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[34.83.5.33]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-233f9e17e3csm10816895ad.177.2025.05.23.10.06.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 May 2025 10:06:37 -0700 (PDT) Date: Fri, 23 May 2025 10:06:33 -0700 From: William McVicker To: Daniel Lezcano Cc: John Stultz , Catalin Marinas , Will Deacon , Peter Griffin , =?iso-8859-1?Q?Andr=E9?= Draszik , Tudor Ambarus , Rob Herring , Krzysztof Kozlowski , Conor Dooley , Alim Akhtar , Thomas Gleixner , Saravana Kannan , Krzysztof Kozlowski , Donghoon Yu , Hosung Kim , kernel-team@android.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Youngmin Nam , linux-samsung-soc@vger.kernel.org, devicetree@vger.kernel.org Subject: Re: [PATCH v2 6/7] clocksource/drivers/exynos_mct: Add module support Message-ID: References: <20250402233407.2452429-1-willmcvicker@google.com> <20250402233407.2452429-7-willmcvicker@google.com> <6e6b0f5f-ac60-48bb-af6c-fa58658d2639@linaro.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <6e6b0f5f-ac60-48bb-af6c-fa58658d2639@linaro.org> 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 > > > > wrote: > > > > > On Wed, Apr 02, 2025 at 04:33:57PM -0700, Will McVicker wrote: > > > > > > From: Donghoon Yu > > > > > > > > > > > > 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