[alsa-devel] [PATCH] alsa-lib: Provide a CLOCK_MONOTONIC_RAW timestamp type

[Takashi Iwai]

At Thu, 10 Jul 2014 10:10:56 -0500,
Pierre-Louis Bossart wrote:
> 
> 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.

As Clemens already pointed, if the application itself refers to the
timestamp and compares with the own timestamp by CLOCK_MONOTONIC,
using CLOCK_MONOTONIC_RAW would break.  So we cannot replace it with
CLOCK_MONOTONIC_RAW silently, unfortunately.


Takashi