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linux-omap OPP management

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									Latency constraint framework - APIs

2010-08-02
Making Wireless

Types of Latency Constraints
   Interrupt or system latency
       Time taken once an interrupt occurs to the time its interrupt handler
       is scheduled
       Depends mainly on system sleep/C-state (MPU+Core power state)
       Any driver can put constraint on the deepest permissible C-state
       system can enter
   Device latency
       Time taken for a device to become accessible
       Includes bringing power domain out of RET or OFF, turning on
       clocks, & bringing clock domain out of inactive
       Depends mainly on power state of parent power domain
       Any driver can put constraint on the deepest power state its power
       domain can enter
       Allows more granular power control than preventing system sleep
Making Wireless

Interrupt Latency APIs
  omap_pm_set_max_mpu_wakeup_lat – set max MPU
  wakeup latency (constraint MPU power state)
      @qos_request: handle for the constraint; initialized to null
      @t: max MPU wakeup latency in microseconds; -1 to release
  omap_pm_set_max_sdma_lat – set max sDMA transfer
  start latency (constraint Core power state)
      @qos_request: handle for the constraint; initialized to null
      @t: max sDMA transfer start latency in microseconds; -1 to release
  Notes:
      Constraint is tracked for MPU and Core domain using PM QoS
      CPU idle selects lowest C-state subject to above constraints
      Files: plat/omap-pm.h, plat-omap/omap-pm.c,
         mach-omap2/cpuidle34xx.c (44xx.c in-progress)
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Device Latency APIs
  omap_pm_set_max_dev_wakeup_lat – Set power domain
  wakeup latency constraint (max device enable latency)
      @req_dev: struct device * requesting the constraint
      @dev: struct device * to set the constraint on
      @t: max wakeup latency constraint in microseconds; -1 to release
  Notes:
      Constraint is put on parent power domain of @dev
      Generally, @req_dev == @dev, i.e. constraint parent power domain
        Devices don’t have access to struct device * of some other device
      APIs to get device * for L3, MPUSS, & IVASS on 3430, similar will
      exist on 4430
      Device HWMOD info needs to be available for API to work
      Files: plat/omap-pm.h, plat-omap/omap-pm.c,
         mach-omap2/powerdomains44xx.h, mach-omap2/powerdomain.c
Making Wireless

Profiled Latencies
  Power domain wakeup latencies defined in powerdomains44xx.h
       static struct powerdomain core_44xx_pwrdm = {
                   .wakeup_lat = {         [PWRDM_FUNC_PWRST_OFF] = 100,
                                           [PWRDM_FUNC_PWRST_OSWR] = UNSUP_STATE,
                                           [PWRDM_FUNC_PWRST_CSWR] = 60,
                                           [PWRDM_FUNC_PWRST_ON] = 0,

  MPU & Core domain power states defined in cpuidle34xx.c
       #define OMAP3_STATE_C1 0 /* C1 - MPU WFI + Core active */
       #define OMAP3_STATE_C2 1 /* C2 - MPU WFI + Core inactive */
       #define OMAP3_STATE_C3 2 /* C3 - MPU CSWR + Core inactive */

  Each C-state has associated sleep, wakeup latencies, & threshold
       valid, sleep lat, wakeup lat, threshold
       {1, 2, 2, 5},           /* C1 */
       {1, 10, 10, 30},        /* C2 */

  Driver owners should refer to these values if they want to force
  specific power state
      We also need to work together & profile latencies for 4430 asap

								
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