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

[Lin, Mengdong]

> -----Original Message-----
> From: David Henningsson [mailto:david.henningsson at canonical.com]
> Sent: Friday, October 23, 2015 6:56 PM
> 
> On 2015-10-23 07:30, 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?
> 
> PulseAudio saves information about profile and volume, and does so on
> what in PA terminology is called a "port". PA binds this port to a specific Jack
> kctl and a PCM device. How this fits together is configured in the files in
> /usr/share/pulseaudio/alsa-mixer/ .
> 
> E g, if both one HDMI and one DP monitor are both connected, the user
> might set the volume (through PA) of the HDMI monitor to high and the DP
> monitor to low. The user then reboots. Now, the PCM-to-monitor (and Jack
> kctl-to-monitor) mapping needs to remain the same after a reboot,
> otherwise PA might restore the volume to the wrong monitor.

I see. Thank you for the info!
Random binding can break static mapping since the order of hot-plug unsol events is out of our control.

> 
> > And how will PA handle PCM 9,10 in a different way?
> 
> PA will handle PCM 9 and 10 in the same way.
> 
> > They are not bound to pins, and even not able to dev indexes.
> > In practice, a platform will usually support either a DP port or a HDMI port
> from the Intel integrated GPU for cost consideration.
> > But theoretically i915 can use same device index on two different pins to
> connect monitors, e.g. pin 0x05, dev index 2 for one monitor and pin 0x06,
> dev index 2 for the other.
> 
> In theory, there are certainly cases where the PCM device allocation can
> cause problems for PA. What I'm trying to achieve is that these cases will be
> so few that no one will ever notice.

I agree. Your suggestion can keep static mapping between monitors and PCM devices during various display mode change and power cycle for common use cases.

Regards
Mengdong

> 
> > On Intel platforms, the max dev indices is 2. Not sure about Nvidia and
> AMD.
> 
> Ok, thanks for the clarification.
> 
> --
> David Henningsson, Canonical Ltd.
> https://launchpad.net/~diwic