Re: [PATCH v3 3/8] drm/msm/dp: Read DPCD and sink count in bridge detect()

[Yongxing Mou <yongxing.mou@oss.qualcomm.com>]

On 1/15/2026 3:29 PM, Dmitry Baryshkov wrote:
> From: Jessica Zhang <jessica.zhang@oss.qualcomm.com>
> 
> Instead of relying on the link_ready flag to specify if DP is connected,
> read the DPCD bits and get the sink count to accurately detect if DP is
> connected.
> 
> Signed-off-by: Jessica Zhang <jessica.zhang@oss.qualcomm.com>
> Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
> ---
>   drivers/gpu/drm/msm/dp/dp_display.c | 60 +++++++++++++++++++++++++++++++++++++
>   drivers/gpu/drm/msm/dp/dp_drm.c     | 20 -------------
>   drivers/gpu/drm/msm/dp/dp_drm.h     |  2 ++
>   3 files changed, 62 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> index 5997cd28ba11..a05144de3b93 100644
> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> @@ -1151,6 +1151,66 @@ static int msm_dp_hpd_event_thread_start(struct msm_dp_display_private *msm_dp_p
>   	return 0;
>   }
>   
> +/**
> + * msm_dp_bridge_detect - callback to determine if connector is connected
> + * @bridge: Pointer to drm bridge structure
> + * @connector: Pointer to drm connector structure
> + * Returns: Bridge's 'is connected' status
> + */
> +enum drm_connector_status msm_dp_bridge_detect(struct drm_bridge *bridge,
> +					       struct drm_connector *connector)
> +{
> +	struct msm_dp_bridge *msm_dp_bridge = to_dp_bridge(bridge);
> +	struct msm_dp *dp = msm_dp_bridge->msm_dp_display;
> +	struct msm_dp_display_private *priv;
> +	int ret = 0;
> +	int status = connector_status_disconnected;
> +	u8 dpcd[DP_RECEIVER_CAP_SIZE];
> +	struct drm_dp_desc desc;
> +
> +	dp = to_dp_bridge(bridge)->msm_dp_display;
> +
> +	priv = container_of(dp, struct msm_dp_display_private, msm_dp_display);
> +
> +	if (!dp->link_ready)
> +		return status;
> +
> +	msm_dp_aux_enable_xfers(priv->aux, true);
> +
> +	ret = pm_runtime_resume_and_get(&dp->pdev->dev);
> +	if (ret) {
> +		DRM_ERROR("failed to pm_runtime_resume\n");
> +		msm_dp_aux_enable_xfers(priv->aux, false);
> +		return status;
> +	}
> +
> +	ret = msm_dp_aux_is_link_connected(priv->aux);
> +	if (dp->internal_hpd && !ret)
> +		goto end;
> +
> +	ret = drm_dp_read_dpcd_caps(priv->aux, dpcd);
> +	if (ret)
> +		goto end;
> +
> +	ret = drm_dp_read_desc(priv->aux, &desc, drm_dp_is_branch(dpcd));
> +	if (ret)
> +		goto end;
> +
> +	status = connector_status_connected;
> +	if (drm_dp_read_sink_count_cap(connector, dpcd, &desc)) {
> +		int sink_count = drm_dp_read_sink_count(priv->aux);
> +
> +		drm_dbg_dp(dp->drm_dev, "sink_count = %d\n", sink_count);
> +
> +		if (sink_count <= 0)
> +			status = connector_status_disconnected;
> +	}
> +
> +end:
> +	pm_runtime_put_sync(&dp->pdev->dev);
> +	return status;
> +}
> +
>   static irqreturn_t msm_dp_display_irq_handler(int irq, void *dev_id)
>   {
>   	struct msm_dp_display_private *dp = dev_id;
> diff --git a/drivers/gpu/drm/msm/dp/dp_drm.c b/drivers/gpu/drm/msm/dp/dp_drm.c
> index fd6443d2b6ce..e4622c85fb66 100644
> --- a/drivers/gpu/drm/msm/dp/dp_drm.c
> +++ b/drivers/gpu/drm/msm/dp/dp_drm.c
> @@ -15,26 +15,6 @@
>   #include "dp_audio.h"
>   #include "dp_drm.h"
>   
> -/**
> - * msm_dp_bridge_detect - callback to determine if connector is connected
> - * @bridge: Pointer to drm bridge structure
> - * @connector: Pointer to drm connector structure
> - * Returns: Bridge's 'is connected' status
> - */
> -static enum drm_connector_status
> -msm_dp_bridge_detect(struct drm_bridge *bridge, struct drm_connector *connector)
> -{
> -	struct msm_dp *dp;
> -
> -	dp = to_dp_bridge(bridge)->msm_dp_display;
> -
> -	drm_dbg_dp(dp->drm_dev, "link_ready = %s\n",
> -		str_true_false(dp->link_ready));
> -
> -	return (dp->link_ready) ? connector_status_connected :
> -					connector_status_disconnected;
> -}
> -
>   static int msm_dp_bridge_atomic_check(struct drm_bridge *bridge,
>   			    struct drm_bridge_state *bridge_state,
>   			    struct drm_crtc_state *crtc_state,
> diff --git a/drivers/gpu/drm/msm/dp/dp_drm.h b/drivers/gpu/drm/msm/dp/dp_drm.h
> index 9eb3431dd93a..6c0426803d78 100644
> --- a/drivers/gpu/drm/msm/dp/dp_drm.h
> +++ b/drivers/gpu/drm/msm/dp/dp_drm.h
> @@ -25,6 +25,8 @@ int msm_dp_bridge_init(struct msm_dp *msm_dp_display, struct drm_device *dev,
>   		   struct drm_encoder *encoder,
>   		   bool yuv_supported);
>   
> +enum drm_connector_status msm_dp_bridge_detect(struct drm_bridge *bridge,
> +					       struct drm_connector *connector);
>   void msm_dp_bridge_atomic_enable(struct drm_bridge *drm_bridge,
>   				 struct drm_atomic_state *state);
>   void msm_dp_bridge_atomic_disable(struct drm_bridge *drm_bridge,
> 
Hi, Dmitry, i want to ask a question about the .detect() here.

https://lore.kernel.org/all/20250802-dp-conn-no-detect-v1-1-2748c2b946da@oss.qualcomm.com/ 

In this change, we moved the hpd-gpios detect logic into the MSM DP 
driver, but it seems like the current detect flow doesn’t really handle 
this setup. what i'm seeing is:

1.On platforms using hpd-gpios, booting the system with the DP cable 
already plugged in doesn’t work. The msm DP driver can’t detect when 
link_ready = false.

2.For hot‑plug cases, it looks like detection is still handled by 
display_connector_detect() instead of the MSM DP driver.

Could you please comment about this? thanks..