RE: [PATCH v4 6/7] ALSA: aloop: Support selection of snd_timer instead of jiffies

[Andrew Gabbasov <andrew_gabbasov@mentor.com>]

Hello Takashi,

> -----Original Message-----
> From: Takashi Iwai [mailto:tiwai@suse.de]
> Sent: Wednesday, November 20, 2019 5:34 PM
> To: Gabbasov, Andrew
> Cc: alsa-devel@alsa-project.org; linux-kernel@vger.kernel.org; Jaroslav
> Kysela; Takashi Iwai; Timo Wischer
> Subject: Re: [PATCH v4 6/7] ALSA: aloop: Support selection of snd_timer
> instead of jiffies
> 
> On Wed, 20 Nov 2019 12:58:55 +0100,
> Andrew Gabbasov wrote:
> > +/* call in loopback->cable_lock */
> > +static int loopback_snd_timer_open(struct loopback_pcm *dpcm)
> > +{
> > +	int err = 0;
> > +	struct snd_timer_id tid = {
> > +		.dev_class = SNDRV_TIMER_CLASS_PCM,
> > +		.dev_sclass = SNDRV_TIMER_SCLASS_APPLICATION,
> > +	};
> > +	struct snd_timer_instance *timeri;
> > +	struct loopback_cable *cable = dpcm->cable;
> > +
> > +	spin_lock_irq(&cable->lock);
> > +
> > +	/* check if timer was already opened. It is only opened once
> > +	 * per playback and capture subdevice (aka cable).
> > +	 */
> > +	if (cable->snd_timer.instance)
> > +		goto unlock;
> > +
> > +	err = loopback_parse_timer_id(dpcm->loopback->timer_source, &tid);
> > +	if (err < 0) {
> > +		pcm_err(dpcm->substream->pcm,
> > +			"Parsing timer source \'%s\' failed with %d",
> > +			dpcm->loopback->timer_source, err);
> > +		goto unlock;
> > +	}
> > +
> > +	cable->snd_timer.stream = dpcm->substream->stream;
> > +	cable->snd_timer.id = tid;
> > +
> > +	timeri = snd_timer_instance_new(dpcm->loopback->card->id);
> > +	if (!timeri) {
> > +		err = -ENOMEM;
> > +		goto unlock;
> > +	}
> > +	/* The callback has to be called from another tasklet. If
> > +	 * SNDRV_TIMER_IFLG_FAST is specified it will be called from the
> > +	 * snd_pcm_period_elapsed() call of the selected sound card.
> > +	 * snd_pcm_period_elapsed() helds snd_pcm_stream_lock_irqsave().
> > +	 * Due to our callback loopback_snd_timer_function() also calls
> > +	 * snd_pcm_period_elapsed() which calls
> snd_pcm_stream_lock_irqsave().
> > +	 * This would end up in a dead lock.
> > +	 */
> > +	timeri->flags |= SNDRV_TIMER_IFLG_AUTO;
> > +	timeri->callback = loopback_snd_timer_function;
> > +	timeri->callback_data = (void *)cable;
> > +	timeri->ccallback = loopback_snd_timer_event;
> > +
> > +	/* snd_timer_close() and snd_timer_open() should not be called with
> > +	 * locked spinlock because both functions can block on a mutex. The
> > +	 * mutex loopback->cable_lock is kept locked. Therefore
> snd_timer_open()
> > +	 * cannot be called a second time by the other device of the same
> cable.
> > +	 * Therefore the following issue cannot happen:
> > +	 * [proc1] Call loopback_timer_open() ->
> > +	 *	   Unlock cable->lock for snd_timer_close/open() call
> > +	 * [proc2] Call loopback_timer_open() -> snd_timer_open(),
> > +	 *	   snd_timer_start()
> > +	 * [proc1] Call snd_timer_open() and overwrite running timer
> > +	 *	   instance
> > +	 */
> > +	spin_unlock_irq(&cable->lock);
> > +	err = snd_timer_open(timeri, &cable->snd_timer.id, current->pid);
> > +	if (err < 0) {
> > +		pcm_err(dpcm->substream->pcm,
> > +			"snd_timer_open (%d,%d,%d) failed with %d",
> > +			cable->snd_timer.id.card,
> > +			cable->snd_timer.id.device,
> > +			cable->snd_timer.id.subdevice,
> > +			err);
> > +		snd_timer_instance_free(timeri);
> > +		return err;
> > +	}
> > +	spin_lock_irq(&cable->lock);
> > +
> > +	cable->snd_timer.instance = timeri;
> > +
> > +	/* initialise a tasklet used for draining */
> > +	tasklet_init(&cable->snd_timer.event_tasklet,
> > +		     loopback_snd_timer_tasklet, (unsigned long)timeri);
> 
> This has to be set before snd_timer_open().  The callback might be
> called immediately after snd_timer_open().

This tasklet is used/scheduled only in ccallback (not regular tick
callback),
and only for SNDRV_TIMER_EVENT_MSTOP event. Can this event really happen
immediately after snd_timer_open()?

Thanks!

Best regards,
Andrew