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Message-ID: <20251001180922.6bbe42ac@booty>
Date: Wed, 1 Oct 2025 18:09:22 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Damon Ding <damon.ding@...k-chips.com>
Cc: andrzej.hajda@...el.com, neil.armstrong@...aro.org, rfoss@...nel.org,
Laurent.pinchart@...asonboard.com, jonas@...boo.se,
jernej.skrabec@...il.com, maarten.lankhorst@...ux.intel.com,
mripard@...nel.org, tzimmermann@...e.de, airlied@...il.com,
simona@...ll.ch, shawnguo@...nel.org, s.hauer@...gutronix.de,
kernel@...gutronix.de, festevam@...il.com, inki.dae@...sung.com,
sw0312.kim@...sung.com, kyungmin.park@...sung.com, krzk@...nel.org,
alim.akhtar@...sung.com, jingoohan1@...il.com, p.zabel@...gutronix.de,
hjc@...k-chips.com, heiko@...ech.de, andy.yan@...k-chips.com,
dmitry.baryshkov@....qualcomm.com, dianders@...omium.org,
m.szyprowski@...sung.com, jani.nikula@...el.com,
linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
linux-samsung-soc@...r.kernel.org, linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH v6 11/18] drm/display: bridge_connector: Ensure last
bridge determines EDID/modes detection capabilities
Hello Damon,
On Tue, 30 Sep 2025 17:09:13 +0800
Damon Ding <damon.ding@...k-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
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