linux-kernel - Re: [PATCH v4 6/7] ALSA: aloop: Support selection of snd

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Date:   Wed, 20 Nov 2019 15:33:50 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Andrew Gabbasov <andrew_gabbasov@...tor.com>
Cc:     <alsa-devel@...a-project.org>, <linux-kernel@...r.kernel.org>,
        "Jaroslav Kysela" <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
        Timo Wischer <twischer@...adit-jv.com>
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().


thanks,

Takashi