[alsa-devel] [PATCH - JACK plugin 4/4] jack: Support snd_pcm_drain()

[Wischer, Timo (ADITG/ESB)]

Hi Jaroslav,

> But it changes the PCM API behaviour, so some apps might have trouble
> with it, because it might cause the unexpected task blocking. The
> correct behaviour is return with -EAGAIN

I have checked all IO plugins (pulse, oss, a52, jack) only a52 could return -EAGAIN.
oss and pulse do always block and it seems that they are not updating the file descriptor in case of draining.
Therefore snd_pcm_wait() would block infinitely.

I think an application developer would not expect that he has to call snd_pcm_drop() after snd_pcm_drain().
Therefore snd_pcm_drop has to be called internally.
(On a real hardware it will be handled in the kernel)

Therefore I think snd_pcm_drop() has to be called from snd_pcm_wait() before returning.
But snd_pcm_wait callback is not available in the IO plug API.
Therefore I think we have to call snd_pcm_drop() from snd_pcm_ioplug_poll_revents().
But this looks a little bit ugly for me.
Do you have another idea?

One approach could be the following
snd_pcm_ioplug_poll_revents()
{
	...
	if (draining)
		snd_pcm_drop()
}

snd_pcm_ioplug_drain()
{
	int err = 0
	if (io->data->callback->drain)
		err = io->data->callback->drain(io->data);
	if (err != -EAGAIN) {
		snd_pcm_lock(pcm);
		err = snd_pcm_ioplug_drop(pcm);
		snd_pcm_unlock(pcm);
	}
	return err;
}

But I am not aware of if this would brake any of the existing IO plugins.
I would prefer to avoid these changes.

Best regards

Timo Wischer

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