linux-kernel - Re: [PATCH] arm64: arch_timer: XGene-1 has 31 bit, not 32 bit, arch timer.

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Date:   Fri, 21 Oct 2022 19:08:50 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Joe Korty <joe.korty@...current-rt.com>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] arm64: arch_timer: XGene-1 has 31 bit, not 32 bit, arch timer.

On Fri, 21 Oct 2022 16:34:24 +0100,
Joe Korty <joe.korty@...current-rt.com> wrote:
> 
> arm64: XGene-1 has a 31 bit, not a 32 bit, arch timer.
> 
> Fixes: 012f188504528b8cb32f441ac3bd9ea2eba39c9e ("clocksource/drivers/arm_arch_timer:
>   Work around broken CVAL implementations")

Sorry, but you'll have to provide a bit more of an analysis here. As
far as I can tell, you're just changing a parameter without properly
describing what breaks and how.

>
> Testing:
>   On an 8-cpu Mustang, the following sequence no longer locks up the system:
> 
>      echo 0 >/proc/sys/kernel/watchdog
>      for i in {0..7}; do taskset -c $i echo hi there $i; done
> 
> Stable:
>   To be applied to 5.16 and above, once accepted by mainline.
> 
> Signed-off-by: Joe Korty <joe.korty@...current-rt.com>
> 
> Index: b/drivers/clocksource/arm_arch_timer.c
> ===================================================================
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -805,7 +805,7 @@ static u64 __arch_timer_check_delta(void
>  	const struct midr_range broken_cval_midrs[] = {
>  		/*
>  		 * XGene-1 implements CVAL in terms of TVAL, meaning
> -		 * that the maximum timer range is 32bit. Shame on them.
> +		 * that the maximum timer range is 31bit. Shame on them.
>  		 */
>  		MIDR_ALL_VERSIONS(MIDR_CPU_MODEL(ARM_CPU_IMP_APM,
>  						 APM_CPU_PART_POTENZA)),
> @@ -813,8 +813,8 @@ static u64 __arch_timer_check_delta(void
>  	};
>  
>  	if (is_midr_in_range_list(read_cpuid_id(), broken_cval_midrs)) {
> -		pr_warn_once("Broken CNTx_CVAL_EL1, limiting width to 32bits");
> -		return CLOCKSOURCE_MASK(32);
> +		pr_warn_once("Broken CNTx_CVAL_EL1, limiting width to 31bits");
> +		return CLOCKSOURCE_MASK(31);
>  	}
>  #endif
>  	return CLOCKSOURCE_MASK(arch_counter_get_width());
> 

Also, this isn't much of a patch. Please see the documentation on how
to properly submit one.

Thanks,

	M.

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