[alsa-devel] [PATCH] alsa-lib: Provide a CLOCK_MONOTONIC_RAW timestamp type
Pierre-Louis Bossart
pierre-louis.bossart at linux.intel.com
Thu Jul 10 17:10:56 CEST 2014
On 7/9/14, 2:40 PM, Clemens Ladisch wrote:
> Takashi Iwai wrote:
>> Currently, the timestamp mode is set implicitly in alsa-lib pcm_hw.c:
>> - When kernel PCM protocol version is high enough, alsa-lib hw prefers
>> the monotonic always (if available), then set pcm->monotonic = 1.
>> - Application can ask whether the current timestamp is monotonic or
>> not via snd_pcm_hw_params_is_monotonic().
>> So, only adding the flag above doesn't suffice. If we need to add a
>> new mode, the API has to be extended as well.
>>
>> But how? The current API assumes that the monotonic mode was already
>> determined before hw_params. We may add a set of new hw_params get
>> and set calls for tstamp mode while keeping the old API. This would
>> be one option. Another option would be to add a new PCM open flag
>> SND_PCM_TSTAMP_MONOTONIC_RAW, and snd_pcm_hw_params_is_monotonic_raw()
>> function. The latter is easier (a simpler addition), while the former
>> is more extensible to newer formats in future.
>
> These timestamps cannot be handled by hardware drivers; they are always
> read by the ALSA framework, in software. Furthermore, switching between
> MONOTONIC and MONOTONIC_RAW is possible at any time. Therefore, the
> timestamp mode should be made a part of sw_params; just add a field
> named tstamp_mode ...
Humm... why would anyone switch modes at run time during
playback/capture? I have seen timestamps being used mainly to track that
playback/capture is happening as predicted, improve A/V sync or sleep
for a predictable time with interrupts disabled (PulseAudio, Android,
ChromeOS, etc). NTP adjustments are really adding noise more than
information for those usages. I have yet to see a case where the use of
those NTP adjustments would matter for userspace, and for now I really
don't see the point of making this dynamically configurable as a
software parameter. I would personally make this new mode the default.
-Pierre
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