Re: [PATCH v3] drm/bridge: Fix refcount shown via debugfs for encoder_bridges_show()

[Luca Ceresoli]

On Wed Mar 18, 2026 at 6:26 AM CET, Liu Ying wrote:
> A typical bridge refcount value is 3 after a bridge chain is formed:
> - devm_drm_bridge_alloc() initializes the refcount value to be 1.
> - drm_bridge_add() gets an additional reference hence 2.
> - drm_bridge_attach() gets the third reference hence 3.
>
> This typical refcount value aligns with allbridges_show()'s behaviour.
> However, since encoder_bridges_show() uses
> drm_for_each_bridge_in_chain_scoped() to automatically get/put the
> bridge reference while iterating, a bogus reference is accidentally
> got when showing the wrong typical refcount value as 4 to users via
> debugfs.  Fix this by caching the refcount value returned from
> kref_read() while iterating and explicitly decreasing the cached
> refcount value by 1 before showing it to users.
>
> Fixes: bd57048e4576 ("drm/bridge: use drm_for_each_bridge_in_chain_scoped()")
> Signed-off-by: Liu Ying <victor.liu@xxxxxxx>

Reviewed-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
Tested-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>

Thanks!

Luca

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