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

[Takashi Iwai]

On Fri, 23 Oct 2015 10:35:26 +0200,
Lin, Mengdong wrote:
> 
> > -----Original Message-----
> > From: Takashi Iwai [mailto:tiwai at suse.de]
> > Sent: Friday, October 23, 2015 1:54 PM
> > 
> > On Fri, 23 Oct 2015 07:30:12 +0200,
> > Lin, Mengdong wrote:
> > >
> > > > -----Original Message-----
> > > > From: David Henningsson [mailto:david.henningsson at canonical.com]
> > > > Sent: Thursday, October 22, 2015 4:53 PM
> > >
> > > [snip]
> > >
> > > > I'll try to explain my suggestion (which I believe Takashi's buying
> > > > too) one more time then:
> > > >
> > > > First, when a monitor is plugged in, we need to dynamically assign
> > > > this monitor to five PCM devices. I believe this scheme will be best:
> > > >
> > > > For a monitor at pin nid 0x05, dev index 0, it will prefer PCM 3.
> > > > For a monitor at pin nid 0x06, dev index 0, it will prefer PCM 7.
> > > > For a monitor at pin nid 0x07, dev index 0, it will prefer PCM 8.
> > > > For a monitor at dev index 1 (any pin), it will prefer PCM 9.
> > > > For a monitor at dev index 2 (any pin), it will prefer PCM 10.
> > > >
> > > > For monitors at dev indices > 2 (can that happen?), or if the PCM is
> > > > already assigned to something else, try PCMs in this order: 9, 10, 3, 7, 8.
> > > > (Subject to discussion perhaps, I don't think the order matters too
> > > > much, because conflicts will be rare in practice.)
> > >
> > > Hi David,
> > >
> > > Would you please clarify why PA needs such a fixed binding between PCM
> > 3,7, 8 and pin 0x05,6,7?
> > 
> > It's not PA's requirement.  The only concern here is for people who uses
> > ALSA device directly, e.g. via aplay -Dhdmi:2.  The first monitor is
> > guaranteed to be mapped to the compatible position for such people.
> 
> Excuse me, could you share more background info here?
> Even before DP MST support, it seems that such people need to know what pin is used to connect a monitor. Why?

On the existing machines, if you connect to a certain DP port, it's
always assigned to the same device.  This is guaranteed, so it's
perfectly fine to use this device statically for your purpose.

> The user can enumerate the PCM devices, and the PCM's jack ioctl can show whether a monitor is available behind this PCM device, no matter the driver uses fixed or dynamic binding between the PCM and pins.
> So people can always choose a PCM to use a monitor. I feel they need not care which pin is used for the monitor.

People *can* choose, but it doesn't mean people *must* reselect
another number out of sudden.  If they have to, it may be called a
regression.


Takashi