omap_pm

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 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)