linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Marc Zyngier <maz@kernel.org>
To: Lukasz Luba <lukasz.luba@arm.com>
Cc: Ionela Voinescu <ionela.voinescu@arm.com>,
	catalin.marinas@arm.com, will@kernel.org, mark.rutland@arm.com,
	suzuki.poulose@arm.com, sudeep.holla@arm.com,
	valentin.schneider@arm.com, rjw@rjwysocki.net,
	peterz@infradead.org, mingo@redhat.com,
	vincent.guittot@linaro.org, viresh.kumar@linaro.org,
	linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org
Subject: Re: [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate arch_timer_rate
Date: Wed, 12 Feb 2020 11:10:51 +0000	[thread overview]
Message-ID: <f01a6384e7297de87a434e83bd1479d8@kernel.org> (raw)
In-Reply-To: <289c6110-b7ea-1d61-d795-551723263803@arm.com>

On 2020-02-12 10:55, Lukasz Luba wrote:
> 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.'

Hint: constant apparent frequency.

> 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.

You're completely missing the point, I'm afraid. Nobody has to know that
this is happening. For all intent and purposes, the counter has always
the same frequency, even if the HW does fewer ticks of larger 
increments.


[...]

>>> 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.

It seems to solve an issue with an assumption made in the AMU driver,
and would help debugging the problem on broken systems. Are you saying
that this is not the case and that the AMU code can perfectly cope with
the frequency being less than 1MHz?

         M.
-- 
Jazz is not dead. It just smells funny...

  reply	other threads:[~2020-02-12 11:10 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-11 18:45 [PATCH v3 0/7] arm64: ARMv8.4 Activity Monitors support Ionela Voinescu
2020-02-11 18:45 ` [PATCH v3 1/7] arm64: add support for the AMU extension v1 Ionela Voinescu
2020-02-12 11:30   ` Suzuki Kuruppassery Poulose
2020-02-12 14:54     ` Valentin Schneider
2020-02-12 16:10     ` Ionela Voinescu
2020-02-12 16:20       ` Suzuki Kuruppassery Poulose
2020-02-12 18:20         ` Ionela Voinescu
2020-02-12 19:24           ` Suzuki K Poulose
2020-02-12 20:19         ` Ionela Voinescu
2020-02-12 16:24       ` Vladimir Murzin
2020-02-12 18:27         ` Ionela Voinescu
2020-02-11 18:45 ` [PATCH v3 2/7] arm64: trap to EL1 accesses to AMU counters from EL0 Ionela Voinescu
2020-02-12 11:44   ` Suzuki Kuruppassery Poulose
2020-02-12 15:36   ` Valentin Schneider
2020-02-11 18:45 ` [PATCH v3 3/7] arm64/kvm: disable access to AMU registers from kvm guests Ionela Voinescu
2020-02-12 15:36   ` Valentin Schneider
2020-02-12 16:33   ` Suzuki Kuruppassery Poulose
2020-02-11 18:45 ` [PATCH v3 4/7] Documentation: arm64: document support for the AMU extension Ionela Voinescu
2020-02-12 15:36   ` Valentin Schneider
2020-02-11 18:45 ` [PATCH v3 5/7] cpufreq: add function to get the hardware max frequency Ionela Voinescu
2020-02-12  4:14   ` Viresh Kumar
2020-02-13 11:59   ` Valentin Schneider
2020-02-13 12:59     ` Ionela Voinescu
2020-02-13 15:22       ` Valentin Schneider
2020-02-11 18:45 ` [PATCH v3 6/7] arm64: use activity monitors for frequency invariance Ionela Voinescu
2020-02-12 18:59   ` Lukasz Luba
2020-02-13  9:47     ` Ionela Voinescu
2020-02-17 16:59   ` Valentin Schneider
2020-02-23 18:49     ` Ionela Voinescu
2020-02-11 18:45 ` [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate arch_timer_rate Ionela Voinescu
2020-02-12  9:30   ` Valentin Schneider
2020-02-12 10:32     ` Ionela Voinescu
2020-02-12 10:01   ` Lukasz Luba
2020-02-12 10:12     ` Marc Zyngier
2020-02-12 10:54       ` Ionela Voinescu
2020-02-12 10:55       ` Lukasz Luba
2020-02-12 11:10         ` Marc Zyngier [this message]
2020-02-12 11:43           ` Lukasz Luba
2020-02-12 11:12         ` Valentin Schneider
2020-02-14  0:35   ` Thomas Gleixner
2020-02-14 15:45     ` Ionela Voinescu
2020-02-14 15:57       ` Ionela Voinescu

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=f01a6384e7297de87a434e83bd1479d8@kernel.org \
    --to=maz@kernel.org \
    --cc=catalin.marinas@arm.com \
    --cc=ionela.voinescu@arm.com \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-pm@vger.kernel.org \
    --cc=lukasz.luba@arm.com \
    --cc=mark.rutland@arm.com \
    --cc=mingo@redhat.com \
    --cc=peterz@infradead.org \
    --cc=rjw@rjwysocki.net \
    --cc=sudeep.holla@arm.com \
    --cc=suzuki.poulose@arm.com \
    --cc=valentin.schneider@arm.com \
    --cc=vincent.guittot@linaro.org \
    --cc=viresh.kumar@linaro.org \
    --cc=will@kernel.org \
    --subject='Re: [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate arch_timer_rate' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).