target/linux/brcm2708/patches-4.14/950-0410-bcm2835-interpolate-audio-delay.patch

   1 From 84ad00eb76af34f8a6c6941dc6da80a3f735b207 Mon Sep 17 00:00:00 2001
   2 From: wm4 <wm4@nowhere>
   3 Date: Wed, 13 Jan 2016 19:44:47 +0100
   4 Subject: [PATCH 410/454] bcm2835: interpolate audio delay
   5
   6 It appears the GPU only sends us a message all 10ms to update
   7 the playback progress. Other than this, the playback position
   8 (what SNDRV_PCM_IOCTL_DELAY will return) is not updated at all.
   9 Userspace will see jitter up to 10ms in the audio position.
  10
  11 Make this a bit nicer for userspace by interpolating the
  12 position using the CPU clock.
  13
  14 I'm not sure if setting snd_pcm_runtime.delay is the right
  15 approach for this. Or if there is maybe an already existing
  16 mechanism for position interpolation in the ALSA core.
  17
  18 I only set SNDRV_PCM_INFO_BATCH because this appears to remove
  19 at least one situation snd_pcm_runtime.delay is used, so I have
  20 to worry less in which place I have to update this field, or
  21 how it interacts with the rest of ALSA.
  22
  23 In the future, it might be nice to use VC_AUDIO_MSG_TYPE_LATENCY.
  24 One problem is that it requires sending a videocore message, and
  25 waiting for a reply, which could make the implementation much
  26 harder due to locking and synchronization requirements.
  27 ---
  28  .../vc04_services/bcm2835-audio/bcm2835-pcm.c       | 13 +++++++++++--
  29  .../staging/vc04_services/bcm2835-audio/bcm2835.h   |  1 +
  30  2 files changed, 12 insertions(+), 2 deletions(-)
  31
  32 --- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
  33 +++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
  34 @@ -22,7 +22,7 @@
  35  /* hardware definition */
  36  static const struct snd_pcm_hardware snd_bcm2835_playback_hw = {
  37         .info = (SNDRV_PCM_INFO_INTERLEAVED | SNDRV_PCM_INFO_BLOCK_TRANSFER |
  38 -       SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_MMAP_VALID),
  39 +       SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_MMAP_VALID | SNDRV_PCM_INFO_BATCH),
  40         .formats = SNDRV_PCM_FMTBIT_U8 | SNDRV_PCM_FMTBIT_S16_LE,
  41         .rates = SNDRV_PCM_RATE_CONTINUOUS | SNDRV_PCM_RATE_8000_48000,
  42         .rate_min = 8000,
  43 @@ -93,6 +93,8 @@ void bcm2835_playback_fifo(struct bcm283
  44                 alsa_stream->pos %= alsa_stream->buffer_size;
  45         }
  46
  47 +       alsa_stream->interpolate_start = ktime_get_ns();
  48 +
  49         if (alsa_stream->substream) {
  50                 if (new_period)
  51                         snd_pcm_period_elapsed(alsa_stream->substream);
  52 @@ -323,6 +325,7 @@ static int snd_bcm2835_pcm_prepare(struc
  53         alsa_stream->buffer_size = snd_pcm_lib_buffer_bytes(substream);
  54         alsa_stream->period_size = snd_pcm_lib_period_bytes(substream);
  55         alsa_stream->pos = 0;
  56 +       alsa_stream->interpolate_start = ktime_get_ns();
  57
  58         audio_debug("buffer_size=%d, period_size=%d pos=%d frame_bits=%d\n",
  59                 alsa_stream->buffer_size, alsa_stream->period_size,
  60 @@ -415,13 +418,19 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_s
  61  {
  62         struct snd_pcm_runtime *runtime = substream->runtime;
  63         struct bcm2835_alsa_stream *alsa_stream = runtime->private_data;
  64 -
  65 +       u64 now = ktime_get_ns();
  66
  67         audio_debug("pcm_pointer... (%d) hwptr=%d appl=%d pos=%d\n", 0,
  68                 frames_to_bytes(runtime, runtime->status->hw_ptr),
  69                 frames_to_bytes(runtime, runtime->control->appl_ptr),
  70                 alsa_stream->pos);
  71
  72 +       /* Give userspace better delay reporting by interpolating between GPU
  73 +        * notifications, assuming audio speed is close enough to the clock
  74 +        * used for ktime */
  75 +       if (alsa_stream->interpolate_start && alsa_stream->interpolate_start < now)
  76 +               runtime->delay = -(int)div_u64((now - alsa_stream->interpolate_start) * runtime->rate,  1000000000);
  77 +
  78         return snd_pcm_indirect_playback_pointer(substream,
  79                 &alsa_stream->pcm_indirect,
  80                 alsa_stream->pos);
  81 --- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
  82 +++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
  83 @@ -137,6 +137,7 @@ struct bcm2835_alsa_stream {
  84         unsigned int pos;
  85         unsigned int buffer_size;
  86         unsigned int period_size;
  87 +       u64 interpolate_start;
  88
  89         atomic_t retrieved;
  90         struct bcm2835_audio_instance *instance;