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 ...
Regards, Clemens