Re: [PATCH] clk: fix self-consuming provider module pinning

[Brian Masney <bmasney@redhat.com>]

Hi Jerome,

On Wed, Jul 22, 2026 at 09:49:33AM +0200, Jerome Brunet wrote:
> On mar. 21 juil. 2026 at 18:48, Brian Masney <bmasney@redhat.com> wrote:
> > On Tue, Jul 21, 2026 at 12:08:17PM +0200, Jerome Brunet wrote:
> >> clk_hw_get_clk() lets a provider get a struct clk for one of its own
> >> struct clk_hw.
> >> 
> >> When a struct clk is created, the module usage count of the provider
> >> is unconditionally increased. For a self-consuming provider, this means
> >> it pins itself and the module can never be unloaded.
> >> 
> >> Increasing the module usage count should only be done when the consumer
> >> lives in a different module from the provider. Use THIS_MODULE to
> >> capture caller's module and increase the module usage count accordingly.
> >> 
> >> It is OK for consumer-only APIs such as clk_get() or of_clk_get() to
> >> pass a NULL owner. As a result, any provider module will get pinned,
> >> same as before.
> >> 
> >> Fixes: 30d6f8c15d2c ("clk: add api to get clk consumer from clk_hw")
> >> Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
> >> ---
> >> This issue has been present for a while. Virtually all users of
> >> clk_hw_get_clk() are affected. The majority are compiled as builtins
> >> according to the defconfigs. It is not problem in this case but it is
> >> if the configuration is changed to module.
> >> 
> >> The following modules are using clk_hw_get_clk() and are compiled as
> >> module with some shipped defconfigs:
> >> * drivers/gpu/drm/msm/disp/mdp4/mdp4_lvds_pll.c
> >> * drivers/phy/cadence/phy-cadence-sierra.c
> >> * drivers/pwm/pwm-meson.c
> >> * sound/soc/codecs/lpass-va-macro.c
> >> 
> >> Currently those module cannot be unloaded once they have been loaded.
> >> 
> >> """
> >> rmmod: ERROR: Module blabla-module is in use
> >> """
> >> 
> >> I initially thought about using the dev parameter and compare it
> >> against the clock own device but this proved unreliable. A clock
> >> does not always have a backing device and some paths, such as
> >> clk_get_sys(), do not provide a device either.
> >> 
> >> With this applied, we can get back to removing the direct usage
> >> of the struct clk in struct clk_hw and eventually remove this
> >> struct member entirely.
> >
> > __clk_register() has this comment:
> >
> >         /*
> >          * Don't call clk_hw_create_clk() here because that would pin the
> >          * provider module to itself and prevent it from ever being removed.
> >          */
> >         hw->clk = alloc_clk(core, NULL, NULL);
> >
> > With your change, can this code path be updated to use clk_hw_create_clk() ?
> >
> 
> Indeed the comment no longer applies but I think keeping alloc
> here is best. It avoids "linking a consumer" here where there should not
> be one. All clocks would appear to have consumer in the clk summary, it
> would weird. 
> 
> The end goal is remove the hw->clk. I've restarted the work to rid the
> kernel of using this. Some rework don't need clk_hw_create_clk() and are
> not blocked by this patch but most will.
> 
> Once this is done, we could probably merge clk_hw_create_clk() and
> alloc(), the split won't have purpose anymore.

That all sounds good.

I know that Stephen likes to see any changes that are made to clk.c have
associated kunit tests to avoid any breakage in the clk core. There's
already a bunch of tests in clk_test.c.

Brian