Re: [PATCH v4 1/4] kernel: param: initialize module_kset before do_initcalls()

[Shashank Balaji <shashank.mahadasyam@sony.com>]

Adding Tegra maintainers.

On Tue, Apr 28, 2026 at 12:10:50PM +0100, Gary Guo wrote:
> On Tue Apr 28, 2026 at 1:37 AM BST, Shashank Balaji wrote:
> > Hi Gary,
> >
> > On Mon, Apr 27, 2026 at 02:29:55PM +0100, Gary Guo wrote:
> >> On Mon Apr 27, 2026 at 3:41 AM BST, Shashank Balaji wrote:
> >> > module_kset is initialized in param_sysfs_init(), a subsys_initcall. A number
> >> > of platform drivers register themselves prior to subsys_initcalls
> >> > (tegra194_cbb_driver registers in a pure_initcall, for example). With an
> >> > upcoming patch ("driver core: platform: set mod_name in driver registration")
> >> > that sets their mod_name in struct device_driver, lookup_or_create_module_kobject()
> >> > will be called for those drivers, which calls kset_find_obj(module_kset, mod_name).
> >> > This causes a null deref because module_kset isn't alive yet.
> >> >
> >> > Fix this by initializing module_kset in do_basic_setup() before do_initcalls().
> >> > Modernize the pr_warn while we're at it.
> >> >
> >> > Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> >> > Suggested-by: Gary Guo <gary@garyguo.net>
> >> 
> >> I didn't suggest this change :)
> >> 
> >> I suggested `pure_initcall`, which is just a one line change.
> >
> > Oops, sorry about the misattribution.
> >
> >> diff --git a/kernel/params.c b/kernel/params.c
> >> index 74d620bc2521..ac088d4b09a9 100644
> >> --- a/kernel/params.c
> >> +++ b/kernel/params.c
> >> @@ -957,7 +957,7 @@ static int __init param_sysfs_init(void)
> >>  
> >>  	return 0;
> >>  }
> >> -subsys_initcall(param_sysfs_init);
> >> +pure_initcall(param_sysfs_init);
> >>  
> >>  /*
> >>   * param_sysfs_builtin_init - add sysfs version and parameter
> >> 
> >> pure_initcall is level 0 so it happens before all other init calls. Does it not
> >> work?
> >
> > tegra194_cbb_driver registers itself in a pure_initcall too. We wouldn't
> > want the ordering of its registration and module_kset init to be link order
> > dependent.
> 
> It's the only device driver that does this. And I don't think it's supposed to.
> 
> >From documentation:
> 
> > A "pure" initcall has no dependencies on anything else, and purely
> > initializes variables that couldn't be statically initialized.
> 
> I understand that given large amount of drivers registering themselves during
> core/arch_initcall that there might be regressions if all of them are moved, but
> surely we can demote these two specific tegra driver to core/postcore_initcall?
> This will still be called earlier than init_machine call which happens during
> arch_initcall.
> 
> Looks like the tegra CBB driver is just doing error logging anyway.

That's a good point, Gary. Thanks!

Hi Thierry and Jonathan,

You can find the context for this email in this patch:
https://lore.kernel.org/all/20260427-acpi_mod_name-v4-1-22b42240c9bf@sony.com/

TL;DR: tegra194_cbb_driver and tegra234_cbb_driver are the only drivers
registering themselves as early as in a pure_initcall. This is a problem
on two fronts:
1. Philosophical: As Gary pointed out, pure_initcalls are intended to purely
initialize variables that couldn't be statically initialized. But these
are doing driver registrations.
2. module_kset not initialized at pure_initcall stage: This is needed to
set the module sysfs symlink. Since module_kset is not alive yet during
pure_initcalls, registering these drivers panics the kernel.

We would like to do the tegra cbb driver registration in a core_initcall
(or some later initcall works too), and move module_kset initialization
to a pure_initcall. Like this:

diff --git a/drivers/soc/tegra/cbb/tegra194-cbb.c b/drivers/soc/tegra/cbb/tegra194-cbb.c
index ab75d50cc85c..2f69e104c838 100644
--- a/drivers/soc/tegra/cbb/tegra194-cbb.c
+++ b/drivers/soc/tegra/cbb/tegra194-cbb.c
@@ -2342,7 +2342,7 @@ static int __init tegra194_cbb_init(void)
 {
        return platform_driver_register(&tegra194_cbb_driver);
 }
-pure_initcall(tegra194_cbb_init);
+core_initcall(tegra194_cbb_init);

 static void __exit tegra194_cbb_exit(void)
 {
diff --git a/drivers/soc/tegra/cbb/tegra234-cbb.c b/drivers/soc/tegra/cbb/tegra234-cbb.c
index fb26f085f691..785072fa4e85 100644
--- a/drivers/soc/tegra/cbb/tegra234-cbb.c
+++ b/drivers/soc/tegra/cbb/tegra234-cbb.c
@@ -1774,7 +1774,7 @@ static int __init tegra234_cbb_init(void)
 {
        return platform_driver_register(&tegra234_cbb_driver);
 }
-pure_initcall(tegra234_cbb_init);
+core_initcall(tegra234_cbb_init);

 static void __exit tegra234_cbb_exit(void)
 {

Would this work?

Thanks,
Shashank