Re: [PATCH 2/4] clocksource/drivers/arm_arch_timer: Add standalone MMIO driver

[Marc Zyngier <maz@kernel.org>]

On Thu, 14 Aug 2025 11:49:26 +0100,
Marc Zyngier <maz@kernel.org> wrote:
> 
> On Thu, 14 Aug 2025 11:13:47 +0100,
> Steven Price <steven.price@arm.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@kernel.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@canb.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.

Of course not. That would just result in a big fat zero.

> 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.

That question still stand, and I wonder whether we have ugly bugs
lurking on 32bit platforms because of that... I'll try and have a
look.

	M.

-- 
Jazz isn't dead. It just smells funny.