linux-kernel - Re: [PATCH v8 1/2] drm/msm/dp: force link training for display resolution change

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Message-ID: <f127060a-5648-fa40-81a0-be1be05b3951@linaro.org>
Date:   Wed, 22 Jun 2022 10:26:39 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Kuogee Hsieh <quic_khsieh@...cinc.com>,
        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
Cc:     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 v8 1/2] drm/msm/dp: force link training for display
 resolution change

On 16/06/2022 20:09, Kuogee Hsieh wrote:
> Display resolution change is implemented through drm modeset. Older
> modeset (resolution) has to be disabled first before newer modeset
> (resolution) can be enabled. Display disable will turn off both
> pixel clock and main link clock so that main link have to be
> re-trained during display enable to have new video stream flow
> again. At current implementation, display enable function manually
> kicks up irq_hpd_handle which will read panel link status and start
> link training if link status is not in sync state.
> 
> However, there is rare case that a particular panel links status keep
> staying in sync for some period of time after main link had been shut
> down previously at display disabled. In this case, main link retraining
> will not be executed by irq_hdp_handle(). Hence video stream of newer
> display resolution will fail to be transmitted to panel due to main
> link is not in sync between host and panel.
> 
> This patch will bypass irq_hpd_handle() in favor of directly call
> dp_ctrl_on_stream() to always perform link training in regardless of
> main link status. So that no unexpected exception resolution change
> failure cases will happen. Also this implementation are more efficient
> than manual kicking off irq_hpd_handle function.
> 
> Changes in v2:
> -- set force_link_train flag on DP only (is_edp == false)
> 
> Changes in v3:
> -- revise commit  text
> -- add Fixes tag
> 
> Changes in v4:
> -- revise commit  text
> 
> Changes in v5:
> -- fix spelling at commit text
> 
> Changes in v6:
> -- split dp_ctrl_on_stream() for phy test case
> -- revise commit text for modeset
> 
> Changes in v7:
> -- drop 0 assignment at local variable (ret = 0)
> 
> Changes in v8:
> -- add patch to remove pixel_rate from dp_ctrl
> 
> Fixes: 62671d2ef24b ("drm/msm/dp: fixes wrong connection state caused by failure of link train")
> Signed-off-by: Kuogee Hsieh <quic_khsieh@...cinc.com>
> ---
>   drivers/gpu/drm/msm/dp/dp_ctrl.c    | 31 +++++++++++++++++++++++--------
>   drivers/gpu/drm/msm/dp/dp_ctrl.h    |  3 ++-
>   drivers/gpu/drm/msm/dp/dp_display.c | 13 ++++++-------
>   3 files changed, 31 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> index af7a80c..01028b5 100644
> --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> @@ -1551,7 +1551,7 @@ static int dp_ctrl_process_phy_test_request(struct dp_ctrl_private *ctrl)
>   
>   	ret = dp_ctrl_on_link(&ctrl->dp_ctrl);
>   	if (!ret)
> -		ret = dp_ctrl_on_stream(&ctrl->dp_ctrl);
> +		ret = dp_ctrl_on_stream_phy_test_report(&ctrl->dp_ctrl);
>   	else
>   		DRM_ERROR("failed to enable DP link controller\n");
>   
> @@ -1807,7 +1807,27 @@ static int dp_ctrl_link_retrain(struct dp_ctrl_private *ctrl)
>   	return dp_ctrl_setup_main_link(ctrl, &training_step);
>   }
>   
> -int dp_ctrl_on_stream(struct dp_ctrl *dp_ctrl)
> +int dp_ctrl_on_stream_phy_test_report(struct dp_ctrl *dp_ctrl)
> +{
> +	int ret;
> +	struct dp_ctrl_private *ctrl;
> +
> +	ctrl = container_of(dp_ctrl, struct dp_ctrl_private, dp_ctrl);
> +
> +	ctrl->dp_ctrl.pixel_rate = ctrl->panel->dp_mode.drm_mode.clock;

Stephen has raised an interesting question. Comparing this to the 
dp_ctrl_on_stream(), he noticed that we do not halve the pixel clock 
here (if the wide bus is supported). So, the question is if this is 
correct or not.

> +
> +	ret = dp_ctrl_enable_stream_clocks(ctrl);
> +	if (ret) {
> +		DRM_ERROR("Failed to start pixel clocks. ret=%d\n", ret);
> +		return ret;
> +	}
> +
> +	dp_ctrl_send_phy_test_pattern(ctrl);
> +
> +	return 0;
> +}
> +
> +int dp_ctrl_on_stream(struct dp_ctrl *dp_ctrl, bool force_link_train)
>   {
>   	int ret = 0;
>   	bool mainlink_ready = false;
> @@ -1843,12 +1863,7 @@ int dp_ctrl_on_stream(struct dp_ctrl *dp_ctrl)
>   		goto end;
>   	}
>   
> -	if (ctrl->link->sink_request & DP_TEST_LINK_PHY_TEST_PATTERN) {
> -		dp_ctrl_send_phy_test_pattern(ctrl);
> -		return 0;
> -	}
> -
> -	if (!dp_ctrl_channel_eq_ok(ctrl))
> +	if (force_link_train || !dp_ctrl_channel_eq_ok(ctrl))
>   		dp_ctrl_link_retrain(ctrl);
>   
>   	/* stop txing train pattern to end link training */


-- 
With best wishes
Dmitry