[alsa-devel] [PATCH] ALSA: hda - delay resume haswell hdmi codec in system resume

[David Henningsson]

On 04/02/2013 02:18 PM, Takashi Iwai wrote:
> At Tue, 02 Apr 2013 13:53:16 +0200,
> David Henningsson wrote:
>>
>> On 03/27/2013 11:01 AM, Lin, Mengdong wrote:
>>>> -----Original Message-----
>>>> From: David Henningsson [mailto:david.henningsson at canonical.com]
>>>> Sent: Wednesday, March 27, 2013 4:19 PM
>>>
>>>>> The Haswell codec set_power_state ops (intel_haswell_set_power_state) will
>>>> only wait if this is a delayed resume and clear this flag after waiting. And
>>>> actually, there is no waiting even in this case. Because when 1st user operation
>>>> after system resume happens, Gfx already finishes resuming and audio
>>>> initialization, so as long as intel_haswell_wait_ready_to_resume() enable the
>>>> unsol event, the unsol event comes and so so waiting finishes. The 300ms time
>>>> out is set for safety consideration in case unsol event is lost. I've not observed
>>>> any unsol event lost till now.
>>>>
>>>> As for the timeout, I suggest you use the codec->bus->workq instead of
>>>> creating a new workq. I think that will also give us some serialisation, i e,
>>>> protection against race conditions if the timeout happen at the same time as
>>>> the unsol event.
>>>
>>> Hi David,
>>>
>>> The new added "resume_wq" for hdmi codec is a wait queue, not a work queue like codec->bus->workq.
>>> It's expected to wake up as soon as the unsol event is got.
>>
>> Sure; but I don't see why you need a wait queue for that? Why don't you
>> just call the resume path from the unsol event handler
>> (hdmi_present_sense, or its caller), and then also cancel the timeout
>> handler (which can then be in the normal workq)?
>
> Because the delayed resume actually fakes as if the resume is done.
> This is necessary not to block other device's resume operation.
>
> Since it looks as if ready, user-space might restart things soon again
> before the delayed resume is really finished.  So, some serialization
> is required there.

Lin, would it be possible to add some chain of events description to the 
bug commit? The different contexts are just boggling my mind :-)

Assume we have two cases, system idle after S3 and immediate playback 
after S3.

Is this correct:

System idle:
  1. System skips hda_call_codec_resume and sets codec->resume_delayed.
  2. Since the codec is not reinitialized, nothing powers up the codec, 
so this is all that happens.
  3. Since unsol events are not enabled (?), we have a bug that jack 
detection does not work and cannot be detected from userspace...?

Immediate playback:
  1. System skips hda_call_codec_resume and sets codec->resume_delayed.
  2. Process context wants to start playback, which powers up the codec 
and calls intel_haswell_set_power_state.
  3. intel_haswell_wait_ready_to_resume enables unsol events and starts 
to wait on ready_to_resume
  4. Gfx init finishes, and workq context fires the unsol event, which 
calls hdmi_intrinsic_event, which triggers the resume_wq.
  5. Process context and workq context now continues in parallel, 
potentially (but hopefully not) leading to race conditions?


-- 
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic