Re: [alsa-devel] [very-RFC 0/8] TSN driver for the kernel

[Pierre-Louis Bossart]

> Presentation time is either set by
> a) Local sound card performing capture (in which case it will be 'capture
>    time')
> b) Local media application sending a stream accross the network
>    (time when the sample should be played out remotely)
> c) Remote media application streaming data *to* host, in which case it will
>    be local presentation time on local  soundcard
>
> > This value is dominant to the number of events included in an IEC 61883-1
> > packet. If this TSN subsystem decides it, most of these items don't need
> > to be in ALSA.
>
> Not sure if I understand this correctly.
>
> TSN should have a reference to the timing-domain of each *local*
> sound-device (for local capture or playback) as well as the shared
> time-reference provided by gPTP.
>
> Unless an End-station acts as GrandMaster for the gPTP-domain, time set
> forth by gPTP is inmutable and cannot be adjusted. It follows that the
> sample-frequency of the local audio-devices must be adjusted, or the
> audio-streams to/from said devices must be resampled.

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.
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.