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

[William McVicker <willmcvicker@google.com>]

On 05/23/2025, Daniel Lezcano wrote:
> 
> Hi William,
> 
> On 15/05/2025 01:16, William McVicker wrote:
> > 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/
> 
> Thanks for the pointer and the heads up regarding the module_exit() problem
> with module_platform_driver_probe().
> 
> After digging into the timekeeping framework it appears if the owner of the
> clockevent device is set to THIS_MODULE, then the framework automatically
> grabs a reference preventing unloading the module when this one is
> registered.
> 
> IMO it was not heavily tested but for me it is enough to go forward with the
> module direction regarding the drivers.

Great! Thanks for looking into that. I'll add that in the next revision and
verify we can't unload the module.

> 
> One point though, the condition:
> 
> +#ifdef MODULE
> [ ... ]
> +static const struct of_device_id exynos4_mct_match_table[] = {
> +	{ .compatible = "samsung,exynos4210-mct", .data = &mct_init_spi, },
> +	{ .compatible = "samsung,exynos4412-mct", .data = &mct_init_ppi, },
> +	{}
> +};
> +MODULE_DEVICE_TABLE(of, exynos4_mct_match_table);
> +
> +static struct platform_driver exynos4_mct_driver = {
> +	.probe		= exynos4_mct_probe,
> +	.driver		= {
> +		.name	= "exynos-mct",
> +		.of_match_table = exynos4_mct_match_table,
> +	},
> +module_platform_driver(exynos4_mct_driver);
> +#else
>  TIMER_OF_DECLARE(exynos4210, "samsung,exynos4210-mct", mct_init_spi);
>  TIMER_OF_DECLARE(exynos4412, "samsung,exynos4412-mct", mct_init_ppi);
> +#endif
> 
>  is not acceptable as is. We don't want to do the same in all the drivers.

Are you suggesting we create a new timer macro to handle if we want to use
TIMER_OF_DECLARE() or builtin_platform_driver()?

> Furthermore, we should not assume if the modules are enabled in the kernel
> that implies the driver is compiled as a module.

Ah yes, that's right. The ifdef should be checking
CONFIG_CLKSRC_EXYNOS_MCT_MODULE.

Thanks,
Will