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

[David Henningsson]

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.

> 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.

> 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