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

On 05/23/2025, Daniel Lezcano wrote:

Hi William,

On 15/05/2025 01:16, William McVicker wrote:

quoted

On 05/13/2025, Daniel Lezcano wrote:

quoted

On Tue, Apr 15, 2025 at 05:48:41PM -0700, John Stultz wrote:

quoted

On Tue, Apr 15, 2025 at 9:50 AM Daniel Lezcano
[off-list ref] wrote:

quoted

On Wed, Apr 02, 2025 at 04:33:57PM -0700, Will McVicker wrote:

quoted

From: Donghoon Yu <redacted>

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/ (local)

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:

quoted

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/ (local)

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