[Sound-open-firmware] Design decisions for PM flow

[Pierre-Louis Bossart]

On 12/14/20 1:28 PM, Daniel Baluta wrote:
> Hi Ranjani,
> 
> I have few questions about PM flow in SOF.
> 
> 1) Firmware is always loaded at SOF device probe.
> 
> Why is that? Is this just an early check that DSP is OK?
> Because after SND_SOF_SUSPEND_DELAY_MS the SOF device
> is suspended and DSP is turned off.

It's standard to bring up audio on startup so that you can play a chime 
and start configuring sound servers.

> 
> 2) What is the correct behavior when immediately after boot
> there is a PM suspend/PM resume cycle.
> 
> Here is what happens on IMX.
> 
> PM suspend -> will do nothing because DSP is already suspended
> PM resume -> will reload the firmware.
> 
> But now the firmware is loaded and DSP is up. And the system stays like this.

Not clear on what you are asking.

If you suspend immediately, i.e. without a couple of ms, despite having 
configured the device with pm_runtime_use_autosuspend and a generous 
delay, you are probably missing a pm_runtime_mark_last_busy() on 
probe/resume to reset counters and prevent an immediate pm_runtime suspend?

> How does this looks like on Intel platforms.

We only suspend 4+ seconds after boot or when PulseAudio or other 
servers are no longer using the device