linux-kernel - Re: [PATCH 6/9] drm/msm/dp: Work around bogus maximum link rate

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Message-ID: <eb0f5be5-3a6f-4969-affd-c01a6216ad0c@oss.qualcomm.com>
Date: Thu, 17 Jul 2025 23:10:17 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Jérôme de Bretagne <jerome.debretagne@...il.com>,
        Xilin Wu <sophon@...xa.com>
Cc: Dale Whinham <daleyo@...il.com>, Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Dmitry Baryshkov <lumag@...nel.org>, 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
Subject: Re: [PATCH 6/9] drm/msm/dp: Work around bogus maximum link rate

On 7/17/25 10:27 PM, Jérôme de Bretagne wrote:
> On 2025/7/17 04:21, Xilin Wu <sophon@...xa.com> wrote :
>>
>> On 2025/7/15 01:35:42, Dale Whinham wrote:
>>> From: Jérôme de Bretagne <jerome.debretagne@...il.com>
>>>
>>> The OLED display in the Surface Pro 11 reports a maximum link rate of
>>> zero in its DPCD, causing it to fail to probe correctly.
>>>
>>> The Surface Pro 11's DSDT table contains some XML with an
>>> "EDPOverrideDPCDCaps" block that defines the max link rate as 0x1E
>>> (8.1Gbps/HBR3).
>>>
>>> Add a quirk to conditionally override the max link rate if its value
>>> is zero specifically for this model.
>>>
>>> Signed-off-by: Jérôme de Bretagne <jerome.debretagne@...il.com>
>>> Signed-off-by: Dale Whinham <daleyo@...il.com>
>>> ---
>>>   drivers/gpu/drm/msm/dp/dp_panel.c | 13 +++++++++++++
>>>   1 file changed, 13 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/msm/dp/dp_panel.c b/drivers/gpu/drm/msm/dp/dp_panel.c
>>> index 4e8ab75c771b..b2e65b987c05 100644
>>> --- a/drivers/gpu/drm/msm/dp/dp_panel.c
>>> +++ b/drivers/gpu/drm/msm/dp/dp_panel.c
>>> @@ -11,6 +11,8 @@
>>>   #include <drm/drm_of.h>
>>>   #include <drm/drm_print.h>
>>>
>>> +#include <linux/dmi.h>
>>> +
>>>   #define DP_MAX_NUM_DP_LANES 4
>>>   #define DP_LINK_RATE_HBR2   540000 /* kbytes */
>>>
>>> @@ -58,6 +60,17 @@ static int msm_dp_panel_read_dpcd(struct msm_dp_panel *msm_dp_panel)
>>>       if (rc)
>>>               return rc;
>>>
>>> +     /*
>>> +      * for some reason the ATNA30DW01-1 OLED panel in the Surface Pro 11
>>> +      * reports a max link rate of 0 in the DPCD. Fix it to match the
>>> +      * EDPOverrideDPCDCaps string found in the ACPI DSDT
>>> +      */
>>> +     if (dpcd[DP_MAX_LINK_RATE] == 0 &&
>>> +         dmi_match(DMI_SYS_VENDOR, "Microsoft Corporation") &&
>>> +         dmi_match(DMI_PRODUCT_NAME, "Microsoft Surface Pro, 11th Edition")) {
>>> +             dpcd[1] = DP_LINK_BW_8_1;
>>> +     }
>>> +
>>
>> My Galaxy Book4 Edge with the ATNA60CL07-0 panel also reports a max link
>> rate of 0. But I think eDP v1.4 panels need a different way to retrieve
>> supported links rates, which could be found in the amdgpu [1], i915 [2]
>> and nouveau [3] drivers.
> 
> Thanks Xilin for the sharing and pointers into 3 other drivers, that
> would explain the current limitation for Adreno GPUs. Fixing it would
> require a big contribution independent of the actual SP11 enablement.

FWIW Adreno is a wholly separate (from DPU - the display engine) block

> 
> Is it a feature planned in the short-medium term within the MSM driver?
> If not, would a quirk like [4] be acceptable upstream in the meanwhile?

I'm not a display guy, but this looks like yet another block of code
begging to be commonized across DP drivers, so I wouldn't expect it to
be a big blocker.

Adding a panel quirk doesn't seem in order, as the panel is /probably/
very much in spec, and it's the driver bit that's missing.

Konrad

> 
> [4] https://github.com/JeromeDeBretagne/linux-surface-pro-11/commit/d265cfb
> 
> Thanks a lot,
> Jérôme
> 
> 
> 
>> [1]:
>> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c#n2098
>> [2]:
>> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/gpu/drm/i915/display/intel_dp.c#n4281
>> [3]:
>> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/gpu/drm/nouveau/nouveau_dp.c#n101
>>
>>
>>>       msm_dp_panel->vsc_sdp_supported = drm_dp_vsc_sdp_supported(panel->aux, dpcd);
>>>       link_info = &msm_dp_panel->link_info;
>>>       link_info->revision = dpcd[DP_DPCD_REV];
>>
>>
>> --
>> Best regards,
>> Xilin Wu <sophon@...xa.com>
>