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

[AngeloGioacchino Del Regno]

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