Re: [PATCH v2 6/7] clocksource/drivers/exynos_mct: Add module support

[William McVicker <willmcvicker@google.com>]

On 05/13/2025, Daniel Lezcano wrote:
> On Tue, Apr 15, 2025 at 05:48:41PM -0700, John Stultz wrote:
> > On Tue, Apr 15, 2025 at 9:50 AM Daniel Lezcano
> > <daniel.lezcano@linaro.org> wrote:
> > > On Wed, Apr 02, 2025 at 04:33:57PM -0700, Will McVicker wrote:
> > > > From: Donghoon Yu <hoony.yu@samsung.com>
> > > >
> > > > On Arm64 platforms the Exynos MCT driver can be built as a module. On
> > > > boot (and even after boot) the arch_timer is used as the clocksource and
> > > > tick timer. Once the MCT driver is loaded, it can be used as the wakeup
> > > > source for the arch_timer.
> > >
> > > From a previous thread where there is no answer:
> > >
> > > https://lore.kernel.org/all/c1e8abec-680c-451d-b5df-f687291aa413@linaro.org/
> > >
> > > I don't feel comfortable with changing the clocksource / clockevent drivers to
> > > a module for the reasons explained in the aforementionned thread.
> > 
> > I wasn't CC'ed on that, but to address a few of your points:
> > 
> > > I have some concerns about this kind of changes:
> > >
> > >   * the core code may not be prepared for that, so loading / unloading
> > > the modules with active timers may result into some issues
> > 
> > That's a fair concern, but permanent modules (which are loaded but not
> > unloaded) shouldn't suffer this issue. I recognize having modules be
> > fully unloadable is generally cleaner and preferred, but I also see
> > the benefit of allowing permanent modules to be one-way loaded so a
> > generic/distro kernel shared between lots of different platforms
> > doesn't need to be bloated with drivers that aren't used everywhere.
> > Obviously any single driver doesn't make a huge difference, but all
> > the small drivers together does add up.
> 
> Perhaps using module_platform_driver_probe() should do the trick with
> some scripts updated for my git hooks to check
> module_platform_driver() is not used.

Using `module_platform_driver_probe()` won't work as that still defines
a `module_exit()` hook. If you want to automatically handle this in code, then
the best approach is to follow what Saravana did in [1] for irqchip drivers.
Basically by using `builtin_platform_driver(drv_name##_driver)`, you will only
define the `module_init()` hook when the driver is compiled as a module which
ensures you always get a permanent module.

[1] https://lore.kernel.org/linux-arm-kernel/20200718000637.3632841-1-saravanak@google.com/

Regards,
Will

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