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

[Pierre-Louis Bossart]

On 7/10/14, 10:33 AM, Takashi Iwai wrote:
> 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.

ok, fine, it's a different mode then. That still doesn't clarify who 
would ever switch modes dynamically while streaming data.