Message-ID: <26c100f2-bf31-7212-5d13-25b6b14b8f61@collabora.com>
Date:   Thu, 16 Feb 2023 12:36:21 +0100
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     Matthias Brugger <matthias.bgg@...il.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

Il 16/02/23 12:23, Matthias Brugger ha scritto:
> 
> 
> 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

Right. I completely forgot about those platforms, even though my intention was
to actually try and migrate them once the CPUX was picked. My bad.

Well, I think that this module conversion will take quite a while, so there
should be no need to rush... I'll send a series later with the split *and* a
conversion of those platforms, so that we will see a removal of that
mediatek_timer_init() function.

Some encouraging words ahead: I'm totally confident that the conversion will
Just Work, because the MT6795 SoC had the same handling for CPUXGPT as the
older MT6589/7623/8127/8135... as that SoC had two implementations initially,
one as ARM, one as ARM64.

Whatever - let's see what I can come up with, then.

Cheers,
Angelo

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