Re: [PATCH] drm/msm/dp: add EDID re-read retry in msm_dp_panel_get_modes()

[Jani Nikula <jani.nikula@linux.intel.com>]

On Sun, 31 May 2026, Jens Glathe via B4 Relay <devnull+jens.glathe.oldschoolsolutions.biz@kernel.org> wrote:
> From: Jens Glathe <jens.glathe@oldschoolsolutions.biz>
>
> drm_edid_read_ddc() can return a structurally valid EDID from which
> drm_edid_connector_add_modes() still returns zero modes. This triggers
> the error:

What are the EDID contents in that case?

Where does it originate from? A valid but "not what you wanted" EDID
doesn't show up from thin air.

Perhaps it's the hub/adapter replying with some placeholder EDID instead
of the actual sink EDID. Maybe it's the timing and/or ordering.

Feels like root causing would be in order before adding hacks like this
that are notoriously difficult to remove in the future, and will easily
block future progress.


BR,
Jani.


>
>   [drm:msm_dp_bridge_get_modes [msm]] *ERROR* failed to get DP sink
>    modes, rc=0
>
> even though the link is ready. Since the EDID is only read once, this
> error persists and the display comes up with 640x480 resolution.
>
> Add a retry of drm_edid_read_ddc() inside get_modes() when the initial
> read produces no usable modes. The bad EDID is freed before retrying
> and container_of ensures access to the DDC channel. This directly
> addresses the observed "valid but empty/garbage" EDID case on
> flaky DP plugs and adapters.
>
> I tested this on a few of my "flaky" type-c to HDMI adapters and hubs,
> getting no "retry failed" messages and the desired resolution. Without
> the patch most plugs would result in 640x480 external display.
>
> Fixes: [5bea90ad9743d2] "drm/msm/dp: switch to struct drm_edid"
>
> Signed-off-by: Jens Glathe <jens.glathe@oldschoolsolutions.biz>
> ---
>  drivers/gpu/drm/msm/dp/dp_panel.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
>
> diff --git a/drivers/gpu/drm/msm/dp/dp_panel.c b/drivers/gpu/drm/msm/dp/dp_panel.c
> index 6bb021820d7c5..4f27c8129b0ef 100644
> --- a/drivers/gpu/drm/msm/dp/dp_panel.c
> +++ b/drivers/gpu/drm/msm/dp/dp_panel.c
> @@ -327,11 +327,39 @@ u32 msm_dp_panel_get_mode_bpp(struct msm_dp_panel *msm_dp_panel,
>  int msm_dp_panel_get_modes(struct msm_dp_panel *msm_dp_panel,
>  	struct drm_connector *connector)
>  {
> +	int modes;
> +	int retry;
> +	struct msm_dp_panel_private *panel;
> +
>  	if (!msm_dp_panel) {
>  		DRM_ERROR("invalid input\n");
>  		return -EINVAL;
>  	}
>  
> +	panel = container_of(msm_dp_panel, struct msm_dp_panel_private, msm_dp_panel);
> +
> +	if (msm_dp_panel->drm_edid) {
> +		modes = drm_edid_connector_add_modes(connector);
> +		if (modes > 0)
> +			return modes;
> +
> +		drm_edid_free(msm_dp_panel->drm_edid);
> +		msm_dp_panel->drm_edid = NULL;
> +	}
> +
> +	for (retry = 0; retry < 5; retry++) {
> +		usleep_range(20000, 25000);
> +		msm_dp_panel->drm_edid =
> +			drm_edid_read_ddc(connector, &panel->aux->ddc);
> +		if (msm_dp_panel->drm_edid)
> +			break;
> +
> +		drm_dbg_dp(connector->dev,
> +			   "get_modes re-read attempt %d/5 failed\n",
> +			   retry + 1);
> +	}
> +
> +	drm_edid_connector_update(connector, msm_dp_panel->drm_edid);
>  	if (msm_dp_panel->drm_edid)
>  		return drm_edid_connector_add_modes(connector);
>  
>
> ---
> base-commit: 7da7f07112610a520567421dd2ffcb51beaefbcc
> change-id: 20260531-drm_plug_flaky_edid-cc7743f6f909
>
> Best regards,

-- 
Jani Nikula, Intel