Re: [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate arch_timer_rate

[Lukasz Luba <lukasz.luba@arm.com>]

On 2/12/20 10:12 AM, Marc Zyngier wrote:
> On 2020-02-12 10:01, Lukasz Luba wrote:
>> Hi Ionela, Valentin
>>
>> On 2/11/20 6:45 PM, Ionela Voinescu wrote:
>>> From: Valentin Schneider <valentin.schneider@arm.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@arm.com>
>>> Cc: Mark Rutland <mark.rutland@arm.com>
>>> Cc: Marc Zyngier <maz@kernel.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...
> 
> Because, as the commit message outlines it, such a frequency is terribly
> out of spec?

I don't see in the RM that < 1MHz is terribly out of spec.
'Frequency
Increments at a fixed frequency, typically in the range 1-50MHz.
Can support one or more alternative operating modes in which it 
increments by larger amounts at a
lower frequency, typically for power-saving.'

There is even an example how to operate at 20kHz and increment by 500.

I don't know the code if it's supported, thought.

> 
>>
>>> +
>>> +    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.
> 
> And? It seems to address a potential issue where the time frequency
> is out of spec, and makes sure we don't end up with additional problems
> in the AMU code.

This patch just prints warning, does not change anything in booting or
in any code related to AMU.

> 
> On its own, it is perfectly sensible and could be merged as part of this
> series with my
> 
> Acked-by: Marc Zyngier <maz@kernel.org>
> 
>          M.

Regards,
Lukasz