Re: [RFC PATCH] ALSA: hda/hdmi: fix race in handling acomp ELD notification at resume

From: Kai Vehmanen <kvehmanen@nosignal.fi>
To: Takashi Iwai <tiwai@suse.de>
Cc: alsa-devel@alsa-project.org,
	pierre-louis.bossart@linux.intel.com,
	Kai Vehmanen <kai.vehmanen@linux.intel.com>
Subject: Re: [RFC PATCH] ALSA: hda/hdmi: fix race in handling acomp ELD notification at resume
Date: Thu, 8 Apr 2021 13:49:15 +0300 (EEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.2104081341410.1629@ecabase.localdomain> (raw)
In-Reply-To: <s5hft0189bi.wl-tiwai@suse.de>

Hi,

On Thu, 8 Apr 2021, Takashi Iwai wrote:
> OK, that's a messy problem, indeed.  It's partly because of ASoC
> referred resume that is completely independent from the rest resume
> via HD-audio bus.  More badly, this can't be managed via the device
> link because the resume callback itself has been processed.
>
> And, IIUC, another part of the problem is that i915 notifies the HPD
> *after* the resume completion, right?  Then indeed it can be racy.

yes, exactly.

>> Seems quite odd indeed, but I've now got reports of systems where this is
>> hit, and unfortunately it's very systematic on these systems. By adding
>> some arbitrary delay to soc_resume_deferred(), I could easily hit this
>> myself as well on the systems I have at hand.
>
> Another possible fix would be to check dev->power.power_state instead
> of the global card state.  This is set in each PM callback in
> hda_codec.c to indicate the current PM state of the codec.  Something
> like below.  Let me know if this works, too.

Thanks, this works in my setup and is much cleaner. I think this is also 
more robust. I realized that with snd_power_get_state() check, there is a 
theoretical race still possible if notify comes before 
soc_resume_deferred() gets scheduled (i.e. delay is not within 
soc_resume_deferred() but in getting it scheduled to begin with). This 
would seem really unlikely, but it's a possible race nevertheless.

I'll update the patch to use dev->power.power_state, ask people with 
affected systems to double check, and I'll send a V2.

Br, Kai