[alsa-devel] [PATCH v2 34/37] ALSA/dummy: Replace tasklet with softirq hrtimer

[Anna-Maria Gleixner]

From: Thomas Gleixner <tglx at linutronix.de>

The tasklet is used to defer the execution of snd_pcm_period_elapsed() to
the softirq context. Using the HRTIMER_MODE_SOFT mode invokes the timer
callback in softirq context as well which renders the tasklet useless.

[o-takashi: avoid stall due to a call of hrtimer_cancel() on a callback
            of hrtimer]
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Anna-Maria Gleixner <anna-maria at linutronix.de>
Cc: Jaroslav Kysela <perex at perex.cz>
Cc: Takashi Iwai <tiwai at suse.com>
Cc: Takashi Sakamoto <o-takashi at sakamocchi.jp>
Cc: alsa-devel at alsa-project.org
Link: http://lkml.kernel.org/r/20170905161820.jtysvxtfleunbbmf@breakpoint.cc

---
On 2017-09-06 01:05:43 [+0900], Takashi Sakamoto wrote:
> As Iwai-san mentioned, in this function, .trigger can be called in two
> cases; XRUN occurs and draining is done. Thus, let you change the comment as
> 'In cases of XRUN and draining, this calls .trigger to stop PCM substream.'.
> I'm sorry to trouble you.
> 
> snd_pcm_period_elapsed()
> ->snd_pcm_update_hw_ptr0()
>   ->snd_pcm_update_state()
>     ->snd_pcm_drain_done()
>       ...
>       ->.trigger(TRIGGER_STOP)
>     ->xrun()
>       ->snd_pcm_stop()
>         ...
>         ->.trigger(TRIGGER_STOP)
> 

I think you asked me just to update the comment. Did I do it right?

v2...v3: updated the comment as per Takashi Sakamoto's suggestion.
v1...v2: merged Takashi Sakamoto fixup of the original patch into v2.

 sound/drivers/dummy.c |   27 ++++++++++++---------------
 1 file changed, 12 insertions(+), 15 deletions(-)

--- a/sound/drivers/dummy.c
+++ b/sound/drivers/dummy.c
@@ -376,17 +376,9 @@ struct dummy_hrtimer_pcm {
 	ktime_t period_time;
 	atomic_t running;
 	struct hrtimer timer;
-	struct tasklet_struct tasklet;
 	struct snd_pcm_substream *substream;
 };
 
-static void dummy_hrtimer_pcm_elapsed(unsigned long priv)
-{
-	struct dummy_hrtimer_pcm *dpcm = (struct dummy_hrtimer_pcm *)priv;
-	if (atomic_read(&dpcm->running))
-		snd_pcm_period_elapsed(dpcm->substream);
-}
-
 static enum hrtimer_restart dummy_hrtimer_callback(struct hrtimer *timer)
 {
 	struct dummy_hrtimer_pcm *dpcm;
@@ -394,7 +386,14 @@ static enum hrtimer_restart dummy_hrtime
 	dpcm = container_of(timer, struct dummy_hrtimer_pcm, timer);
 	if (!atomic_read(&dpcm->running))
 		return HRTIMER_NORESTART;
-	tasklet_schedule(&dpcm->tasklet);
+	/*
+	 * In cases of XRUN and draining, this calls .trigger to stop PCM
+	 * substream.
+	 */
+	snd_pcm_period_elapsed(dpcm->substream);
+	if (!atomic_read(&dpcm->running))
+		return HRTIMER_NORESTART;
+
 	hrtimer_forward_now(timer, dpcm->period_time);
 	return HRTIMER_RESTART;
 }
@@ -404,7 +403,7 @@ static int dummy_hrtimer_start(struct sn
 	struct dummy_hrtimer_pcm *dpcm = substream->runtime->private_data;
 
 	dpcm->base_time = hrtimer_cb_get_time(&dpcm->timer);
-	hrtimer_start(&dpcm->timer, dpcm->period_time, HRTIMER_MODE_REL);
+	hrtimer_start(&dpcm->timer, dpcm->period_time, HRTIMER_MODE_REL_SOFT);
 	atomic_set(&dpcm->running, 1);
 	return 0;
 }
@@ -414,14 +413,14 @@ static int dummy_hrtimer_stop(struct snd
 	struct dummy_hrtimer_pcm *dpcm = substream->runtime->private_data;
 
 	atomic_set(&dpcm->running, 0);
-	hrtimer_cancel(&dpcm->timer);
+	if (!hrtimer_callback_running(&dpcm->timer))
+		hrtimer_cancel(&dpcm->timer);
 	return 0;
 }
 
 static inline void dummy_hrtimer_sync(struct dummy_hrtimer_pcm *dpcm)
 {
 	hrtimer_cancel(&dpcm->timer);
-	tasklet_kill(&dpcm->tasklet);
 }
 
 static snd_pcm_uframes_t
@@ -466,12 +465,10 @@ static int dummy_hrtimer_create(struct s
 	if (!dpcm)
 		return -ENOMEM;
 	substream->runtime->private_data = dpcm;
-	hrtimer_init(&dpcm->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	hrtimer_init(&dpcm->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_SOFT);
 	dpcm->timer.function = dummy_hrtimer_callback;
 	dpcm->substream = substream;
 	atomic_set(&dpcm->running, 0);
-	tasklet_init(&dpcm->tasklet, dummy_hrtimer_pcm_elapsed,
-		     (unsigned long)dpcm);
 	return 0;
 }