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

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Message-ID: <CACSVV00-DDnQYp-65Pi-XwpEKT1_jYik2=zH_bK_oJiGLxX48A@mail.gmail.com>
Date: Mon, 14 Jul 2025 12:50:54 -0700
From: Rob Clark <rob.clark@....qualcomm.com>
To: Dale Whinham <daleyo@...il.com>
Cc: 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>,
        Jérôme de Bretagne <jerome.debretagne@...il.com>,
        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 Mon, Jul 14, 2025 at 10:36 AM Dale Whinham <daleyo@...il.com> 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;
> +       }

Not a dp expert myself, but..

In drm_dp_helpers.c there is dpcd_quirk_list[].. which applies quirks
based on the oui ("Organizational Unique ID") of the dp sink.  I think
this would be the correct way to handle this.  Although I guess you'll
need to add a new quirk for this.

Idk if the surface pro 11 has multiple different panel options.  If so
you defn wouldn't want to match on the DMI.

BR,
-R


> +
>         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];
> --
> 2.50.1
>