linux-kernel - RE: [alsa-devel] Timeout issues in wait_for

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Date:   Thu, 31 May 2018 07:43:10 +0000
From:   "Zengtao (B)" <prime.zeng@...ilicon.com>
To:     Takashi Iwai <tiwai@...e.de>
CC:     "perex@...ex.cz" <perex@...ex.cz>,
        "tiwai@...e.com" <tiwai@...e.com>,
        "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [alsa-devel] Timeout issues in wait_for_avail function

Hi Takashi:

Thank you for your reply.

>-----Original Message-----
>From: Takashi Iwai [mailto:tiwai@...e.de]
>Sent: Sunday, May 13, 2018 6:40 PM
>To: Zengtao (B) <prime.zeng@...ilicon.com>
>Cc: perex@...ex.cz; tiwai@...e.com; alsa-devel@...a-project.org;
>linux-kernel@...r.kernel.org
>Subject: Re: [alsa-devel] Timeout issues in wait_for_avail function
>
>On Mon, 07 May 2018 12:49:34 +0200,
>Zengtao (B) wrote:
>>
>> Hi perex and tiwai:
>>
>> I have met a timeout case when capture audio from snd-usb-audio
>> device, when the host call the pcm_read and get into the wait_for_avail
>function.
>> The following happends
>> 1.  No available data for capture(maybe because of the late response
>> audio data by the uac device)
>
>Hrm, in the case of capture, the data must be available.
>If it's not the case, something is wrong.
>
>> 2.  The current thread falls into sleep state and no one wakes up it.
>> 3.  The current thread will sleep 10s(schedule_timeout(1000)) and then
>wakeup.
>>
>> I have two question about the wait_for_avail:
>> 1. The timeout value too long, is it a reasonable value?
>> 2. Is there any mechanism to wake up the thread if there is data from the
>hw.
>
>The scenario above shouldn't happen, so no need for discussion.
>Rather we should check why it's woken up even though no data is available.
>

It really happens on my platform, and anyway 10 seconds timeout seems
not a reasonable value. And I don't there is any guarantee that there must be avail
data when we reach wait_for_avail, in fact, inside the wait_for_avail, there is branch
when no data is available.

>You can check the tracepoints to see the action of PCM stream, and confirm
>whether it's really no data, or it's just lost by some reason (or looks as if so).
>
I trace the data flow, we reach the wait_for_avail function before the usb snd data arrives, 
so we wait until 10 seconds timeout. 

>
>Takashi


Regards
Zengtao