[Bug 785937] alsasrc timestamps is sensitive to context switches.

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=785937

--- Comment #2 from Göran Jönsson <goran.jonsson at axis.com> ---
If I run Ftrace with attached Instrumentation I got.

...
   irq/79-SSI-RX-3150  [003] ....   568.856203: snd_pcm_period_elapsed: Hello 1 
   irq/79-SSI-RX-3150  [003] d..2   568.856251: update_audio_tstamp: Hello 2 1 
   irq/79-SSI-RX-3150  [003] d..2   568.856261: update_audio_tstamp: Hello 3
568.829260520 
   irq/79-SSI-RX-3150  [003] d..2   568.856266: update_audio_tstamp: Hello 3.1
9.088000000 
    alsasrc7:src-3151  [002] d..2   568.856464: update_audio_tstamp: Hello 2 1 
    alsasrc7:src-3151  [002] d..2   568.856475: update_audio_tstamp: Hello 3
568.829483940 
    alsasrc7:src-3151  [002] d..2   568.856480: update_audio_tstamp: Hello 3.1
9.088000000 
    alsasrc7:src-3151  [002] d..2   568.856637: update_audio_tstamp: Hello 2 1 
    alsasrc7:src-3151  [002] d..2   568.856642: update_audio_tstamp: Hello 3
568.829662690 
    alsasrc7:src-3151  [002] d..2   568.856653: update_audio_tstamp: Hello 3.1
9.088000000 
    alsasrc7:src-3151  [002] d..2   568.856675: update_audio_tstamp: Hello 2 1 
    alsasrc7:src-3151  [002] d..2   568.856680: update_audio_tstamp: Hello 3
568.829700740 
    alsasrc7:src-3151  [002] d..2   568.856685: update_audio_tstamp: Hello 3.1
9.088000000 
    alsasrc7:src-3151  [002] d..2   568.856743: update_audio_tstamp: Hello 2 1 
    alsasrc7:src-3151  [002] d..2   568.856749: update_audio_tstamp: Hello 3
568.829768200 
    alsasrc7:src-3151  [002] d..2   568.856754: update_audio_tstamp: Hello 3.1
9.088000000 
    alsasrc7:src-3151  [002] ...1   568.856818: tracing_mark_write: USER_SPACE
Hello 2
    alsasrc7:src-3151  [002] ...1   568.856858: tracing_mark_write: USER_SPACE
Hello 3
    alsasrc7:src-3151  [002] d..2   568.856886: update_audio_tstamp: Hello 2 1 
    alsasrc7:src-3151  [002] d..2   568.856892: update_audio_tstamp: Hello 3
568.829910820 
    alsasrc7:src-3151  [002] d..2   568.856897: update_audio_tstamp: Hello 3.1
9.088000000 
    alsasrc7:src-3151  [002] d..2   568.856919: update_audio_tstamp: Hello 2 1 
    alsasrc7:src-3151  [002] d..2   568.856924: update_audio_tstamp: Hello 3
568.829944705 
    alsasrc7:src-3151  [002] d..2   568.856928: update_audio_tstamp: Hello 3.1
9.088000000 
    alsasrc7:src-3151  [002] ...1   568.856950: tracing_mark_write: USER_SPACE
Hello 4
    alsasrc7:src-3151  [002] ...1   568.857632: tracing_mark_write: USER_SPACE
Hello 1
    alsasrc7:src-3151  [002] d..2   568.857812: update_audio_tstamp: Hello 2 1 
    alsasrc7:src-3151  [002] d..2   568.857829: update_audio_tstamp: Hello 3
568.830811315 
    alsasrc7:src-3151  [002] d..2   568.857836: update_audio_tstamp: Hello 3.1
9.088000000 
    alsasrc7:src-3151  [002] d..2   568.857878: update_audio_tstamp: Hello 2 1 
    alsasrc7:src-3151  [002] d..2   568.857885: update_audio_tstamp: Hello 3
568.830901195 
    alsasrc7:src-3151  [002] d..2   568.857890: update_audio_tstamp: Hello 3.1
9.088000000 
...

With this trace we can see that the IRQ is updating the tstamp and audio stamp.
We can also se that snd_pcm_readi and snd_pcm_status_get_state also updating
the tstamp . So if a context switch occur the time stamp will not be as when
the interrupt occur . Also notice this is not true for audio timestamp.

Please find attached patch that uses audio stamp instead of tstamp.

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