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

Jani Nikula
2015-06-29T07:58:00Z

On Sat, 27 Jun 2015, Raymond Yau <superquad.vortex2@gmail.com> wrote:
>> > >
>> > > 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 ?

Please elaborate.

BR,
Jani.

--
Jani Nikula, Intel Open Source Technology Center