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 =====================
 The OMAP PM interface
 =====================
 
 This document describes the temporary OMAP PM interface.  Driver
 authors use these functions to communicate minimum latency or
 throughput constraints to the kernel power management code.
 Over time, the intention is to merge features from the OMAP PM
 interface into the Linux PM QoS code.
 
 Drivers need to express PM parameters which:
 
 - support the range of power management parameters present in the TI SRF;
 
 - separate the drivers from the underlying PM parameter
   implementation, whether it is the TI SRF or Linux PM QoS or Linux
   latency framework or something else;
 
 - specify PM parameters in terms of fundamental units, such as
   latency and throughput, rather than units which are specific to OMAP
   or to particular OMAP variants;
 
 - allow drivers which are shared with other architectures (e.g.,
   DaVinci) to add these constraints in a way which won't affect non-OMAP
   systems,
 
 - can be implemented immediately with minimal disruption of other
   architectures.
 
 
 This document proposes the OMAP PM interface, including the following
 five power management functions for driver code:
 
 1. Set the maximum MPU wakeup latency::
 
    (*pdata->set_max_mpu_wakeup_lat)(struct device *dev, unsigned long t)
 
 2. Set the maximum device wakeup latency::
 
    (*pdata->set_max_dev_wakeup_lat)(struct device *dev, unsigned long t)
 
 3. Set the maximum system DMA transfer start latency (CORE pwrdm)::
 
    (*pdata->set_max_sdma_lat)(struct device *dev, long t)
 
 4. Set the minimum bus throughput needed by a device::
 
    (*pdata->set_min_bus_tput)(struct device *dev, u8 agent_id, unsigned long r)
 
 5. Return the number of times the device has lost context::
 
    (*pdata->get_dev_context_loss_count)(struct device *dev)
 
 
 Further documentation for all OMAP PM interface functions can be
 found in arch/arm/plat-omap/include/mach/omap-pm.h.
 
 
 The OMAP PM layer is intended to be temporary
 ---------------------------------------------
 
 The intention is that eventually the Linux PM QoS layer should support
 the range of power management features present in OMAP3.  As this
 happens, existing drivers using the OMAP PM interface can be modified
 to use the Linux PM QoS code; and the OMAP PM interface can disappear.
 
 
 Driver usage of the OMAP PM functions
 -------------------------------------
 
 As the 'pdata' in the above examples indicates, these functions are
 exposed to drivers through function pointers in driver .platform_data
 structures.  The function pointers are initialized by the `board-*.c`
 files to point to the corresponding OMAP PM functions:
 
 - set_max_dev_wakeup_lat will point to
   omap_pm_set_max_dev_wakeup_lat(), etc.  Other architectures which do
   not support these functions should leave these function pointers set
   to NULL.  Drivers should use the following idiom::
 
         if (pdata->set_max_dev_wakeup_lat)
             (*pdata->set_max_dev_wakeup_lat)(dev, t);
 
 The most common usage of these functions will probably be to specify
 the maximum time from when an interrupt occurs, to when the device
 becomes accessible.  To accomplish this, driver writers should use the
 set_max_mpu_wakeup_lat() function to constrain the MPU wakeup
 latency, and the set_max_dev_wakeup_lat() function to constrain the
 device wakeup latency (from clk_enable() to accessibility).  For
 example::
 
         /* Limit MPU wakeup latency */
         if (pdata->set_max_mpu_wakeup_lat)
             (*pdata->set_max_mpu_wakeup_lat)(dev, tc);
 
         /* Limit device powerdomain wakeup latency */
         if (pdata->set_max_dev_wakeup_lat)
             (*pdata->set_max_dev_wakeup_lat)(dev, td);
 
         /* total wakeup latency in this example: (tc + td) */
 
 The PM parameters can be overwritten by calling the function again
 with the new value.  The settings can be removed by calling the
 function with a t argument of -1 (except in the case of
 set_max_bus_tput(), which should be called with an r argument of 0).
 
 The fifth function above, omap_pm_get_dev_context_loss_count(),
 is intended as an optimization to allow drivers to determine whether the
 device has lost its internal context.  If context has been lost, the
 driver must restore its internal context before proceeding.
 
 
 Other specialized interface functions
 -------------------------------------
 
 The five functions listed above are intended to be usable by any
 device driver.  DSPBridge and CPUFreq have a few special requirements.
 DSPBridge expresses target DSP performance levels in terms of OPP IDs.
 CPUFreq expresses target MPU performance levels in terms of MPU
 frequency.  The OMAP PM interface contains functions for these
 specialized cases to convert that input information (OPPs/MPU
 frequency) into the form that the underlying power management
 implementation needs:
 
 6. `(*pdata->dsp_get_opp_table)(void)`
 
 7. `(*pdata->dsp_set_min_opp)(u8 opp_id)`
 
 8. `(*pdata->dsp_get_opp)(void)`
 
 9. `(*pdata->cpu_get_freq_table)(void)`
 
 10. `(*pdata->cpu_set_freq)(unsigned long f)`
 
 11. `(*pdata->cpu_get_freq)(void)`
 
 Customizing OPP for platform
 ============================
 Defining CONFIG_PM should enable OPP layer for the silicon
 and the registration of OPP table should take place automatically.
 However, in special cases, the default OPP table may need to be
 tweaked, for e.g.:
 
  * enable default OPPs which are disabled by default, but which
    could be enabled on a platform
  * Disable an unsupported OPP on the platform
  * Define and add a custom opp table entry
    in these cases, the board file needs to do additional steps as follows:
 
 arch/arm/mach-omapx/board-xyz.c::
 
         #include "pm.h"
         ....
         static void __init omap_xyz_init_irq(void)
         {
                 ....
                 /* Initialize the default table */
                 omapx_opp_init();
                 /* Do customization to the defaults */
                 ....
         }
 
 NOTE:
   omapx_opp_init will be omap3_opp_init or as required
   based on the omap family.

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