Re: [PATCH 02/26] drm/bridge: deprecate of_drm_find_bridge()

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

Hi Maxime,

On Mon Nov 24, 2025 at 11:18 AM CET, Maxime Ripard wrote:
> On Wed, Nov 19, 2025 at 02:28:41PM +0000, Louis Chauvet wrote:
>> On 11/19/25 13:05, Luca Ceresoli wrote:
>> > of_drm_find_bridge() does not increment the returned bridge
>> > refcount. drm_of_find_bridge() is to be used as a replacement.
>> >
>> > Suggested-by: Maxime Ripard <mripard@kernel.org>
>> > Link: https://lore.kernel.org/dri-devel/20250319-stylish-lime-mongoose-0a18ad@houat/
>> > Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
>> > ---
>> >   drivers/gpu/drm/drm_bridge.c | 7 +++++--
>> >   1 file changed, 5 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
>> > index d98a7b4a83c0..6debbf20aaa8 100644
>> > --- a/drivers/gpu/drm/drm_bridge.c
>> > +++ b/drivers/gpu/drm/drm_bridge.c
>> > @@ -299,7 +299,7 @@ EXPORT_SYMBOL(__devm_drm_bridge_alloc);
>> >    * @bridge: bridge control structure
>> >    *
>> >    * Add the given bridge to the global list of bridges, where they can be
>> > - * found by users via of_drm_find_bridge().
>> > + * found by users via drm_of_find_bridge().
>> >    *
>> >    * The bridge to be added must have been allocated by
>> >    * devm_drm_bridge_alloc().
>> > @@ -360,7 +360,7 @@ EXPORT_SYMBOL(devm_drm_bridge_add);
>> >    * @bridge: bridge control structure
>> >    *
>> >    * Remove the given bridge from the global list of registered bridges, so
>> > - * it won't be found by users via of_drm_find_bridge(), and add it to the
>> > + * it won't be found by users via drm_of_find_bridge(), and add it to the
>> >    * lingering bridge list, to keep track of it until its allocated memory is
>> >    * eventually freed.
>> >    */
>> > @@ -1452,6 +1452,9 @@ EXPORT_SYMBOL(drm_of_find_bridge);
>> >    *
>> >    * @np: device node
>> >    *
>> > + * This function is deprecated. Use drm_of_find_bridge() instead for proper
>> > + * refcounting.
>> > + *
>>
>> I think it should be more explicit that the refcounting is not done by this
>> function, like:
>>
>> 	This function is deprecated. The returned bridge is not refcounted, you
>> should not use drm_bridge_put(). Use drm_of_find_bridge() instead for proper
>> refcounting.
>
> I'd rather say that the callers must take care of the refcounting by
> themselves but that it's racy, or at least that it *must* not use
> drm_bridge_put().

I was rather leaning towards "Deprecated. Convert to drm_of_get_bridge().".

But if we want to still support existing (lazy) users, with yours and
Louis' comments taken into account, what about:

 * This function is deprecated. Convert to of_drm_get_bridge() instead for
 * proper refcounting.
 *
 * The bridge returned by this function is not refcounted. This is
 * dangerous because the bridge might be deallocated even before the caller
 * has a chance to use it. To use this function you have to do one of:
 * - get a reference with drm_bridge_get() as soon as possible to
 *   minimize the race window, and then drm_bridge_put() when no longer
 *   using the pointer
 * - not call drm_bridge_get() or drm_bridge_put() at all, which used to
 *   be the correct practice before dynamic bridge lifetime was introduced
 * - again, convert to of_drm_get_bridge(), which is the only safe thing
 *   to do

Luca

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