linux-kernel - Re: [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate arch_timer

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Date:   Wed, 12 Feb 2020 10:01:54 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Ionela Voinescu <ionela.voinescu@....com>, catalin.marinas@....com,
        will@...nel.org, mark.rutland@....com, maz@...nel.org,
        suzuki.poulose@....com, sudeep.holla@....com,
        valentin.schneider@....com, rjw@...ysocki.net
Cc:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
        viresh.kumar@...aro.org, linux-arm-kernel@...ts.infradead.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate
 arch_timer_rate

Hi Ionela, Valentin

On 2/11/20 6:45 PM, Ionela Voinescu wrote:
> From: Valentin Schneider <valentin.schneider@....com>
> 
> Using an arch timer with a frequency of less than 1MHz can result in an
> incorrect functionality of the system which assumes a reasonable rate.
> 
> One example is the use of activity monitors for frequency invariance
> which uses the rate of the arch timer as the known rate of the constant
> cycle counter in computing its ratio compared to the maximum frequency
> of a CPU. For arch timer frequencies less than 1MHz this ratio could
> end up being 0 which is an invalid value for its use.
> 
> Therefore, warn if the arch timer rate is below 1MHz which contravenes
> the recommended architecture interval of 1 to 50MHz.
> 
> Signed-off-by: Ionela Voinescu <ionela.voinescu@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Marc Zyngier <maz@...nel.org>
> ---
>   drivers/clocksource/arm_arch_timer.c | 18 +++++++++++++++---
>   1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index 9a5464c625b4..4faa930eabf8 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -885,6 +885,17 @@ static int arch_timer_starting_cpu(unsigned int cpu)
>   	return 0;
>   }
>   
> +static int validate_timer_rate(void)
> +{
> +	if (!arch_timer_rate)
> +		return -EINVAL;
> +
> +	/* Arch timer frequency < 1MHz can cause trouble */
> +	WARN_ON(arch_timer_rate < 1000000);

I don't see a big value of having a patch just to add one extra warning,
in a situation which we handle in our code with in 6/7 with:

+	if (!ratio) {
+		pr_err("System timer frequency too low.\n");
+		return -EINVAL;
+	}

Furthermore, the value '100000' here is because of our code and
calculation in there, so it does not belong to arch timer. Someone
might ask why it's not 200000 or a define in our header...
Or questions asking why do you warn when that arch timer and cpu is not
AMU capable...

> +
> +	return 0;
> +}
> +
>   /*
>    * For historical reasons, when probing with DT we use whichever (non-zero)
>    * rate was probed first, and don't verify that others match. If the first node
> @@ -900,7 +911,7 @@ static void arch_timer_of_configure_rate(u32 rate, struct device_node *np)
>   		arch_timer_rate = rate;
>   
>   	/* Check the timer frequency. */
> -	if (arch_timer_rate == 0)
> +	if (validate_timer_rate())
>   		pr_warn("frequency not available\n");
>   }
>   
> @@ -1594,9 +1605,10 @@ static int __init arch_timer_acpi_init(struct acpi_table_header *table)
>   	 * CNTFRQ value. This *must* be correct.
>   	 */
>   	arch_timer_rate = arch_timer_get_cntfrq();
> -	if (!arch_timer_rate) {
> +	ret = validate_timer_rate();
> +	if (ret) {
>   		pr_err(FW_BUG "frequency not available.\n");
> -		return -EINVAL;
> +		return ret;
>   	}
>   
>   	arch_timer_uses_ppi = arch_timer_select_ppi();
> 

Lastly, this is arch timer.
To increase chances of getting merge soon, I would recommend to drop
the patch from this series.

Regards,
Lukasz