Re: [PATCH RESEND v7 2/3] riscv: clocksource: Add readq options to clocksource mmio

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Using `readq_relaxed()` to read a live 64-bit clocksource counter is racy and unsafe on 32-bit systems, leading to torn reads and broken timekeeping.
--

commit 21048c9043069be78a033341f4f13739837c7833
Author: Aleksa Paunovic <aleksa.paunovic@htecgroup.com>

riscv: clocksource: Add readq options to clocksource mmio

This commit adds generic read functions for 64-bit memory-mapped
clocksources covering both up and down counters.

> diff --git a/drivers/clocksource/mmio.c b/drivers/clocksource/mmio.c
> index 9de7515318312..6329d8ce2c091 100644
> --- a/drivers/clocksource/mmio.c
> +++ b/drivers/clocksource/mmio.c
[ ... ]
> @@ -17,6 +17,20 @@ static inline struct clocksource_mmio *to_mmio_clksrc(struct clocksource *c)
>  	return container_of(c, struct clocksource_mmio, clksrc);
>  }
>  
> +#if defined(readq_relaxed)

Is it safe to use readq_relaxed() directly for a live 64-bit clocksource
counter on 32-bit systems?

readq_relaxed() is not guaranteed to be atomic on 32-bit platforms. For
example, 32-bit SuperH implements __raw_readq as *(volatile u64 *), which
results in two separate 32-bit memory accesses. Other 32-bit platforms
might define it via <linux/io-64-nonatomic-lo-hi.h>.

> +
> +u64 clocksource_mmio_readq_up(struct clocksource *c)
> +{
> +	return (u64)readq_relaxed(to_mmio_clksrc(c)->reg);

If the lower 32 bits of the live hardware counter roll over between the two
32-bit reads, will the resulting 64-bit value be severely corrupted (torn)?

To safely support 64-bit MMIO clocksources on 32-bit platforms, a software
retry loop is typically used. Otherwise, shouldn't the generic functions be
strictly guarded by #ifdef CONFIG_64BIT to prevent accidental usage on
32-bit systems, rather than just #if defined(readq_relaxed)?

> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260513-riscv-time-mmio-v7-0-f8925ffc226b@htecgroup.com?part=2