On 6/20/16 5:18 AM, Richard Cochran wrote:
On Mon, Jun 20, 2016 at 01:08:27PM +0200, Pierre-Louis Bossart wrote:
The ALSA API provides support for 'audio' timestamps (playback/capture rate defined by audio subsystem) and 'system' timestamps (typically linked to TSC/ART) with one option to take synchronized timestamps should the hardware support them.
Thanks for the info. I just skimmed Documentation/sound/alsa/timestamping.txt.
That is fairly new, only since v4.1. Are then any apps in the wild that I can look at? AFAICT, OpenAVB, gstreamer, etc, don't use the new API.
The ALSA API supports a generic .get_time_info callback, its implementation is for now limited to a regular 'DMA' or 'link' timestamp for HDaudio - the difference being which counters are used and how close they are to the link serializer. The synchronized part is still WIP but should come 'soon'
The intent was that the 'audio' timestamps are translated to a shared time reference managed in userspace by gPTP, which in turn would define if (adaptive) audio sample rate conversion is needed. There is no support at the moment for a 'play_at' function in ALSA, only means to control a feedback loop.
Documentation/sound/alsa/timestamping.txt says:
If supported in hardware, the absolute link time could also be used to define a precise start time (patches WIP)
Two questions:
Where are the patches? (If some are coming, I would appreciate being on CC!)
Can you mention specific HW that would support this?
You can experiment with the 'dma' and 'link' timestamps today on any HDaudio-based device. Like I said the synchronized part has not been upstreamed yet (delays + dependency on ART-to-TSC conversions that made it in the kernel recently)