Re: [PATCH] drm/msm/dp: don't fall back to a link that can't carry the eDP mode

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found no new issues
in this patch, but noticed 1 related pre-existing issue it does not
introduce:
- [Medium] drm/msm/dp: unpropagated eDP 1.4 link_info fields in
  msm_dp_ctrl_on_link() [1]
--

commit 2e586911d41c02809032afda8e106d6b81011527
Author: Jesse Casco <jesse.casco@gmail.com>

drm/msm/dp: don't fall back to a link that can't carry the eDP mode

When eDP link training downshifts to a link rate or lane count that lacks
enough bandwidth for the panel mode, this commit restores the previous
link parameters and retries training instead of training a link that
leaves the panel dark.

> diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> index 47647fdec9de5..c77427aadcb46 100644
> --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c

[ ... ]

> @@ -2420,6 +2433,7 @@ int msm_dp_ctrl_on_link(struct msm_dp_ctrl *msm_dp_ctrl,
>  		return rc;
>  
>  	while (--link_train_max_retries) {
> +		prev_params = ctrl->link->link_params;
>  		training_step = DP_TRAINING_NONE;
>  		rc = msm_dp_ctrl_setup_main_link(ctrl, panel, &training_step);
>  		if (rc == 0) {

[Severity: Medium]
This problem wasn't introduced by this patch, but does
msm_dp_ctrl_on_link() drop the eDP 1.4 supported_rates, rate_set, and
use_rate_set fields from panel->link_info when initializing
ctrl->link->link_params [1]?

Earlier in msm_dp_ctrl_on_link(), only rate and num_lanes are copied into
ctrl->link->link_params (and msm_dp_ctrl_link_lane_down_shift() likewise
restores only rate from panel->link_info):

drivers/gpu/drm/msm/dp/dp_ctrl.c:msm_dp_ctrl_on_link() {
    ...
	} else {
		ctrl->link->link_params.rate = rate;
		ctrl->link->link_params.num_lanes =
			panel->link_info.num_lanes;
    ...
}

In addition, msm_dp_ctrl_link_train() copies only num_lanes, rate, and
capabilities into the local link_info struct passed to
msm_dp_aux_link_configure().

Because use_rate_set and rate_set remain zero in both places,
msm_dp_aux_link_configure() never writes DP_LINK_RATE_SET on eDP 1.4+
panels, and msm_dp_ctrl_link_rate_down_shift() bypasses supported_rates[]
and fails with -EINVAL on intermediate eDP 1.4 link rates.

[1] https://sashiko.dev/#/bug/linux-bd6dd9b8-1fc3-40aa-82c1-7f5b2c6afbc1

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261115000021.78869-1-jesse.casco@gmail.com?part=1