[alsa-devel] [PATCH 23/25] ALSA/dummy: Replace tasklet with softirq hrtimer

[Takashi Sakamoto]

On p 1 2017 20:58, Takashi Iwai wrote:
>> >From 07d61ba2a1c0e06e914443225e194d99f2d8c58d Mon Sep 17 00:00:00 2001
>> From: Takashi Sakamoto <o-takashi at sakamocchi.jp>
>> Date: Fri, 1 Sep 2017 19:10:18 +0900
>> Subject: [PATCH] ALSA: dummy: avoid stall due to a call of hrtimer_cancel() on
>>   a callback of hrtimer
>>
>> A call of 'htrimer_cancel()' on a callback of hrtimer brings endless loop
>> because 'struct hrtimer_clock_base.running' is not NULL on the callback.
>> In hrtimer subsystem, this member is used to indicate the instance of
>> hrtimer gets callbacks and there's a helper function,
>> 'hrtimer_callback_running()' to check it.
>>
>> ALSA dummy driver uses hrtimer to emulate hardware interrupt per period
>> of PCM buffer. When XRUN occurs on PCM substream, in a call of
>> 'snd_pcm_period_elapsed()', 'struct snd_pcm_ops.stop()' is called to
>> stop the substream. In current implementation, 'hrtimer_cancel()' is
>> used to wait for cancellation of hrtimer. However, as described, this
>> brings endless loop.
> 
> It's not only about XRUN.  When the stream finishes the draining, it
> stops the stream gracefully -- that is the very normal operation.

I overlooked it. Thanks for your indication.

>> For this problem, this commit uses 'hrtimer_callback_running()' to
>> detect whether to be on a callback of hrtimer or not, then skip
>> cancellation of hrtimer in hrtimer callbacks. Furthermore, at a case of
>> XRUN, hrtimer callback returns HRTIMER_NORESTART after a call of
>> 'snd_pcm_period_elapsed()' to discontinue hrtimr because cancellation is
>> skipped.
>>
>> Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
> 
> It's better to fold the fix into the original patch instead of
> introducing a bug and fixing it.

Yep. I request the authors to include this fix.


Well, in sound subsystem, there're a few drivers which uses hrtimer:
  - snd-pcsp
  - snd-sh-dac-audio
  - snd-soc-imx-pcm-fiq

As a quick glance, 'snd-sh-dac-audio' includes the same bug, too. 
Additionally, 'snd-soc-imx-pcm-fiq' maintains hrtimer with loose manner 
in a point of state of PCM substream and it shall gain the same bug if 
improved. Later, I posted some patches for them.


Thanks

Takashi Sakamoto