[alsa-devel] Can I stop sound from driver as system error ?

[Takashi Sakamoto]

Hi,

On Dec 15 2015 11:47, Kuninori Morimoto wrote:
>>> I wonder can I stop sound from driver side (not from user-land)
>>> as system error during playback or capture ?
>>> How to do it if we can ?
>>
>> Stop PCM substreams with SNDRV_PCM_STATE_XRUN. In detail, see
>> implementation of snd_pcm_stop_xrun() available in 3.19 or
>> later. Then, userspace applications cannot continue to read/write PCM
>> frames via ALSA uapi.
>>
>> When using alsa-lib, then, userspace applications receive -EPIPE as
>> return value of calling PCM APIs. The applications can stop their
>> operation for PCM character devices, or can also recover the XRUN
>> state of PCM substream by calling snd_pcm_prepare(). In this case,
>> struct snd_pcm_ops.prepare() is called again in driver side.
>>
>> I think tinyalsa works almost the same.
>
> Thank you for detail explanation !
> I will try/investigate it

The state of PCM substreams can be changed by process context, software 
IRQ (softIRQ) context and hardware IRQ (hardIRQ) context. Therefore, 
ALSA PCM core uses spin_lock_irqsave()/spin_lock_irqrestore() to manage 
the race condition with softIRQ disabled, hardIRQ disabled and 
kernel-preemption disabled in local processor core.

As a glance, snd-soc-rcar in upstream pushes many operations to PCM 
trigger processing, while this trigger processing is protected with the 
critical section.

When you program the xrun handling in hardware/software IRQ context, 
it's better to pay enough attention to the lock, because any process 
contexts can spin in some processor cores.


Regards

Takashi Sakamoto