Re: [PATCH v5] drm/bridge: imx8qxp-pixel-link: get/put the next bridge

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

Hello Liu,

On Thu Feb 5, 2026 at 10:26 AM CET, Liu Ying wrote:
> On Thu, Feb 05, 2026 at 09:52:04AM +0100, Luca Ceresoli wrote:
>> Hello Liu,
>
> Hello Luca,
>
>>
>> On Wed Feb 4, 2026 at 7:27 AM CET, Liu Ying wrote:
>>> Hi Luca,
>>>
>>> On Tue, Feb 03, 2026 at 11:35:25AM +0100, Luca Ceresoli wrote:
>>>> This driver obtains a bridge pointer from of_drm_find_bridge() in the probe
>>>> function and stores it until driver removal. of_drm_find_bridge() is
>>>> deprecated. Move to of_drm_find_and_get_bridge() for the bridge to be
>>>> refcounted and use bridge->next_bridge to put the reference on
>>>> deallocation.
>>>>
>>>> To keep the code as simple and reliable as possible, get a reference for
>>>> each pointer that stores a drm_bridge address when it is stored and release
>>>> it when the pointer is overwritten or goes out of scope. Also remove the
>>>> intermediate selected_bridge variable to reduce the refcounted variables in
>>>> the function. The involved pointers are:
>>>>
>>>>  * next_bridge loop-local variable:
>>>>    - get reference by of_drm_find_and_get_bridge()
>>>>    - put reference at the end of the loop iteration (__free)
>>>>
>>>>  * pl->bridge.next_bridge, tied to struct imx8qxp_pixel_link lifetime:
>>>>    - get reference when assigned (by copy from next_bridge)
>>>>    - put reference before reassignment if reassignment happens
>>>>    - put reference when the struct imx8qxp_pixel_link embedding the
>>>>      struct drm_bridge is destroyed (struct drm_bridge::next_bridge)
>>>>
>>>> Additionally, split the somewhat complex if() for readability.
>>>>
>>>> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
>>>>
>>>> ---
>>>>
>>>> Changes in v5:
>>>> - rewrite commit message after Liu's review to clarify the per-pointer
>>>>   get/put idea
>>>> - split the if()s involved in selcting the bridge
>>>> - remove intermediate selected_bridge pointer
>>>> - removed Maxime's R-by, patch changed
>>>> ---
>>>>  drivers/gpu/drm/bridge/imx/imx8qxp-pixel-link.c | 17 ++++++++++-------
>>>>  1 file changed, 10 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/bridge/imx/imx8qxp-pixel-link.c b/drivers/gpu/drm/bridge/imx/imx8qxp-pixel-link.c
>>>> index 91e4f4d55469..e29e099b893a 100644
>>>> --- a/drivers/gpu/drm/bridge/imx/imx8qxp-pixel-link.c
>>>> +++ b/drivers/gpu/drm/bridge/imx/imx8qxp-pixel-link.c
>>>> @@ -23,7 +23,6 @@
>>>>
>>>>  struct imx8qxp_pixel_link {
>>>>  	struct drm_bridge bridge;
>>>> -	struct drm_bridge *next_bridge;
>>>>  	struct device *dev;
>>>>  	struct imx_sc_ipc *ipc_handle;
>>>>  	u8 stream_id;
>>>> @@ -140,7 +139,7 @@ static int imx8qxp_pixel_link_bridge_attach(struct drm_bridge *bridge,
>>>>  	}
>>>>
>>>>  	return drm_bridge_attach(encoder,
>>>> -				 pl->next_bridge, bridge,
>>>> +				 pl->bridge.next_bridge, bridge,
>>>>  				 DRM_BRIDGE_ATTACH_NO_CONNECTOR);
>>>>  }
>>>>
>>>> @@ -260,7 +259,6 @@ static int imx8qxp_pixel_link_find_next_bridge(struct imx8qxp_pixel_link *pl)
>>>>  {
>>>>  	struct device_node *np = pl->dev->of_node;
>>>>  	struct device_node *port;
>>>> -	struct drm_bridge *selected_bridge = NULL;
>>>>  	u32 port_id;
>>>>  	bool found_port = false;
>>>>  	int reg;
>>>> @@ -297,7 +295,8 @@ static int imx8qxp_pixel_link_find_next_bridge(struct imx8qxp_pixel_link *pl)
>>>>  			continue;
>>>>  		}
>>>>
>>>> -		struct drm_bridge *next_bridge = of_drm_find_bridge(remote);
>>>> +		struct drm_bridge *next_bridge __free(drm_bridge_put) =
>>>> +			of_drm_find_and_get_bridge(remote);
>>>>  		if (!next_bridge)
>>>>  			return -EPROBE_DEFER;
>>>>
>>>> @@ -305,12 +304,16 @@ static int imx8qxp_pixel_link_find_next_bridge(struct imx8qxp_pixel_link *pl)
>>>>  		 * Select the next bridge with companion PXL2DPI if
>>>>  		 * present, otherwise default to the first bridge
>>>>  		 */
>>>> -		if (!selected_bridge || of_property_present(remote, "fsl,companion-pxl2dpi"))
>>>> -			selected_bridge = next_bridge;
>>>> +		if (!pl->bridge.next_bridge)
>>>> +			pl->bridge.next_bridge = drm_bridge_get(next_bridge);
>>>> +
>>>> +		if (of_property_present(remote, "fsl,companion-pxl2dpi")) {
>>>> +			drm_bridge_put(pl->bridge.next_bridge);
>>>> +			pl->bridge.next_bridge = drm_bridge_get(next_bridge);
>>>> +		}
>>>
>>> Can you drop the intermediate next_bridge variable to simplify the code?
>>>
>>> -8<-
>>> if (!pl->bridge.next_bridge) {
>>>         pl->bridge.next_bridge = of_drm_find_and_get_bridge(remote);
>>>         if (!pl->bridge.next_bridge)
>>>                 return -EPROBE_DEFER;
>>> }
>>>
>>> if (of_property_present(remote, "fsl,companion-pxl2dpi")) {
>>>         drm_bridge_put(pl->bridge.next_bridge);
>>>         pl->bridge.next_bridge = of_drm_find_and_get_bridge(remote);
>>>         if (!pl->bridge.next_bridge)
>>>                 return -EPROBE_DEFER;
>>> }
>>> -8<-
>>
>> Potentially calling of_drm_find_and_get_bridge() twice on the same node,
>> with a put in the middle, looks poorly readable to me, even though it still
>> looks correct code.
>>
>> However I think we can do even better with an 'else if':
>>
>>   if (!pl->bridge.next_bridge) {
>>          pl->bridge.next_bridge = of_drm_find_and_get_bridge(remote);
>>          if (!pl->bridge.next_bridge)
>>                  return -EPROBE_DEFER;
>>   } else if (of_property_present(remote, "fsl,companion-pxl2dpi")) {   <===
>>          drm_bridge_put(pl->bridge.next_bridge);
>>          pl->bridge.next_bridge = of_drm_find_and_get_bridge(remote);
>>          if (!pl->bridge.next_bridge)
>>                  return -EPROBE_DEFER;
>>   }
>>
>> Looks OK?
>
> Both are fine to me.  TBH, I feel my version with two 'if's is a bit easier
> to read.  But, I'd say up to you.

I think this is really a minor detail and there is no obvious "best"
version, so I'll send v6 as I had it ready and build-tested already,
i.e. with the 'else if' version.

Thanks for the discussion!

Luca

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