Re: [PATCH v7 00/35] drm/msm/dp: Add MST support for MSM chipsets

[Dmitry Baryshkov]

On Tue, Oct 27, 2026 at 10:04:54PM +0100, Robert Meijers wrote:
> Hi,
> 
> ((Sorry, re-sending because the first was rejected for not being plain
> text))
> (First up this is the first time I'm participating on a mailing list,
> so sorry in advance if I'm doing something wrong).
> 
> > Add support for Multi-stream transport for MSM chipsets that allow > a
> > single instance of DP controller to send multiple streams. > >
> This series has been validated on sa8775p ride platform using multiple > MST
> dongles and also daisy chain method on both DP0 and DP1 upto 1080P. > > With
> 4x4K monitors, due to lack of layer mixers that combination will > not >
> work but this can be supported as well after some rework on the DPU > side.
> > > In addition, SST was re-validated with all these changes to ensure >
> there > were no regressions.
> I have applied this patch series onto a near mainline 7.3 tree for the
> Xiaomi Pad 6s Pro ("Sheng"), with SM8550. But with these patches
> applied I don't have working audio, over USB-C to my (DisplayPort)
> monitor, anymore. If I disable MST support on the monitor audio works
> again. Debugging (using AI) reveals that the DP0 Jack is not reported as
> connected anymore when connected to the monitor, with these patches
> applied and MST enabled on the monitor. As shown, for example, by
> amixer:
> 
> $ amixer -c 0 contents | grep -i -A8 -B3 -E 'hdmi|displayport|jack'
> numid=208,iface=CARD,name='DP0 Jack'
> ; type=BOOLEAN,access=r-------,values=1
> : values=off
> 
> With MST on the monitor disabled this outputs "values=on". The same is
> also reflected by "pactl list cards" and "pactl list sinks" where
> HDMI/DisplayPort is reported as "not available" with MST enabled (and
> "available" when disabling MST).
> 
> (Again, suggested by AI) I also checked the ELD, which in both
> cases (MST on and off on the monitor) is identical, including audio
> information (like "speakers [0x1] FL/FR"). (Also, audio works both using
> an AMD and an Intel based laptop, in which case I can also choose
> whether to use "HDMI" or "HDMI 2" to switch audio output between two
> monitors, in daisy chain).
> 
> So my foremost question is: should audio be supported in case of MST,
> after applying these patches? Or is this possibly out of scope?

I'd say, it was out of scope for now.

> For clarity I would also like to note that without these patches
> applied audio over DP does work as well, irrespective of MST being
> on or off on the monitor (obviously always running as SST because of
> missing kernel support). Which would make me believe that no audio
> support in the same setup might be considered a regression?

Technically you are right, it is a regression in your case.

Yongxing, how difficult would it be to support audio over the MST?

> 
> Note that I do understand that me running a forked kernel on a
> (by upstream) unsupported device might also lead to issues or require
> changes for the fork. And that if audio does work with MST then this is
> something the maintainers of the fork should, most likely, fix. But
> from the cover letter and commit messages it is unclear to me whether
> audio support should work and/or has been tested when MST is in use.
> 
> Kind regards,
> Robert
> 

-- 
With best wishes
Dmitry