ALSA: dummy - Better jiffies handling

In the system-timer mode, snd-dummy driver issues each tick to update
the position.  This is highly inefficient and even inaccurate if the
timer can't be triggered at each tick.

Now rewritten to wake up only at the period boundary.  The position
is calculated from the current jiffies.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/drivers/dummy.c

@@ -205,21 +205,43 @@ struct snd_dummy {
 struct dummy_systimer_pcm {
 	spinlock_t lock;
 	struct timer_list timer;
-	unsigned int pcm_buffer_size;
-	unsigned int pcm_period_size;
-	unsigned int pcm_bps;		/* bytes per second */
-	unsigned int pcm_hz;		/* HZ */
-	unsigned int pcm_irq_pos;	/* IRQ position */
-	unsigned int pcm_buf_pos;	/* position in buffer */
+	unsigned long base_time;
+	unsigned int frac_pos;	/* fractional sample position (based HZ) */
+	unsigned int frac_buffer_size;	/* buffer_size * HZ */
+	unsigned int frac_period_size;	/* period_size * HZ */
+	unsigned int rate;
 	struct snd_pcm_substream *substream;
 };
 
+static void dummy_systimer_rearm(struct dummy_systimer_pcm *dpcm)
+{
+	unsigned long frac;
+
+	frac = dpcm->frac_pos % dpcm->frac_period_size;
+	dpcm->timer.expires = jiffies +
+		(dpcm->frac_period_size + dpcm->rate - 1) / dpcm->rate;
+	add_timer(&dpcm->timer);
+}
+
+static void dummy_systimer_update(struct dummy_systimer_pcm *dpcm)
+{
+	unsigned long delta;
+
+	delta = jiffies - dpcm->base_time;
+	if (!delta)
+		return;
+	dpcm->base_time = jiffies;
+	dpcm->frac_pos += delta * dpcm->rate;
+	while (dpcm->frac_pos >= dpcm->frac_buffer_size)
+		dpcm->frac_pos -= dpcm->frac_buffer_size;
+}
+
 static int dummy_systimer_start(struct snd_pcm_substream *substream)
 {
 	struct dummy_systimer_pcm *dpcm = substream->runtime->private_data;
 	spin_lock(&dpcm->lock);
-	dpcm->timer.expires = 1 + jiffies;
-	add_timer(&dpcm->timer);
+	dpcm->base_time = jiffies;
+	dummy_systimer_rearm(dpcm);
 	spin_unlock(&dpcm->lock);
 	return 0;
 }
@@ -237,20 +259,11 @@ static int dummy_systimer_prepare(struct snd_pcm_substream *substream)
 {
 	struct snd_pcm_runtime *runtime = substream->runtime;
 	struct dummy_systimer_pcm *dpcm = runtime->private_data;
-	int bps;
 
-	bps = snd_pcm_format_width(runtime->format) * runtime->rate *
-		runtime->channels / 8;
-
-	if (bps <= 0)
-		return -EINVAL;
-
-	dpcm->pcm_bps = bps;
-	dpcm->pcm_hz = HZ;
-	dpcm->pcm_buffer_size = snd_pcm_lib_buffer_bytes(substream);
-	dpcm->pcm_period_size = snd_pcm_lib_period_bytes(substream);
-	dpcm->pcm_irq_pos = 0;
-	dpcm->pcm_buf_pos = 0;
+	dpcm->frac_pos = 0;
+	dpcm->rate = runtime->rate;
+	dpcm->frac_buffer_size = runtime->buffer_size * HZ;
+	dpcm->frac_period_size = runtime->period_size * HZ;
 
 	return 0;
 }
@@ -261,26 +274,21 @@ static void dummy_systimer_callback(unsigned long data)
 	unsigned long flags;
 	
 	spin_lock_irqsave(&dpcm->lock, flags);
-	dpcm->timer.expires = 1 + jiffies;
-	add_timer(&dpcm->timer);
-	dpcm->pcm_irq_pos += dpcm->pcm_bps;
-	dpcm->pcm_buf_pos += dpcm->pcm_bps;
-	dpcm->pcm_buf_pos %= dpcm->pcm_buffer_size * dpcm->pcm_hz;
-	if (dpcm->pcm_irq_pos >= dpcm->pcm_period_size * dpcm->pcm_hz) {
-		dpcm->pcm_irq_pos %= dpcm->pcm_period_size * dpcm->pcm_hz;
-		spin_unlock_irqrestore(&dpcm->lock, flags);
-		snd_pcm_period_elapsed(dpcm->substream);
-	} else
-		spin_unlock_irqrestore(&dpcm->lock, flags);
+	dummy_systimer_update(dpcm);
+	dummy_systimer_rearm(dpcm);
+	spin_unlock_irqrestore(&dpcm->lock, flags);
+	snd_pcm_period_elapsed(dpcm->substream);
 }
 
 static snd_pcm_uframes_t
 dummy_systimer_pointer(struct snd_pcm_substream *substream)
 {
-	struct snd_pcm_runtime *runtime = substream->runtime;
-	struct dummy_systimer_pcm *dpcm = runtime->private_data;
+	struct dummy_systimer_pcm *dpcm = substream->runtime->private_data;
 
-	return bytes_to_frames(runtime, dpcm->pcm_buf_pos / dpcm->pcm_hz);
+	spin_lock(&dpcm->lock);
+	dummy_systimer_update(dpcm);
+	spin_unlock(&dpcm->lock);
+	return dpcm->frac_pos / HZ;
 }
 
 static int dummy_systimer_create(struct snd_pcm_substream *substream)