Re: [PATCH v6 11/18] drm/display: bridge_connector: Ensure last bridge determines EDID/modes detection capabilities

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

Hello Damon,

On Tue, 30 Sep 2025 17:09:13 +0800
Damon Ding <damon.ding@rock-chips.com> wrote:

> When multiple bridges are present, EDID detection capability
> (DRM_BRIDGE_OP_EDID) takes precedence over modes detection
> (DRM_BRIDGE_OP_MODES). To ensure the above two capabilities are
> determined by the last bridge in the chain, we handle three cases:
> 
> Case 1: The later bridge declares only DRM_BRIDGE_OP_MODES
>  - If the previous bridge declares DRM_BRIDGE_OP_EDID, set
>    &drm_bridge_connector.bridge_edid to NULL and set
>    &drm_bridge_connector.bridge_modes to the later bridge.
>  - Ensure modes detection capability of the later bridge will not
>    be ignored.
> 
> Case 2: The later bridge declares only DRM_BRIDGE_OP_EDID
>  - If the previous bridge declares DRM_BRIDGE_OP_MODES, set
>    &drm_bridge_connector.bridge_modes to NULL and set
>    &drm_bridge_connector.bridge_edid to the later bridge.
>  - Although EDID detection capability has higher priority, this
>    operation is for balance and makes sense.
> 
> Case 3: the later bridge declares both of them
>  - Assign later bridge as &drm_bridge_connector.bridge_edid and
>    and &drm_bridge_connector.bridge_modes to this bridge.
>  - Just leave transfer of these two capabilities as before.

I think the whole explanation can be more concisely rewritten as:

If the later bridge declares OP_EDID, OP_MODES or both, then both
.bridge_modes and .bridge_edid should be set to NULL (if any was set
from a previous bridge), and then .bridge_modes and/or .bridge_edid be
set to the later bridge as is done already.

Does this look correct (i.e. does it convey the same meaning)?

> --- a/drivers/gpu/drm/display/drm_bridge_connector.c
> +++ b/drivers/gpu/drm/display/drm_bridge_connector.c
> @@ -640,6 +640,7 @@ struct drm_connector *drm_bridge_connector_init(struct drm_device *drm,
>  	struct drm_connector *connector;
>  	struct i2c_adapter *ddc = NULL;
>  	struct drm_bridge *bridge, *panel_bridge = NULL;
> +	struct drm_bridge *pre_bridge_edid, *pre_bridge_modes;
>  	unsigned int supported_formats = BIT(HDMI_COLORSPACE_RGB);
>  	unsigned int max_bpc = 8;
>  	bool support_hdcp = false;
> @@ -668,6 +669,9 @@ struct drm_connector *drm_bridge_connector_init(struct drm_device *drm,
>  	 */
>  	connector_type = DRM_MODE_CONNECTOR_Unknown;
>  	drm_for_each_bridge_in_chain(encoder, bridge) {
> +		pre_bridge_edid = bridge_connector->bridge_edid;
> +		pre_bridge_modes = bridge_connector->bridge_modes;
> +
>  		if (!bridge->interlace_allowed)
>  			connector->interlace_allowed = false;
>  		if (!bridge->ycbcr_420_allowed)
> @@ -681,6 +685,44 @@ struct drm_connector *drm_bridge_connector_init(struct drm_device *drm,
>  			bridge_connector->bridge_detect = bridge;
>  		if (bridge->ops & DRM_BRIDGE_OP_MODES)
>  			bridge_connector->bridge_modes = bridge;
> +
> +		/*
> +		 * When multiple bridges are present, EDID detection capability
> +		 * (DRM_BRIDGE_OP_EDID) takes precedence over modes detection
> +		 * (DRM_BRIDGE_OP_MODES). To ensure the above two capabilities
> +		 * are determined by the last bridge in the chain, we handle
> +		 * three cases:
> +		 *
> +		 * Case 1: The later bridge declares only DRM_BRIDGE_OP_MODES
> +		 *  - If the previous bridge declares DRM_BRIDGE_OP_EDID, set
> +		 *    &drm_bridge_connector.bridge_edid to NULL and set
> +		 *    &drm_bridge_connector.bridge_modes to the later bridge.
> +		 *  - Ensure modes detection capability of the later bridge
> +		 *    will not be ignored.
> +		 *
> +		 * Case 2: The later bridge declares only DRM_BRIDGE_OP_EDID
> +		 *  - If the previous bridge declares DRM_BRIDGE_OP_MODES, set
> +		 *    &drm_bridge_connector.bridge_modes to NULL and set
> +		 *    &drm_bridge_connector.bridge_edid to the later bridge.
> +		 *  - Although EDID detection capability has higher priority,
> +		 *    this operation is for balance and makes sense.
> +		 *
> +		 * Case 3: the later bridge declares both of them
> +		 *  - Assign later bridge as &drm_bridge_connector.bridge_edid
> +		 *    and &drm_bridge_connector.bridge_modes to this bridge.
> +		 *  - Just leave transfer of these two capabilities as before.
> +		 */
> +		if (bridge->ops & DRM_BRIDGE_OP_EDID &&
> +		    !(bridge->ops & DRM_BRIDGE_OP_MODES)) {
> +			if (pre_bridge_modes)
> +				bridge_connector->bridge_modes = NULL;
> +		}
> +		if (bridge->ops & DRM_BRIDGE_OP_MODES &&
> +		    !(bridge->ops & DRM_BRIDGE_OP_EDID)) {
> +			if (pre_bridge_edid)
> +				bridge_connector->bridge_edid = NULL;
> +		}
> +

If the above rewrite is correct, then I think this patch can be
rewritten in a simple way (build tested only):

diff --git a/drivers/gpu/drm/display/drm_bridge_connector.c b/drivers/gpu/drm/display/drm_bridge_connector.c
index a5bdd6c10643..bd5dbafe88bc 100644
--- a/drivers/gpu/drm/display/drm_bridge_connector.c
+++ b/drivers/gpu/drm/display/drm_bridge_connector.c
@@ -672,14 +672,18 @@ struct drm_connector *drm_bridge_connector_init(struct drm_device *drm,
                if (!bridge->ycbcr_420_allowed)
                        connector->ycbcr_420_allowed = false;
 
-               if (bridge->ops & DRM_BRIDGE_OP_EDID)
-                       bridge_connector->bridge_edid = bridge;
+               if (bridge->ops & DRM_BRIDGE_OP_EDID || bridge->ops & DRM_BRIDGE_OP_MODES) {
+                       bridge_connector->bridge_edid = NULL;
+                       bridge_connector->bridge_modes = NULL;
+                       if (bridge->ops & DRM_BRIDGE_OP_EDID)
+                               bridge_connector->bridge_edid = bridge;
+                       if (bridge->ops & DRM_BRIDGE_OP_MODES)
+                               bridge_connector->bridge_modes = bridge;
+               }
                if (bridge->ops & DRM_BRIDGE_OP_HPD)
                        bridge_connector->bridge_hpd = bridge;
                if (bridge->ops & DRM_BRIDGE_OP_DETECT)
                        bridge_connector->bridge_detect = bridge;
-               if (bridge->ops & DRM_BRIDGE_OP_MODES)
-                       bridge_connector->bridge_modes = bridge;
                if (bridge->ops & DRM_BRIDGE_OP_HDMI) {
                        if (bridge_connector->bridge_hdmi)
                                return ERR_PTR(-EBUSY);

Another thing to note is that this patch conflicts with [0], which I
plan to apply in the next few days. The two patches are orthogonal but
they insist on the same lines (those assigning
bridge_connector->bridge_* = bridge). Not a big deal, whichever patch
comes later will be easily adapted. Just wanted to ensure you are aware.

[0] https://lore.kernel.org/all/20250926-drm-bridge-alloc-getput-bridge-connector-v2-1-138b4bb70576@bootlin.com/

Best regards,
Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com