linux-kernel - Re: [PATCH 2/4] clocksource/drivers/arm_arch

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Message-ID: <86ldnmdvpl.wl-maz@kernel.org>
Date: Thu, 14 Aug 2025 11:49:26 +0100
From: Marc Zyngier <maz@...nel.org>
To: 	Daniel Lezcano <daniel.lezcano@...aro.org>, Steven Price <steven.price@....com>
Cc: linux-kernel@...r.kernel.org,
	linux-acpi@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Hanjun Guo <guohanjun@...wei.com>,
	Sudeep Holla <sudeep.holla@....com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 2/4] clocksource/drivers/arm_arch_timer: Add standalone MMIO driver

On Thu, 14 Aug 2025 11:13:47 +0100,
Steven Price <steven.price@....com> wrote:
> 
> On 07/08/2025 17:02, Marc Zyngier wrote:
> > Add a new driver for the MMIO side of the ARM architected timer.
> > Most of it has been lifted from the existing arch timer code,
> > massaged, and finally rewritten.
> > 
> > It supports both DT and ACPI as firmware descriptions.
> > 
> > Signed-off-by: Marc Zyngier <maz@...nel.org>
> > ---
> >  MAINTAINERS                               |   1 +
> >  drivers/clocksource/arm_arch_timer_mmio.c | 420 ++++++++++++++++++++++
> >  2 files changed, 421 insertions(+)
> >  create mode 100644 drivers/clocksource/arm_arch_timer_mmio.c
> > 
> [...]
> > +static void arch_timer_mmio_setup(struct arch_timer *at, int irq)
> > +{
> > +	at->evt = (struct clock_event_device) {
> > +		.features		   = (CLOCK_EVT_FEAT_ONESHOT |
> > +					      CLOCK_EVT_FEAT_DYNIRQ),
> > +		.name			   = "arch_mem_timer",
> > +		.rating			   = 400,
> > +		.cpumask		   = cpu_possible_mask,
> > +		.irq 			   = irq,
> > +		.set_next_event		   = arch_timer_mmio_set_next_event,
> > +		.set_state_oneshot_stopped = arch_timer_mmio_shutdown,
> > +		.set_state_shutdown	   = arch_timer_mmio_shutdown,
> > +	};
> > +
> > +	at->evt.set_state_shutdown(&at->evt);
> > +
> > +	clockevents_config_and_register(&at->evt, at->rate, 0xf, CLOCKSOURCE_MASK(56));
> 
> This doesn't work on 32 bit - clockevents_config_and_register()'s final
> argument is an unsigned long, and a 56 bit mask doesn't fit. This
> triggers a compiler warning:

Already reported, see 20250814111657.7debc9f1@...b.auug.org.au.

> Possible this should really be min(CLOCKSOURCE_MASK(56), ULONG_MAX)? But
> I'm not familiar enough with this code. Most likely it's dead code on a
> 32 bit platform.

No, this definitely exists on 32bit crap, since it has been part of
the architecture from the ARMv7+VE days.

I think this is more of an impedance mismatch between the
CLOCKSOURCE_MASK() helper and the clockevents_config_and_register(),
and a (unsigned long) cast would do the trick.

But it also means that the per-cpu timer also gets truncated the same
way, and that has interesting impacts on how often the timer is
reprogrammed.

Daniel, do you want a patch on top or a new series?

	M.

-- 
Without deviation from the norm, progress is not possible.