linux-kernel - Re: [PATCH v2 00/38] drm/msm/dp: Add MST support for MSM chipsets

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Message-ID: <xrgrnjnu4ngod6uizhyx2jtti5xjjhskey7id7xzhpuc5suqbk@g3a57j42ukty>
Date: Wed, 11 Jun 2025 17:35:59 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Yongxing Mou <quic_yongmou@...cinc.com>
Cc: Rob Clark <robin.clark@....qualcomm.com>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jessica.zhang@....qualcomm.com>,
        Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Abhinav Kumar <quic_abhinavk@...cinc.com>
Subject: Re: [PATCH v2 00/38] drm/msm/dp: Add MST support for MSM chipsets

On Wed, Jun 11, 2025 at 08:08:49PM +0800, Yongxing Mou wrote:
> 
> 
> On 2025/6/10 16:30, Dmitry Baryshkov wrote:
> > On Tue, Jun 10, 2025 at 12:47:00PM +0800, Yongxing Mou wrote:
> > > 
> > > 
> > > On 2025/6/9 20:36, Dmitry Baryshkov wrote:
> > > > On Mon, Jun 09, 2025 at 08:21:19PM +0800, Yongxing Mou wrote:
> > > > > 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.
> > > > 
> > > > Which means that you didn't validate the MST interaction with the USB-C
> > > > stack (there is a significant difference in the way HPD event is handled
> > > > in the Linux kernel).
> > > > 
> > > Yes. this patch series not test with USB-DP. Actually, our initial plan was
> > > 2x2 MST on SA8775P and 4 MST on ACS8300. All of them are native DP
> > > interface, not USB-DP. So can we only support SA8775P/QCS8300 in this
> > > series. We don't plan to support other platform in this patch series.
> > 
> > I'm sorry, it doesn't work this way. This is not a product kernel,
> > limited to a selected set of platforms. So, you have to hook MST support
> > in both paths. The series is not going to be merged tomorrow, so you
> > have enough time to validate MST on the platforms providing DP over
> > USB-C.
> > 
> Would it be okay to implement USB-C DP MST in a separate patch series later?

What will happen if I plug in MST hub after applying this series?
Also... There were several items beforehand, which were left out 'to be
fixed later'. Up to now, it didn't happen. So... I'm really tempted to
say 'No' here.

Also, as I wrote, HPD rework would need to go first (and it wasn't even
posted). One of the reasons why I demand that is to be able to
streamline HPD handling and use the same code path for both USB-DP and
DP-connector.

> > > > > 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.
> > > > > 
> > > > > This patch series was made on top of:
> > > > > 
> > > > > [1] : https://patchwork.freedesktop.org/seriedds/142010/ (v2 to fix up HPD)
> > > > 
> > > > This series has serious concerns and most likely will not be merged. Not
> > > > to mention that the URL is invalid.
> > > > 
> > > Got it. Sorry for the typo in the URL. So should we drop this patch series
> > > or wait the state machine rework from Jessica zhang ?
> > 
> > Please work with Jessica. As I wrote, I do not intend to land the
> > mentioned series.
> > 
> > > > > 
> > > > > Bindings for the pixel clock for additional stream is available at :
> > > > > 
> > > > > [2] : https://patchwork.freedesktop.org/series/142016/
> > > > 
> > > > This series needs another revision.
> > > > 
> > > Got it.
> > > > Not to mention that I plan to land [3] this cycle
> > > > 
> > > > [3] http://lore.kernel.org/dri-devel/20250518-fd-dp-audio-fixup-v6-0-2f0ec3ec000d@oss.qualcomm.com
> > > > 
> > > > 
> > > Got it. will rebase on this patch series.
> > > > > Overall, the patch series has been organized in the following way:
> > > > > 
> > > > > 1) First set are a couple of fixes made while debugging MST but applicable
> > > > > to SST as well so go ahead of everything else
> > > > > 2) Prepare the DP driver to get ready to handle multiple streams. This is the bulk
> > > > > of the work as current DP driver design had to be adjusted to make this happen.
> > > > > 3) Finally, new files to handle MST related operations
> > > > > 
> > > > > Validation was done on the latest linux-next on top of above changes and
> > > > > both FB console and weston compositors were validated with these changes.
> > > > 
> > > > Validation should be using IGT for testing. Please ensure that there are
> > > > no regressions.

-- 
With best wishes
Dmitry