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

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

Hello Liu,

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?

Luca

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