata->set_max_mpu_wakeup_lat)(struct device *dev, unsigned long t)
 37
 382. Set the maximum device wakeup latency::
 39
 40   (*pdata->set_max_dev_wakeup_lat)(struct device *dev, unsigned long t)
 41
 423. Set the maximum system DMA transfer start latency (CORE pwrdm)::
 43
 44   (*pdata->set_max_sdma_lat)(struct device *dev, long t)
 45
 464. Set the minimum bus throughput needed by a device::
 47
 48   (*pdata->set_min_bus_tput)(struct device *dev, u8 agent_id, unsigned long r)
 49
 505. Return the number of times the device has lost context::
 51
 52   (*pdata->get_dev_context_loss_count)(struct device *dev)
 53
 54
 55Further documentation for all OMAP PM interface functions can be
 56found in arch/arm/plat-omap/include/mach/omap-pm.h.
 57
 58
 59The OMAP PM layer is intended to be temporary
 60---------------------------------------------
 61
 62The intention is that eventually the Linux PM QoS layer should support
 63the range of power management features present in OMAP3.  As this
 64happens, existing drivers using the OMAP PM interface can be modified
 65to use the Linux PM QoS code; and the OMAP PM interface can disappear.
 66
 67
 68Driver usage of the OMAP PM functions
 69-------------------------------------
 70
 71As the 'pdata' in the above examples indicates, these functions are
 72exposed to drivers through function pointers in driver .platform_data
 73structures.  The function pointers are initialized by the `board-*.c`
 74files to point to the corresponding OMAP PM functions:
 75
 76- set_max_dev_wakeup_lat will point to
 77  omap_pm_set_max_dev_wakeup_lat(), etc.  Other architectures which do
 78  not support these functions should leave these function pointers set
 79  to NULL.  Drivers should use the following idiom::
 80
 81        if (pdata->set_max_dev_wakeup_lat)
 82            (*pdata->set_max_dev_wakeup_lat)(dev, t);
 83
 84The most common usage of these functions will probably be to specify
 85the maximum time from when an interrupt occurs, to when the device
 86becomes accessible.  To accomplish this, driver writers should use the
 87set_max_mpu_wakeup_lat() function to constrain the MPU wakeup
 88latency, and the set_max_dev_wakeup_lat() function to constrain the
 89device wakeup latency (from clk_enable() to accessibility).  For
 90example::
 91
 92        /* Limit MPU wakeup latency */
 93        if (pdata->set_max_mpu_wakeup_lat)
 94            (*pdata->set_max_mpu_wakeup_lat)(dev, tc);
 95
 96        /* Limit device powerdomain wakeup latency */
 97        if (pdata->set_max_dev_wakeup_lat)
 98            (*pdata->set_max_dev_wakeup_lat)(dev, td);
 99
100        /* total wakeup latency in this example: (tc + td) */
101
102The PM parameters can be overwritten by calling the function again
103with the new value.  The settings can be removed by calling the
104function with a t argument of -1 (except in the case of
105set_max_bus_tput(), which should be called with an r argument of 0).
106
107The fifth function above, omap_pm_get_dev_context_loss_count(),
108is intended as an optimization to allow drivers to determine whether the
109device has lost its internal context.  If context has been lost, the
110driver must restore its internal context before proceeding.
111
112
113Other specialized interface functions
114-------------------------------------
115
116The five functions listed above are intended to be usable by any
117device driver.  DSPBridge and CPUFreq have a few special requirements.
118DSPBridge expresses target DSP performance levels in terms of OPP IDs.
119CPUFreq expresses target MPU performance levels in terms of MPU
120frequency.  The OMAP PM interface contains functions for these
121specialized cases to convert that input information (OPPs/MPU
122frequency) into the form that the underlying power management
123implementation needs:
124
1256. `(*pdata->dsp_get_opp_table)(void)`
126
1277. `(*pdata->dsp_set_min_opp)(u8 opp_id)`
128
1298. `(*pdata->dsp_get_opp)(void)`
130
1319. `(*pdata->cpu_get_freq_table)(void)`
132
13310. `(*pdata->cpu_set_freq)(unsigned long f)`
134
13511. `(*pdata->cpu_get_freq)(void)`
136
137Customizing OPP for platform
138============================
139Defining CONFIG_PM should enable OPP layer for the silicon
140and the registration of OPP table should take place automatically.
141However, in special cases, the default OPP table may need to be
142tweaked, for e.g.:
143
144 * enable default OPPs which are disabled by default, but which
145   could be enabled on a platform
146 * Disable an unsupported OPP on the platform
147 * Define and add a custom opp table entry
148   in these cases, the board file needs to do additional steps as follows:
149
150arch/arm/mach-omapx/board-xyz.c::
151
152	#include "pm.h"
153	....
154	static void __init omap_xyz_init_irq(void)
155	{
156		....
157		/* Initialize the default table */
158		omapx_opp_init();
159		/* Do customization to the defaults */
160		....
161	}
162
163NOTE:
164  omapx_opp_init will be omap3_opp_init or as required
165  based on the omap family.