On 14/08/2025 12:14, Marc Zyngier wrote:
> On Thu, 14 Aug 2025 11:49:26 +0100,
> Marc Zyngier <maz@xxxxxxxxxx> wrote:
>>
>> On Thu, 14 Aug 2025 11:13:47 +0100,
>> Steven Price <steven.price@xxxxxxx> 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@xxxxxxxxxx>
>>>> ---
>>>> 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@xxxxxxxxxxxxxxxx.
>>
>>> 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
