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Message-ID: <a42ebdca-1cbc-498c-b859-336557823f26@rock-chips.com>
Date: Thu, 9 Oct 2025 12:10:42 +0800
From: Damon Ding <damon.ding@...k-chips.com>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>,
Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.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, 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
Hi Luca,
On 10/2/2025 12:09 AM, Luca Ceresoli wrote:
> 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);
>
Yes, this is correct and maintains functional equivalence with the
previous implementation.
I previously attempted to implement this feature by modifying the logic
in this section. However, that approach would obscure the explicit
propagation semantics of the bridge chain flags
(OP_EDID/OP_HPD/OP_DETECT/OP_MODES). Therefore, I finally decided to
implemented it as a specific check after this code block.
Dmitry, what's your take on this?
> 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/
>
This is indeed a clever approach to the managing bridge resource cleanup
in drm_bridge_connector. Thanks a lot for the heads-up! I'll resolve
this conflict and rebase the patch series.
Apologies for the delayed reply as I was on vacation. ;-)
Best regards,
Damon
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