linux-kernel - Re: [PATCH v1 3/3] drm/msm/dp: place edp at head of drm bridge chain to fix screen corruption

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Date:   Sat, 25 Jun 2022 02:56:34 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Kuogee Hsieh <quic_khsieh@...cinc.com>
Cc:     dri-devel@...ts.freedesktop.org, robdclark@...il.com,
        sean@...rly.run, swboyd@...omium.org, dianders@...omium.org,
        vkoul@...nel.org, daniel@...ll.ch, airlied@...ux.ie,
        agross@...nel.org, bjorn.andersson@...aro.org,
        quic_abhinavk@...cinc.com, quic_aravindh@...cinc.com,
        quic_sbillaka@...cinc.com, freedreno@...ts.freedesktop.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 3/3] drm/msm/dp: place edp at head of drm bridge chain
 to fix screen corruption

On Fri, 24 Jun 2022 at 20:15, Kuogee Hsieh <quic_khsieh@...cinc.com> wrote:
>
> The msm_dp_modeset_init() is used to attach DP driver to drm bridge chain.
> msm_dp_modeset_init() is executed in the order of index (dp->id) of DP
> descriptor table.
>
> Currently, DP is placed at first entry (dp->id = 0) of descriptor table
> and eDP is placed at secondary entry (dp->id = 1 ) of descriptor table.
> This means DP will be placed at head of bridge chain and eDP will be
> placed right after DP at bridge chain.

No, the dp->ids do not have anything to do with the bridge chains.

> Drm screen update is happen sequentially in the order from head to tail
> of bridge chain. Therefore external DP display will have screen updated
> happen before primary eDP display if external DP display presented.
> This is wrong screen update order and cause one frame time screen
> corruption happen at primary display during external DP plugged in.
>
> This patch place eDP at first entry (dp->id = 0) of descriptor table and
> place DP at secondary entry (dp->id = 1) to have primary eDP locate at
> head of bridge chain. This correct screen update order and eliminated
> the one frame time screen corruption happen d at primary display.
>
> Signed-off-by: Kuogee Hsieh <quic_khsieh@...cinc.com>
> ---
>  drivers/gpu/drm/msm/dp/dp_display.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> index a87a9d8..2755ff3 100644
> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> @@ -143,10 +143,10 @@ static const struct msm_dp_config sc7180_dp_cfg = {
>
>  static const struct msm_dp_config sc7280_dp_cfg = {
>         .descs = (const struct msm_dp_desc[]) {
> -               { .io_start = 0x0ae90000, .connector_type = DRM_MODE_CONNECTOR_DisplayPort,
> -               .controller_id = MSM_DP_CONTROLLER_0, .wide_bus_en = true },
>                 { .io_start = 0x0aea0000, .connector_type = DRM_MODE_CONNECTOR_eDP,
>                 .controller_id = MSM_DP_CONTROLLER_1, .wide_bus_en = true },
> +               { .io_start = 0x0ae90000, .connector_type = DRM_MODE_CONNECTOR_DisplayPort,
> +               .controller_id = MSM_DP_CONTROLLER_0, .wide_bus_en = true },

If the correctness of DP display depends on the order of entries in
the dp_desc table, something is terribly wrong in the driver. Please
fix that rather than working around it by shuffling the entries in the
array.

>         },
>         .num_descs = 2,
>  };
> --
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
>


-- 
With best wishes
Dmitry