[PATCH] maximum latency tracking: ALSA support

Add maximum latency tracking to the ALSA subsystem for PCM playback.  In
ALSA, the playback application controls the buffer size and thus indirectly
the period of latency that it can deal with.  This patch uses 75% of the
total available latency as threshold to announce to the latency subsystem;
While 75% is a crude heuristic it's a quite reasonable one; the remaining
25% can be used for all driver processing for the next samples which is
also proportional to the size of the buffer.

With ogg123 a latency setting of about 4msec was seen (at 44Khz), while
with the "play" command a much longer maximum tolerable latency was seen.
Other, more multimedia oriented players as well as games, will have a lot
smaller buffers to allow better synchronization and those will actually get
into the latency domains where there is impact on the power management
rules.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/sound/pcm.h

@@ -347,6 +347,7 @@ struct snd_pcm_substream {
 	int number;
 	char name[32];			/* substream name */
 	int stream;			/* stream (direction) */
+	char latency_id[20];		/* latency identifier */
 	size_t buffer_bytes_max;	/* limit ring buffer size */
 	struct snd_dma_buffer dma_buffer;
 	unsigned int dma_buf_id;

sound/core/pcm.c

@@ -629,6 +629,9 @@ int snd_pcm_new_stream(struct snd_pcm *pcm, int stream, int substream_count)
 		substream->number = idx;
 		substream->stream = stream;
 		sprintf(substream->name, "subdevice #%i", idx);
+		snprintf(substream->latency_id, sizeof(substream->latency_id),
+			 "ALSA-PCM%d-%d%c%d", pcm->card->number, pcm->device,
+			 (stream ? 'c' : 'p'), idx);
 		substream->buffer_bytes_max = UINT_MAX;
 		if (prev == NULL)
 			pstr->substream = substream;

sound/core/pcm_native.c

@@ -25,6 +25,7 @@
 #include <linux/file.h>
 #include <linux/slab.h>
 #include <linux/time.h>
+#include <linux/latency.h>
 #include <linux/uio.h>
 #include <sound/core.h>
 #include <sound/control.h>
@@ -347,11 +348,26 @@ static int snd_pcm_hw_refine_user(struct snd_pcm_substream *substream,
 	return err;
 }
 
+static int period_to_usecs(struct snd_pcm_runtime *runtime)
+{
+	int usecs;
+
+	if (! runtime->rate)
+		return -1; /* invalid */
+
+	/* take 75% of period time as the deadline */
+	usecs = (750000 / runtime->rate) * runtime->period_size;
+	usecs += ((750000 % runtime->rate) * runtime->period_size) /
+		runtime->rate;
+
+	return usecs;
+}
+
 static int snd_pcm_hw_params(struct snd_pcm_substream *substream,
 			     struct snd_pcm_hw_params *params)
 {
 	struct snd_pcm_runtime *runtime;
-	int err;
+	int err, usecs;
 	unsigned int bits;
 	snd_pcm_uframes_t frames;
 
@@ -431,6 +447,10 @@ static int snd_pcm_hw_params(struct snd_pcm_substream *substream,
 
 	snd_pcm_timer_resolution_change(substream);
 	runtime->status->state = SNDRV_PCM_STATE_SETUP;
+
+	remove_acceptable_latency(substream->latency_id);
+	if ((usecs = period_to_usecs(runtime)) >= 0)
+		set_acceptable_latency(substream->latency_id, usecs);
 	return 0;
  _error:
 	/* hardware might be unuseable from this time,
@@ -490,6 +510,7 @@ static int snd_pcm_hw_free(struct snd_pcm_substream *substream)
 	if (substream->ops->hw_free)
 		result = substream->ops->hw_free(substream);
 	runtime->status->state = SNDRV_PCM_STATE_OPEN;
+	remove_acceptable_latency(substream->latency_id);
 	return result;
 }