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authorRussell King <rmk+kernel@arm.linux.org.uk>2009-09-24 20:22:33 (GMT)
committerRussell King <rmk+kernel@arm.linux.org.uk>2009-09-24 20:22:33 (GMT)
commitbaea7b946f00a291b166ccae7fcfed6c01530cc6 (patch)
tree4aa275fbdbec9c7b9b4629e8bee2bbecd3c6a6af /Documentation/arm
parentae19ffbadc1b2100285a5b5b3d0a4e0a11390904 (diff)
parent94e0fb086fc5663c38bbc0fe86d698be8314f82f (diff)
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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 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
+ /* 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)

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