linux-kernel - Re: [PATCH v2 4/4] clocksource/drivers/timer-mediatek: Make timer-mediatek become loadable module

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Message-ID: <96d6f405-22bc-f112-2c88-76da280eed46@gmail.com>
Date:   Thu, 16 Feb 2023 12:23:06 +0100
From:   Matthias Brugger <matthias.bgg@...il.com>
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Sudeep Holla <sudeep.holla@....com>
Cc:     walter.chang@...iatek.com,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Maciej W . Rozycki" <macro@...am.me.uk>,
        John Stultz <jstultz@...gle.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        wsd_upstream@...iatek.com, stanley.chu@...iatek.com,
        Chun-hung.Wu@...iatek.com, Freddy.Hsin@...iatek.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v2 4/4] clocksource/drivers/timer-mediatek: Make
 timer-mediatek become loadable module



On 16/02/2023 11:22, AngeloGioacchino Del Regno wrote:
> Il 15/02/23 15:46, Sudeep Holla ha scritto:
>> On Wed, Feb 15, 2023 at 02:30:51PM +0100, AngeloGioacchino Del Regno wrote:
>>>
>>> Both. I mean that these platforms do have architected timers, but they are 
>>> stopped
>>> before the bootloader jumps to the kernel, or they are never started at all.
>>>
>>> Please refer to:
>>>
>>> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/drivers/clocksource/timer-mediatek.c?h=next-20230215&id=327e93cf9a59b0d04eb3a31a7fdbf0f11cf13ecb
>>>
>>> For a nice explanation.
>>>
>>
>> Thanks for that. Well then I see no point in making these modules if you
>> can't have generic Image that boots on all the platform. I now tend to think
>> that these are made modules just because GKI demands and it *might* work
>> on one or 2 platforms. One we move this as modules, how will be know the
>> Image without these timers or with them built as modules will boot or not
>> on a given mediatek platform. Sorry, I initially saw some point in making
>> these timers as modules but if they are required for boot on some systems
>> then I see no point. So if that is the case, NACK for these as it just
>> creates more confusion after these are merged as why some Images or
>> even why defconfig image(if we push the config change as well) is not
>> booting on these platforms.
>>
>> It is no longer just for system timer useful in low power CPU idle states
>> as I initial thought.
>>
> 
> I think that there is still a point in modularization for this driver and I
> can propose a rather simple solution, even though this may add some, rather
> little, code duplication to the mix.
> 
> The platforms that I've described (like mt6795) need the system timer to be
> initialized as early as possible - that's true - but that timer is always
> "CPUXGPT".
> 
> On those platforms, you *still* have multiple timers:
>   - CPUX (short for cpuxgpt), used only as system timer;
>   - SYST, as another system timer implementation (additional timers) but
>     those are always initialized (AFAIK) from the bootloader before booting;
>   - GPT (General Purpose Timer).
> 
> On one SoC, you may have:
>   - CPUX *and* SYST
>   - CPUX *and* GPT
>   - CPUX *and* SYST *and* GPT
> 
> ... where the only one that is boot critical and needs to be initialized early
> is always only CPUX.
> 
> Hence this proposal: to still allow modularization of timers on MediaTek platforms,
> we could eventually split the CPUX as a separated driver that *cannot be*, due to
> the previously explained constraints, compiled as module, hence always built-in,
> from a timer-mediatek driver that could be a module and capable of handling only
> SYST and GPT timers.
> 
> In that case, we'd hence have...
>   - timer-mediatek-cpux.o (bool)
>   - timer-mediatek.c (tristate)
> 
> Counting that the CPUX timers are actually even using different `tick_resume`
> and `set_state_shutdown` callbacks (doing only a IRQ clear/restore and nothing
> else), the amount of duplication would be .. well, again, minimal, but then
> this means that timer-mediatek-cpux would be `default y if ARCH_MEDIATEK`, or
> even selected by ARCH_MEDIATEK itself.
> 
> If you think that this could be a good solution, I can send a "fast" patch to
> split it out, preparing the ground for the people doing this module work.
> 
> Any considerations?
> 

I think your proposal sounds acceptable, but we would need to make sure that all 
SoCs can boot with the CPUX driver. I'm aware of some armv7 SoCs that use a kind 
of hack to enable the architecture timer [1]. This, for one part, should be 
moved to CPUX, if possible. For the other part it makes me wonder if really all 
supported MediaTek platforms will boot with SYST/GPT being a module. I think we 
will need some effort from the community to test that.

So as a resume, yes I think your approach is feasible but we should collect 
tested-by tags before merging it.

Regards,
Matthias


[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/mach-mediatek/mediatek.c?h=v6.2-rc8#n16