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

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Message-ID: <4570ecc8-d991-4160-aba7-250f5580d58d@quicinc.com>
Date: Wed, 12 Feb 2025 19:03:55 -0800
From: Abhinav Kumar <quic_abhinavk@...cinc.com>
To: Marijn Suijten <marijn.suijten@...ainline.org>,
        "James A. MacInnes"
	<james.a.macinnes@...il.com>
CC: Rob Clark <robdclark@...il.com>,
        Dmitry Baryshkov
	<dmitry.baryshkov@...aro.org>,
        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 2/12/2025 3:41 PM, 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.
> 

Yes, this is good description. Thanks Marijn!

> 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
> 

Its indeed a bug introduced due to msm_dp_desc_sc7180 re-use. There is 
no widebus on this chipset.


With the commit text fixed like above,

Reviewed-by: Abhinav Kumar <quic_abhinavk@...cinc.com>
>>
>> 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
>>