[alsa-devel] DP1.2 MST audio support discussion

[Raymond Yau]

> >
> > > One could then allocate two extra PCMs from the start (9 and 10) to
> > > try
> > > in case the other PCM is busy (my preference), or one could steal one
> > > of
> > > the existing non-busy ones (Takashi's preference).
> >
> > We will create the PCMs based on converter. This means we will
> > create 3 PCMs. And it will not support dynamically allocating PCM.
> > As there are only 3 converters, no more PCMs will be supported.
> > Each PCM will use one converter. If 3 PCMs are all used, connecting
> > monitor will not create new PCM.
> >
> > Yes, if we are not using the PCM, we can re-assign the PCM to another
> > monitor. Currently, user can't decide which PCM is used for which
> > monitor. Image the scenario PCM 3 is assigned to monitor 1, PCM 7 is
> > assigned to monitor 2, PCM 8 is assigned to monitor 3. Monitor 4 is
> > connected, no PCM is available, and driver don't know whether it
> > should steal one PCM for the monitor 4 and user can't change the
> > mapping currently.
>
> Actually, there are a few things to implement properly:
>
> 1. The plug/unplug events are triggered at the actual graphics output
>    switching via xrandr or such, not at the time the monitor is
>    physically plugged/unplugged.
>
> 2. At switching, the unplug event must be handled at first, and then
>    the plug event.
>
> 3. At unplug, detach per_pin object properly from PCM.
>    We currently just notify it via jack ctl, and doesn't care the
>    rest.  Ideally, we should force to stop the PCM and detach
>    internally from the assigned pin.
>
> 4. At plug, assign the monitor to the free per_pin object.
>
> So what we've discussed is about the last step; how to map the new
> monitor to per_pin object.  This requires the other steps beforehand
> which assures a free per_pin is present at plug time.
>

Do it mean that only one DP MST port and no HDMI port on the same graphic
card ?

If DP MST are dasiy chained, the unplug of the first monitor from Display
Port will cause all DP monitor in the dasiy chain disconnect