[Sound-open-firmware] About sof and D0 D3 states, 'standby' and 'suspend to RAM'

[Pierre-Louis Bossart]

Hi Josep,

> I have found[1]  that sof driver works (for now?) only two power
> states: D0 (DSP ON) and D3 (DSP OFF).
> In a laptop with battery, not connected to AC, I have many
> "D0->D3->D0" state changes during system operation [3].When entering
> into to D0, all firmware is loaded again, DSP set up and so (obvious,
> as DSPs where off).
> 
> My guess is that this transition happens in a power state transition
> lower than Suspend to RAM, i.e., Suspend-to-idle or Standby. [2]
> 
> Should not be the transition to D3 state done in a kernel transition
> to S3 or higher only (Suspend to RAM, hibernate)?

Adding to Paul Olaru's answer: it's a feature not a bug ;-)

There are two types of suspend/resume operations, tied to transitions 
between S (platform) and D (device) states.

a) system suspend/resume, where the platform goes to a different S 
state. On system suspend, the platform goes e.g. to S3 to hibernate, and 
that forces the device state to go to D3 - platform and device states 
are not completely independent.

b) pm_runtime suspend/resume. Here the platform remains in S0 (active) 
but since when the audio subsystem is not used, the pm_runtime framework 
will trigger a transition to D3, typically after a timeout. Conversely 
every time you want to play/record the ALSA/ASoC framework will do a 
pm_runtime_get_sync() and force the DSP to be powered, initialized, etc.

Not all platforms have the same power management capabilities. On Intel 
recent hardware, the DSP is power-gated so all state is lost. That's why 
we have to re-download it on each D0 transition. If you look at older 
hardware, we didn't have power gating, only clock gating so 
re-downloading the firmware was not necessarily needed for pm_runtime, 
and only needed on system resume.

If you look at sound/soc/sof/pm.c you will see we have two hooks for 
each type of suspend/resume, and calls into the platform specific parts 
for the implementation.

To be complete I should also add that there are additional states such 
as S0ix and D0ix but that's likely not enabled on your HDaudio-based 
platform. There's also G, P and C states if you really want to get into 
the complicated stuff [1]

Hope this helps
-Pierre

[1] 
https://en.wikipedia.org/wiki/Advanced_Configuration_and_Power_Interface#Power_states