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

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Message-ID: <86jz36dqh1.wl-maz@kernel.org>
Date: Thu, 14 Aug 2025 13:42:34 +0100
From: Marc Zyngier <maz@...nel.org>
To: Steven Price <steven.price@....com>
Cc: Daniel Lezcano <daniel.lezcano@...aro.org>,
	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 13:23:26 +0100,
Steven Price <steven.price@....com> wrote:
> 
> 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.

Huh. yes, Can't think today. Sorry for the noise.

> 
> >> 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.

It really isn't about the warning, but how we express the maximum
deadline to the core infrastructure. The MMIO driver is the least of
our worries, and the CPU timer is the thing I'm concerned about, as it
uses the same construct.

	M.

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