Re: Sndio support in MPV

On Tue, Nov 24, 2020 at 11:04:22PM -0300, Érico Nogueira wrote:
> On Tue Nov 24, 2020 at 10:52 AM -03, Érico Nogueira wrote:
> > Hi!
> >
> > I don't keep up with MPV [1] development, so I was quite surprised that
> > the latest release dropped support for sndio [2]. I was looking into
> > whether it would be possible to simply revert the commit, but it turns
> > out they made quite a few changes after it, so additional fixes are
> > needed. I already "fixed" it enough to at least build successfully, but
> > it doesn't yet work, and I will admit that I don't really understand all
> > that's going on in that codebase.
> >
> > - [1] https://github.com/mpv-player/mpv
> > - [2]
> > https://github.com/mpv-player/mpv/commit/71d218eae4b4d93ada34ff74906f71ad359c84bc
> >
> > So, in order to avoid repeated work, I was wondering if anyone already
> > tried to go this route? I couldn't find any open PRs in their
> > repository.
> >
> > My hope is that they would consider adding the backend back once someone
> > has done the work of porting it over to their new architecture. If
> > anyone is pessimisitic about that, let me know too, I guess :P
> >
> > Thanks,
> > Érico
> 
> Quick update, I have a branch for it that works (at least on my machine :).
> I don't know if it's the best design for their new AO API, but it's (I
> think) a natural evolution from the previous version. Michael Forney has
> suggested using their pull API instead of the current push (it uses
> callbacks called by the audio library for feeding audio when requested
> instead of feeding audio into it ever so often), but I believe that
> would require a threaded implementation inside the ao_sndio.c file,
> which might not be accepted upstream.
> 
> Has there been discussion into providing any such API inside libsndio?

There were no discussions about this. Probably because push/pull are
different APIs for the same thing, and one API is easy to implement
using the other. On Unix-like systems, all call-backs are necessarily
implemented using a thread (explicit or hidden) which will call
write() and the call-back in a loop.

> >From what I can see, the ao_jack.c and ao_sdl.c backends use this model,
> and it seems to result in pretty clean code.

I'll check this one; note that on OpenBSD sdl uses sndio, so if sdl
works well, it would be easy to make sndio work and look clean as
well.

> One other thing I found to be "missing" was a `sio_pause()` call; mpv's
> AO wants to write() before calling start(), which I worked around by
> doing my own buffering and flushing it when start() is called. If
> there's some trick or obvious functionality I missed, please let me
> know.

In play-only mode, sio_start() doesn't start immediately. It waits
until the buffer is full and starts automatically only once the buffer
is ready.

> The branch is in [1], and since upstream has removed sndio support once
> already, I'd like to guarantee that the code is reasonable and that it
> works fine before making a PR their way. Therefore, I'd highly
> appreciate testing and code review, if anyone's up to it.
> 
> - [1] https://github.com/ericonr/mpv/tree/sndio
> 

thank you very much, I'll check this