linux-kernel - Re: [PATCH v2 1/2] drm/msm/dp: Disable wide bus support for SDM845

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Message-ID: <20250227084614.527e428d@jamesmacinnes-VirtualBox>
Date: Thu, 27 Feb 2025 08:46:14 -0800
From: "James A. MacInnes" <james.a.macinnes@...il.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: Marijn Suijten <marijn.suijten@...ainline.org>, Rob Clark
 <robdclark@...il.com>, Abhinav Kumar <quic_abhinavk@...cinc.com>, Sean Paul
 <sean@...rly.run>, David Airlie <airlied@...il.com>, Simona Vetter
 <simona@...ll.ch>, Chandan Uddaraju <chandanu@...eaurora.org>, Stephen Boyd
 <swboyd@...omium.org>, Vara Reddy <quic_varar@...cinc.com>, Tanmay Shah
 <tanmay@...eaurora.org>, linux-arm-msm@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, Guenter Roeck <groeck@...omium.org>, Rob
 Clark <robdclark@...omium.org>
Subject: Re: [PATCH v2 1/2] drm/msm/dp: Disable wide bus support for SDM845

On Thu, 13 Feb 2025 01:58:06 +0200
Dmitry Baryshkov <dmitry.baryshkov@...aro.org> wrote:

> On Thu, Feb 13, 2025 at 12:41:02AM +0100, Marijn Suijten wrote:
> > On 2025-02-12 15:03:46, James A. MacInnes wrote:  
> > > SDM845 DPU hardware is rev 4.0.0 per hardware documents.
> > > Original patch to enable wide_bus operation did not take into
> > > account the SDM845 and it got carried over by accident.
> > > 
> > > - Incorrect setting caused inoperable DisplayPort.
> > > - Corrected by separating SDM845 into its own descriptor.  
> > 
> > If anything I'd have appreciated to see our conversation in v1
> > pasted here verbatim which is of the right verbosity to explain
> > this.  I can't do much with a list of two items.
> > 
> > I don't have a clearer way of explaining what all I find confusing
> > about this description, so let me propose what I would have written
> > if this was my patch instead:
> > 
> > 	When widebus was enabled for DisplayPort in commit
> > c7c412202623 ("drm/msm/dp: enable widebus on all relevant
> > chipsets") it was clarified that it is only supported on DPU 5.0.0
> > onwards which includes SC7180 on DPU revision 6.2. However, this
> > patch missed that the description structure for SC7180 is also
> > reused for SDM845 (because of identical io_start address) which is
> > only DPU 4.0.0, leading to a wrongly enbled widebus feature and
> > corruption on that platform.
> > 
> > 	Create a separate msm_dp_desc_sdm845 structure for this SoC
> > compatible, with the wide_bus_supported flag turned off.
> > 
> > 	Note that no other DisplayPort compatibles currently exist
> > for SoCs older than DPU 4.0.0 besides SDM845.  
> 
> With more or less similar commit message:
> 
> 
> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
> 
> 
> > 
> > Hope I'm not considered being too picky.  I first sketch **how**
> > the original patch created a problem, then explain how this patch
> > is intending to fix it, and finally describe that we went a step
> > further and ensured no other SoCs are suffering from a similar
> > problem.
> > 
> > - Marijn
> >
Not too picky at all. I will use your text. I apologize as I had changed
the cover instead of the patch. I will do my best to balance too many
words and not enough.

Would it be appropriate to split this patch and the other into separate
submissions?

Thank you again.

 - James

> > > 
> > > Fixes: c7c412202623 ("drm/msm/dp: enable widebus on all relevant
> > > chipsets") Signed-off-by: James A. MacInnes
> > > <james.a.macinnes@...il.com> ---
> > >  drivers/gpu/drm/msm/dp/dp_display.c | 7 ++++++-
> > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/msm/dp/dp_display.c
> > > b/drivers/gpu/drm/msm/dp/dp_display.c index
> > > aff51bb973eb..e30cccd63910 100644 ---
> > > a/drivers/gpu/drm/msm/dp/dp_display.c +++
> > > b/drivers/gpu/drm/msm/dp/dp_display.c @@ -126,6 +126,11 @@ static
> > > const struct msm_dp_desc msm_dp_desc_sa8775p[] = { {}
> > >  };
> > >  
> > > +static const struct msm_dp_desc msm_dp_desc_sdm845[] = {
> > > +	{ .io_start = 0x0ae90000, .id = MSM_DP_CONTROLLER_0 },
> > > +	{}
> > > +};
> > > +
> > >  static const struct msm_dp_desc msm_dp_desc_sc7180[] = {
> > >  	{ .io_start = 0x0ae90000, .id = MSM_DP_CONTROLLER_0,
> > > .wide_bus_supported = true }, {}
> > > @@ -178,7 +183,7 @@ static const struct of_device_id
> > > msm_dp_dt_match[] = { { .compatible = "qcom,sc8180x-edp", .data =
> > > &msm_dp_desc_sc8180x }, { .compatible = "qcom,sc8280xp-dp", .data
> > > = &msm_dp_desc_sc8280xp }, { .compatible = "qcom,sc8280xp-edp",
> > > .data = &msm_dp_desc_sc8280xp },
> > > -	{ .compatible = "qcom,sdm845-dp", .data =
> > > &msm_dp_desc_sc7180 },
> > > +	{ .compatible = "qcom,sdm845-dp", .data =
> > > &msm_dp_desc_sdm845 }, { .compatible = "qcom,sm8350-dp", .data =
> > > &msm_dp_desc_sc7180 }, { .compatible = "qcom,sm8650-dp", .data =
> > > &msm_dp_desc_sm8650 }, { .compatible = "qcom,x1e80100-dp", .data
> > > = &msm_dp_desc_x1e80100 },
> > > 
> > > -- 
> > > 2.43.0
> > >   
>