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.