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

[David Henningsson]

On 2015-10-22 09:40, Yang, Libin wrote:
>
>>>>>> From: Raymond Yau [mailto:superquad.vortex2 at gmail.com]
>>>>>> Sent: Friday, October 16, 2015 8:33 AM
>>>>>> To: Takashi Iwai
>>>>>> Cc: airlied at linux.ie; tanuk at iki.fi; David Henningsson; Yang, Libin;
>>>> Lin,
>>>>>> Mengdong; Girdwood, Liam R; ALSA Development Mailing List
>>>>>> Subject: Re: [alsa-devel] DP1.2 MST audio support discussion
>>>>>>
>>>>>>
>>>>>>>>
>>>>>>>> Do it mean that only one DP MST port and no HDMI port on
>> the
>>>>>> same graphic
>>>>>>>> card ?
>>>>>>>
>>>>>>> No.
>>>>>> If there is only one HDMI and one Display Port, this mean that
>> there
>>>>>> are two pin complexes
>>>>>> How about the name of jack detection kctl of three Display Port
>>>>>> monitors  which are created on the same pin complex but
>> different
>>>>>> dev_index ?
>>>>>
>>>>> For the jack name, what do you think if we change to
>>>>> "HDMI/DP, pin=n, dev=m" format? Will it impact on pulseaudio?
>>>>
>>>> Yes, it will impact PulseAudio. It will require changes to files in
>>>>
>>>>
>> http://cgit.freedesktop.org/pulseaudio/pulseaudio/tree/src/modules/
>>>> alsa/mixer/paths
>>>
>>> So does this mean we should not change the name "HDMI/DP,pcm=3
>> Jack"
>>> to "HDMI/DP,pin=n, dev=m Jack", otherwise the old PA will not work
>> with
>>> the new driver?
>>
>> Yes. And I don't understand why you need to do it, either - if you have
>> three display port monitors on one pin, then they will all get different
>> PCMs (8, 9 and 10), and they will signal their Jack status through
>> "HDMI/DP,pcm=8 Jack", "HDMI/DP,pcm=9 Jack" and
>> "HDMI/DP,pcm=10 Jack".
>
> OK, I see. Thanks. I will not change the jack kctl name.
> BTW, the PCM number will be the same as converter, which means
> it will be 3 on Intel platforms.

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.)

The jack and ELD kctls - one per PCM device - will follow what monitor 
the PCM is currently assigned to. (kctls belonging to a PCM device that 
is unassigned will report presence_detect=0 and eld_valid=0.)

Then, at playback open time, a free converter node will be dynamically 
assigned to the PCM. If there are no free converter nodes, return -EBUSY.

Now remains the case when an unassigned PCM is opened. This needs 
additional thinking, but the more backwards compatibility we can keep 
for this case, the better. But I'm not sure about how possible it is to 
let streams "freewheel" in this new DSP MST world?

>> Given that my proposed mapping algorithm will never change pin to
>> PCM
>> mapping (unless you have two displayport outputs and use MST on
>> both), I
>> think we can get away with not exposing the actual pin to userspace.
>> And should this later become necessary, we can add a separate kctl for
>> that.
>
> My current thought is to expose the same jack kctls (same number and
> same name) to userspace while we will create hda_jack_tbl for each device
> entry. When pcm is bound to the device entry, the corresponding
> jack kctl will be bound to the device entry.

-- 
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic