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

[Luca Ceresoli]

Hello Liu, Maxime,

On Thu Mar 12, 2026 at 7:05 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.

Good point, indeed the refcount shown by
<debugfs>/dri/<card>/encoder-0/bridges is by one unit higher than the one
shown in <debugfs>/dri/bridges. I understand it's puzzling from a debugfs
user point of view.

As you noticed, this is because the _scoped loop holds an extra ref on the
current bridge.

For other reasons I proposed a mutex for stronger protection around the
bridge chain [v2]. With the mutex the extra ref is redundant, so in [v2]
the extra ref is removed, thus making your patch unneeded. However Maxime
asked to keep the extra ref, and so my latest iteration [v4] still has the
extra ref.

That series is still on the mailing list, we are still in time to rediscuss
it.

@Maxime: based on the issue Liu is trying to work around, do you think it
would make sense to go back to the initial approach for that series?
I.e. drm_for_each_bridge_in_chain_scoped() grabs the chain lock, which is a
superset of the per-bridge refcount, and thus the refcount can be dropped?
This would remove the debugfs issue, slightly simplify
drm_for_each_bridge_in_chain_scoped(), and introduce no new issues AFAIK.

[v2] https://lore.kernel.org/all/20251003-drm-bridge-alloc-encoder-chain-mutex-v2-4-78bf61580a06@xxxxxxxxxxx/
[v4] https://lore.kernel.org/all/20260113-drm-bridge-alloc-encoder-chain-mutex-v4-4-60f3135adc45@xxxxxxxxxxx/

Luca

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