staging: bcm2835-audio: interpolate audio delay

When the BCM2835 audio output is used, userspace sees a jitter up to 10ms
in the audio position, aka "delay" -- the number of frames that must
be output before a new frame would be played.
Make this a bit nicer for userspace by interpolating the position
using the CPU clock.
The overhead is small -- an extra ktime_get() every time a GPU message
is sent -- and another call and a few calculations whenever the delay
is sought from userland.
At 48,000 frames per second, i.e. approximately 20 microseconds per
frame, it would take a clock inaccuracy of
20 microseconds in 10 milliseconds -- 2,000 parts per million --
to result in an inaccurate estimate, whereas
crystal- or resonator-based clocks typically have an
inaccuracy of 10s to 100s of parts per million.

Signed-off-by: Mike Brady <mikebrady@eircom.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
index 98b6977bdce705005d58a412002ae56cf17a79e5..bc1eaa3a07731797e10186b6ec60b73e9c608310 100644 (file)
--- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
+++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
@@ -74,6 +74,7 @@ void bcm2835_playback_fifo(struct bcm2835_alsa_stream *alsa_stream,
        atomic_set(&alsa_stream->pos, pos);
 
        alsa_stream->period_offset += bytes;
+       alsa_stream->interpolate_start = ktime_get();
        if (alsa_stream->period_offset >= alsa_stream->period_size) {
                alsa_stream->period_offset %= alsa_stream->period_size;
                snd_pcm_period_elapsed(substream);
@@ -237,6 +238,7 @@ static int snd_bcm2835_pcm_prepare(struct snd_pcm_substream *substream)
        atomic_set(&alsa_stream->pos, 0);
        alsa_stream->period_offset = 0;
        alsa_stream->draining = false;
+       alsa_stream->interpolate_start = ktime_get();
 
        return 0;
 }
@@ -286,6 +288,24 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_substream *substream)
 {
        struct snd_pcm_runtime *runtime = substream->runtime;
        struct bcm2835_alsa_stream *alsa_stream = runtime->private_data;
+       ktime_t now = ktime_get();
+
+       /* Give userspace better delay reporting by interpolating between GPU
+        * notifications, assuming audio speed is close enough to the clock
+        * used for ktime
+        */
+
+       if ((ktime_to_ns(alsa_stream->interpolate_start)) &&
+           (ktime_compare(alsa_stream->interpolate_start, now) < 0)) {
+               u64 interval =
+                       (ktime_to_ns(ktime_sub(now,
+                               alsa_stream->interpolate_start)));
+               u64 frames_output_in_interval =
+                       div_u64((interval * runtime->rate), 1000000000);
+               snd_pcm_sframes_t frames_output_in_interval_sized =
+                       -frames_output_in_interval;
+               runtime->delay = frames_output_in_interval_sized;
+       }
 
        return snd_pcm_indirect_playback_pointer(substream,
                &alsa_stream->pcm_indirect,

diff --git a/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h b/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
index 34a0125ce646ab30ecfe666f0e3d1b8cfb7989cf..ed0feb34b6c8830983a63a70196a3054c4c17023 100644 (file)
--- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
+++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
@@ -77,6 +77,7 @@ struct bcm2835_alsa_stream {
        unsigned int period_offset;
        unsigned int buffer_size;
        unsigned int period_size;
+       ktime_t interpolate_start;
 
        struct bcm2835_audio_instance *instance;
        int idx;