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Message-ID: <16181573-f53b-4e76-abe9-953c1e967d75@rock-chips.com>
Date: Sat, 11 Oct 2025 09:04:16 +0800
From: Damon Ding <damon.ding@...k-chips.com>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>,
Luca Ceresoli <luca.ceresoli@...tlin.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,
On 10/10/2025 10:02 PM, Dmitry Baryshkov wrote:
> On Thu, Oct 09, 2025 at 12:10:42PM +0800, Damon Ding wrote:
>> 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?
>
> I think I prefer Luca's code, it is simpler and easier to understand. It
> doesn't need a huge comment, something like "leave the last bridge which
> provides either OP_EDID or OP_MODES" should be enough.
>
Yes, I will update the code in v7.
>>
>>> 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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