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

Daniel Lezcano
2025-05-23T15:17:39Z

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.

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

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