On Thu Jan 29, 2026 at 9:18 AM CET, Liu Ying wrote:
>
>
> On Thu, Jan 29, 2026 at 03:49:38PM +0800, Liu Ying wrote:
>>
>>
>> On Wed, Jan 28, 2026 at 04:58:18PM +0100, Luca Ceresoli wrote:
>>> On Tue Jan 27, 2026 at 4:54 AM CET, Liu Ying wrote:
>>>
>>> ...
>>>
>>>>>>> @@ -260,7 +259,7 @@ 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;
>>>>>>> + struct drm_bridge *selected_bridge __free(drm_bridge_put) =
>>>>>>> NULL;
>>>>>>> u32 port_id;
>>>>>>> bool found_port = false;
>>>>>>> int reg;
>>>>>>> @@ -297,7 +296,8 @@ 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 +305,14 @@ 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 (!selected_bridge || of_property_present(remote,
>>>>>>> "fsl,companion-pxl2dpi")) {
>>>>>>> + drm_bridge_put(selected_bridge);
>>>>>>> + selected_bridge = drm_bridge_get(next_bridge);
>>>>>>
>>>>>> Considering selecting the first bridge without the companion pxl2dpi,
>>>>>> there would be a superfluous refcount for the selected bridge:
>>>>>>
>>>>>> 1) of_drm_find_and_get_bridge: refcount = 1
>>>>>> 2) drm_bridge_put: noop, since selected_bridge is NULL, refcount = 1
>>>>>> 3) drm_bridge_get: refcount = 2
>>>>>> 4) drm_bridge_put(__free): refcount = 1
>>>>>> 5) drm_bridge_get: for the pl->bridge.next_bridge, refcount = 2
>>>>>
>>>>> Here you are missing one put. There are two drm_bridge_put(__free), one
>>>>> for
>>>>> next_bridge and one for selected_bridge. So your list should rather be:
>>>>>
>>>>> 1) next_bridge = of_drm_find_and_get_bridge: refcount = 1
>>>>> 2) drm_bridge_put(selected_bridge): noop, since selected_bridge is NULL,
>>>>> refcount = 1
>>>>> 3) selected_bridge = drm_bridge_get: refcount = 2
>>>>> 4) drm_bridge_put(next_bridge) [__free at loop scope end]: refcount = 1
>>>>> 5) pl->bridge.next_bridge = drm_bridge_get(), refcount = 2
>>>>> 6) drm_bridge_put(selected_bridge) [__free at function scope end]:
>>>>> refcount = 1
>>>>
>>>> Ah, right, I did miss this last put because selected_bridge is declared
>>>> with
>>>> __free a bit far away from the loop at the very beginning of
>>>> imx8qxp_pixel_link_find_next_bridge() - that's my problem I guess, but I'm
>>>> not even sure if I'll fall into this same pitfall again after a while,
>>>> which
>>>> makes the driver difficult to maintain.
>>>>
>>>> Also, it seems that the refcount dance(back and forth bewteen 1 and 2) is
>>>> not
>>>> something straightforward for driver readers to follow.
>>>
>>> I thing the whole logic becomes straightforward if you think it this way:
>>>
>>> * when a pointer is assigned = a new reference starts existing ->
>>> refcount++
>>> * when a pointer is cleared/overwritten or goes out of scope = a reference
>>> stops existing -> refcount--
>>>
>>> In short: one pointer, one reference, one refcount.
>>>
>>> If you re-read the patch with this in mind, does it become clearer?
>>
>> Thanks for more explaination, maybe it becomes a bit clearer, I'm not sure:/
>>
>> Anyway, to simplify things with another try, I came up with the below
>> snippet in that loop, which drops the two intermediate bridges(local
>> next_bridge and selected_bridge) and uses pl->next_bridge only.
>
> Fix a typo:
> s/pl->next_bridge/pl->bridge.next_bridge/
>
>> It looks ok to me(at least, refcount dance is much simpler).
>>
>> -8<-
>> if (!pl->next_bridge || of_property_present(remote,
>> "fsl,companion-pxl2dpi")) {
>> drm_bridge_put(pl->next_bridge);
>> pl->next_bridge = of_drm_find_and_get_bridge(remote);
>> if (!pl->next_bridge)
>> return -EPROBE_DEFER;
>> }
>> -8<-
>
> -8<-
> if (!pl->bridge.next_bridge || 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<-
It's OK enough, so in v5 I'm going to split the if() and skip the
intermediate selected_bridge variable.
Luca
--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
