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

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Message-ID: <d3bd4234-2053-45ce-aad4-6fc577665d72@arm.com>
Date: Thu, 14 Aug 2025 13:23:26 +0100
From: Steven Price <steven.price@....com>
To: Marc Zyngier <maz@...nel.org>, Daniel Lezcano <daniel.lezcano@...aro.org>
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 14/08/2025 12:14, Marc Zyngier wrote:
> On Thu, 14 Aug 2025 11:49:26 +0100,
> Marc Zyngier <maz@...nel.org> wrote:
>>
>> 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.
> 
> Of course not. That would just result in a big fat zero.

No - CLOCKSOURCE_MASK() turns into GENMASK_ULL so a simple truncation to
unsigned long would be ULONG_MAX (all 1s). So an unsigned long cast
should be fine. My suggestion of MIN(, ULONG_MAX) was just to make that
more clear.

>> 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.

I don't know whether there are other bugs due to the capping to
ULONG_MAX, but I don't think there's an (additional) bug here, it's
"just" a ugly warning.

Thanks,
Steve