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

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Message-ID: <8e58b01b-772d-4ca7-a681-34f10baa07e6@arm.com>
Date: Thu, 14 Aug 2025 11:13:47 +0100
From: Steven Price <steven.price@....com>
To: Marc Zyngier <maz@...nel.org>, linux-kernel@...r.kernel.org,
 linux-acpi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc: Lorenzo Pieralisi <lpieralisi@...nel.org>,
 Hanjun Guo <guohanjun@...wei.com>, Sudeep Holla <sudeep.holla@....com>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Daniel Lezcano <daniel.lezcano@...aro.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 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:

 drivers/clocksource/arm_arch_timer_mmio.c: In function
'arch_timer_mmio_setup':
 ./include/linux/bits.h:47:9: error: conversion from 'long long unsigned
int' to 'long unsigned int' changes value from '72057594037927935' to
'4294967295' [-Werror=overflow]
    47 |         ((t)(GENMASK_INPUT_CHECK(h, l) +                        \
       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    48 |              (type_max(t) << (l) &                              \
       |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    49 |               type_max(t) >> (BITS_PER_TYPE(t) - 1 - (h)))))
       |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 ./include/linux/bits.h:52:33: note: in expansion of macro 'GENMASK_TYPE'
    52 | #define GENMASK_ULL(h, l)       GENMASK_TYPE(unsigned long
long, h, l)
       |                                 ^~~~~~~~~~~~
 ./include/linux/clocksource.h:153:32: note: in expansion of macro
'GENMASK_ULL'
   153 | #define CLOCKSOURCE_MASK(bits) GENMASK_ULL((bits) - 1, 0)
       |                                ^~~~~~~~~~~
 drivers/clocksource/arm_arch_timer_mmio.c:255:66: note: in expansion of
macro 'CLOCKSOURCE_MASK'
   255 |         clockevents_config_and_register(&at->evt, at->rate,
0xf, CLOCKSOURCE_MASK(56));
       |
 ^~~~~~~~~~~~~~~~

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.

Thanks,
Steve