Re: [alsa-devel] [PATCH v4 6/7] ALSA: aloop: Support selection of snd_timer instead of jiffies

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