Power Management for DSP Cores

DSP cores are managed by using PM_RUNTIME_DSP context id and core index to the pm-runtime functions.

Some platforms may provide advanced power management of the DSP cores, including clock gating in idle, full power gating in idle, etc. depending on how the cores are integrated on that platform.

An implementation of platform API may provide customized idle entry function platform_wait_for_interrupt() which might simply call arch_wait_for_interrupt() or perform more sophisticated power transition to lower the power consumed when a DSP core is in idle.

An advanced power transitions may take more time to complete since the DSP core / platform has to return from a deeper power state. This additional latency is not always acceptable, for instance it might be better to complete the initial platform setup faster, with a quicker IPC request handling. Therefore the platform initialization code (platform_init()) may disable the advanced pm-runtime of the DSP core by calling pm_runtime_disable(PM_RUNTIME_DSP, 0) and wait for the driver to enable it once the initialization is complete by sending PM_GATE (PreventPowerGating=0) IPC. The PM_GATE handler calls either pm_runtime_enable(PM_RUNTIME_DSP, 0) or pm_runtime_disable(PM_RUNTIME_DSP, 0) depending on the value of the PPG flag. Note that enable/disable calls are ref-counted as there might be other internal clients interested in locking the DSP core in the highest state in order to keep the waiti entry/exit latency minimal and get better performance.

pm_runtime_is_active(PM_RUNTIME_DSP, 0) may be used to query the state of ref-counter and decide whether transitions to deeper power states are allowed inside the platform_wait_for_interrupt().

scale max 800 width

participant drv as "Driver"
participant ipc
participant mct as "master-core-task"
participant platform
participant pm_rt as "pm-runtime"

mct -> platform : platform_init()
   activate platform

   platform -> pm_rt :<b> pm_runtime_disable(PM_RUNTIME_DSP, 0)</b>
      activate pm_rt
      note right: Make latency of dsp core 0 int handler minimal
      pm_rt -> pm_rt : ref_cnt++
         note right: 0 -> 1
   platform <-- pm_rt
   deactivate pm_rt

mct <-- platform
deactivate platform

drv -> ipc : building topology
   ipc -> mct
      mct -> platform : <i>platform_wait_for_interrupt()</i>
         activate platform
         platform -> pm_rt : <b>pm_runtime_is_active(PM_RUNTIME_DSP, 0)</b>
            activate pm_rt
         platform <-- pm_rt : true
         deactivate pm_rt
         platform -> platform : d0-idle state
         deactivate platform
   ipc <-- mct
drv <-- ipc

Figure 22 Pm-runtime: DSP Core 0 initial setup

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participant drv as "Driver"
participant ipc
participant mct as "master-core-task"
participant platform
participant pm_rt as "pm-runtime"

drv -> ipc : PM_GATE (PPG = 0)
note left: Entering idle...
   activate ipc
   ipc -> pm_rt : <b>pm_runtime_enable(PM_RUNTIME_DSP, 0)</b>
      activate pm_rt
      pm_rt -> pm_rt : ref_cnt--
         note right: 1 -> 0 (if there was no other disable calls)
   ipc <-- pm_rt
   deactivate pm_rt
drv <-- ipc
deactivate ipc

mct -> platform : <i>platform_wait_for_interrupt()</i>
   activate platform
   platform -> pm_rt : <b>pm_runtime_is_active(PM_RUNTIME_DSP, 0)</b>
      activate pm_rt
   platform <-- pm_rt : false
   deactivate pm_rt
   platform -> platform : d?-idle state if available
   deactivate platform

Figure 23 Pm-runtime: DSP Core 0 enters idle