target/linux/brcm2708/patches-4.4/0135-bcm2835-interpolate-audio-delay.patch

   1 From d53d9217b5906b4046c12c7ddd56911e447de9f2 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 135/156] 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  sound/arm/bcm2835-pcm.c | 12 +++++++++++-
  29  sound/arm/bcm2835.h     |  1 +
  30  2 files changed, 12 insertions(+), 1 deletion(-)
  31
  32 --- a/sound/arm/bcm2835-pcm.c
  33 +++ b/sound/arm/bcm2835-pcm.c
  34 @@ -25,7 +25,7 @@
  35  /* hardware definition */
  36  static 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 @@ -99,6 +99,8 @@ static irqreturn_t bcm2835_playback_fifo
  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 @@ -399,6 +401,7 @@ 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 @@ -495,6 +498,7 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_s
  61  {
  62         struct snd_pcm_runtime *runtime = substream->runtime;
  63         bcm2835_alsa_stream_t *alsa_stream = runtime->private_data;
  64 +       u64 now = ktime_get_ns();
  65
  66         audio_info(" .. IN\n");
  67
  68 @@ -503,6 +507,12 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_s
  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         audio_info(" .. OUT\n");
  79         return snd_pcm_indirect_playback_pointer(substream,
  80                                                  &alsa_stream->pcm_indirect,
  81 --- a/sound/arm/bcm2835.h
  82 +++ b/sound/arm/bcm2835.h
  83 @@ -137,6 +137,7 @@ typedef struct bcm2835_alsa_stream {
  84         unsigned int pos;
  85         unsigned int buffer_size;
  86         unsigned int period_size;
  87 +       u64 interpolate_start;
  88
  89         uint32_t enable_fifo_irq;
  90         irq_handler_t fifo_irq_handler;