[alsa-devel] [Intel-gfx] [PATCH 3/4] snd: add support for displayport multi-stream to hda codec.

[Raymond Yau]

> > >
> > > Shall we move or cc this discussion on audio driver side to ALSA ML?
> >
> > Oops I thought I had cc'ed these patches to alsa-devel as well when I
sent them.
> >
> > > I think we also need to decide how to manage PCM devices for DP MST.
> > > Now the HD-A driver create a PCM device for each pin, and the
substream
> > > number is 1 for each PCM. Now with DP MST enabled, each pin can
support
> > > multiple streams (e.g. 3 on Intel HSW/BDW/SKL).
> > >
> > > There may be 2 options:
> > > -#1: Let an HDMI codec specify number of substreams, same as the
number
> > > of device entries on a pin. We can specify 3 for HSW/BDW/SKL. Other
> > > vendors can also specify a value according to actual HW capabilities.
> > >
> > > So for HSW, we have 3x3 subtreams totally. But we only have 3
convertors
> > > (for 3 display pipelines), so we can open up to 3 substreams at the
same
> > > time. When the audio driver finds all 3 convertors are used when
opening
> > > a new substream, it will fail.
> >
> > One thing I noticed is the number of devices on a PIN is only updated
when
> > the MST device is plugged in so normally pins 5,6,7 have 0 devices, and
when
> > I plug in MST device, I get the 3 devices on port 6. So it seems dynamic
> > enough at this point, though I guess it'll always be 0 or 3.
> > >
> > > - #2: Create PCM device dynamically. Only create a PCM devices for a
device
> > > entry which connects to monitor with audio support. When the monitor
> > > is removed, the PCM device will be disconnected, closed and removed,
> > > similar to the USB case.

Do the driver really need dynamic PCM since display ports are dasiy chained
and the locations within the dasiy chain are fixed ?