Re: drm/bridge: adv7511: Attach next bridge without creating connector

[Sui Jingfeng <sui.jingfeng@linux.dev>]

Hi,


On 5/14/24 23:12, Laurent Pinchart wrote:
> Hello,
> 
> On Tue, May 14, 2024 at 12:26:15AM +0800, Sui Jingfeng wrote:
>> On 5/13/24 16:02, Liu Ying wrote:
>>> The connector is created by either this ADV7511 bridge driver or
>>> any DRM device driver/previous bridge driver, so this ADV7511
>>> bridge driver should not let the next bridge driver create connector.
>>>
>>> If the next bridge is a HDMI connector, the next bridge driver
>>> would fail to attach bridge from display_connector_attach() without
>>> the DRM_BRIDGE_ATTACH_NO_CONNECTOR flag.
> 
> In theory we could have another HDMI-to-something bridge connected to
> the ADV7511 output, and that bridge could create a connector. However,
> before commit 14b3cdbd0e5b the adv7511 driver didn't try to attach to
> the next bridge, so it's clear that no platform support in mainline had
> such a setup. It should be safe to set DRM_BRIDGE_ATTACH_NO_CONNECTOR
> unconditionally here.


But what if there is a drm bridge prior to adv7511 but after the KMS
engine? Even though we are still safe if it doesn't create connector
by obeying modern rule.


> 
> Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
> 
>>>
>>> Add that flag to drm_bridge_attach() function call in
>>> adv7511_bridge_attach() to fix the issue.
>>>
>>> This fixes the issue where the HDMI connector bridge fails to attach
>>> to the previous ADV7535 bridge on i.MX8MP EVK platform:
>>>
>>> [    2.216442] [drm:drm_bridge_attach] *ERROR* failed to attach bridge /hdmi-connector to encoder None-37: -22
>>> [    2.220675] mmc1: SDHCI controller on 30b50000.mmc [30b50000.mmc] using ADMA
>>> [    2.226262] [drm:drm_bridge_attach] *ERROR* failed to attach bridge /soc@0/bus@30800000/i2c@30a30000/hdmi@3d to encoder None-37: -22
>>> [    2.245204] [drm:drm_bridge_attach] *ERROR* failed to attach bridge /soc@0/bus@32c00000/dsi@32e60000 to encoder None-37: -22
>>> [    2.256445] imx-lcdif 32e80000.display-controller: error -EINVAL: Failed to attach bridge for endpoint0
>>> [    2.265850] imx-lcdif 32e80000.display-controller: error -EINVAL: Cannot connect bridge
>>> [    2.274009] imx-lcdif 32e80000.display-controller: probe with driver imx-lcdif failed with error -22
>>>
>>> Fixes: 14b3cdbd0e5b ("drm/bridge: adv7511: make it honour next bridge in DT")
>>> Signed-off-by: Liu Ying <victor.liu@nxp.com>
>>> ---
>>>    drivers/gpu/drm/bridge/adv7511/adv7511_drv.c | 3 ++-
>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/bridge/adv7511/adv7511_drv.c b/drivers/gpu/drm/bridge/adv7511/adv7511_drv.c
>>> index dd21b81bd28f..66ccb61e2a66 100644
>>> --- a/drivers/gpu/drm/bridge/adv7511/adv7511_drv.c
>>> +++ b/drivers/gpu/drm/bridge/adv7511/adv7511_drv.c
>>> @@ -953,7 +953,8 @@ static int adv7511_bridge_attach(struct drm_bridge *bridge,
>>>    	int ret = 0;
>>>    
>>>    	if (adv->next_bridge) {
>>> -		ret = drm_bridge_attach(bridge->encoder, adv->next_bridge, bridge, flags);
>>> +		ret = drm_bridge_attach(bridge->encoder, adv->next_bridge, bridge,
>>> +					flags | DRM_BRIDGE_ATTACH_NO_CONNECTOR);
>>
>> As a side note, I think, maybe you could do better in the future.
>>
>> If we know that the KMS display driver side has the HDMI connector
>> already created for us, we should pass DRM_BRIDGE_ATTACH_NO_CONNECTOR
>> from the root KMS driver side. Which is to forbidden all potential
>> drm bridge drivers to create a connector in the middle.
> 
> That's the recommended way for new drivers. Using the
> drm_bridge_connector helper handles all this for you.
> 
>> The KMS display driver side could parse the DT to know if there is
>> a hdmi connector, or merely just hdmi connector device node, or
>> something else.
> 
> No, that would violate the basic principle of not peeking into the DT of
> devices you know nothing about. The display engine driver can't walk the
> pipeline in DT and expect to understand all the DT nodes on the path,
> and what their properties mean.
> 

The (next) bridge at the remote port is not necessary a display bridge.
Or it is a bridge from the perspective of hardware viewpoint, but under
controlled by a more complex foreign driver which generic drm bridge
driver has no authority to attach.

> What KMS drivers should do is to use the drm_bridge_connector helper.
> Calling drm_bridge_connector_init() will create a connector for a chain
> of bridges. The KMS driver should then attach to the first bridge with
> DRM_BRIDGE_ATTACH_NO_CONNECTOR, unconditionally.
>

OK, thanks for teaching us the modern way to use drm_bridge_connector
helper, also thanks Ying for providing the patch.


>> However, other maintainer and/or reviewer's opinion are of cause
>> more valuable. I send a A-b because I thought the bug is urgency
>> and it's probably more important to solve this bug first. And
>> maybe you can Cc: <stable@vger.kernel.org> if you like.
>>
>>>    		if (ret)
>>>    			return ret;
>>>    	}
>

-- 
Best regards
Sui