target/linux/brcm2708/patches-4.19/950-0408-staging-bcm2835-audio-interpolate-audio-delay.patch

   1 From 2ba82d9516203ce41f33e98adb667bedee3622bc Mon Sep 17 00:00:00 2001
   2 From: Mike Brady <mikebrady@eircom.net>
   3 Date: Mon, 22 Oct 2018 20:17:08 +0100
   4 Subject: [PATCH] staging: bcm2835-audio: interpolate audio delay
   5
   6 commit a105a3a72824e0ac685a0711a67e4dbe29de62d0 upstream.
   7
   8 When the BCM2835 audio output is used, userspace sees a jitter up to 10ms
   9 in the audio position, aka "delay" -- the number of frames that must
  10 be output before a new frame would be played.
  11 Make this a bit nicer for userspace by interpolating the position
  12 using the CPU clock.
  13 The overhead is small -- an extra ktime_get() every time a GPU message
  14 is sent -- and another call and a few calculations whenever the delay
  15 is sought from userland.
  16 At 48,000 frames per second, i.e. approximately 20 microseconds per
  17 frame, it would take a clock inaccuracy of
  18 20 microseconds in 10 milliseconds -- 2,000 parts per million --
  19 to result in an inaccurate estimate, whereas
  20 crystal- or resonator-based clocks typically have an
  21 inaccuracy of 10s to 100s of parts per million.
  22
  23 Signed-off-by: Mike Brady <mikebrady@eircom.net>
  24 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  25 ---
  26  .../vc04_services/bcm2835-audio/bcm2835-pcm.c | 20 +++++++++++++++++++
  27  .../vc04_services/bcm2835-audio/bcm2835.h     |  1 +
  28  2 files changed, 21 insertions(+)
  29
  30 --- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
  31 +++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
  32 @@ -74,6 +74,7 @@ void bcm2835_playback_fifo(struct bcm283
  33         atomic_set(&alsa_stream->pos, pos);
  34
  35         alsa_stream->period_offset += bytes;
  36 +       alsa_stream->interpolate_start = ktime_get();
  37         if (alsa_stream->period_offset >= alsa_stream->period_size) {
  38                 alsa_stream->period_offset %= alsa_stream->period_size;
  39                 snd_pcm_period_elapsed(substream);
  40 @@ -237,6 +238,7 @@ static int snd_bcm2835_pcm_prepare(struc
  41         atomic_set(&alsa_stream->pos, 0);
  42         alsa_stream->period_offset = 0;
  43         alsa_stream->draining = false;
  44 +       alsa_stream->interpolate_start = ktime_get();
  45
  46         return 0;
  47  }
  48 @@ -286,6 +288,24 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_s
  49  {
  50         struct snd_pcm_runtime *runtime = substream->runtime;
  51         struct bcm2835_alsa_stream *alsa_stream = runtime->private_data;
  52 +       ktime_t now = ktime_get();
  53 +
  54 +       /* Give userspace better delay reporting by interpolating between GPU
  55 +        * notifications, assuming audio speed is close enough to the clock
  56 +        * used for ktime
  57 +        */
  58 +
  59 +       if ((ktime_to_ns(alsa_stream->interpolate_start)) &&
  60 +           (ktime_compare(alsa_stream->interpolate_start, now) < 0)) {
  61 +               u64 interval =
  62 +                       (ktime_to_ns(ktime_sub(now,
  63 +                               alsa_stream->interpolate_start)));
  64 +               u64 frames_output_in_interval =
  65 +                       div_u64((interval * runtime->rate), 1000000000);
  66 +               snd_pcm_sframes_t frames_output_in_interval_sized =
  67 +                       -frames_output_in_interval;
  68 +               runtime->delay = frames_output_in_interval_sized;
  69 +       }
  70
  71         return snd_pcm_indirect_playback_pointer(substream,
  72                 &alsa_stream->pcm_indirect,
  73 --- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
  74 +++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
  75 @@ -78,6 +78,7 @@ struct bcm2835_alsa_stream {
  76         unsigned int period_offset;
  77         unsigned int buffer_size;
  78         unsigned int period_size;
  79 +       ktime_t interpolate_start;
  80
  81         struct bcm2835_audio_instance *instance;
  82         int idx;