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