lkml.org 
[lkml]   [2020]   [Feb]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate arch_timer_rate
Hi Thomas,

On Friday 14 Feb 2020 at 01:35:58 (+0100), Thomas Gleixner wrote:
> Ionela Voinescu <ionela.voinescu@arm.com> writes:
>
> > 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>
>
> So this patch is from Valentin. Where is his Signed-off-by?
>

Yes, sorry about this. This was based on a diff that Valentin provided
in v2. I'll change the author as agreed at:
https://lore.kernel.org/lkml/20200212103249.GA19041@arm.com/

> >
> > +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);
>
> This does not make sense to me. If the rate is out of bounds then why
> warn an just continue instead of making it fail?
>

Because it's not a hard restriction, it's just atypical for the rate to
be below 1Mhz. The spec only mentions a typical range of 1 to 50MHz and
the warning is only here to flag a potentially problematic rate, below
what is assumed typical in the spec.

In [1], where I'm actually relying on arch_timer_rate being higher than
than 1/SCHED_CAPACITY_SCALE² of the maximum frequency, I am making it
fail, as, for that scenario, it is a hard restriction.


+ * We use a factor of 2 * SCHED_CAPACITY_SHIFT -> SCHED_CAPACITY_SCALE²
+ * in order to ensure a good resolution for arch_max_freq_scale for
+ * very low arch timer frequencies (up to the KHz range which should be
+ * unlikely).
+ */
+ ratio = (u64)arch_timer_get_rate() << (2 * SCHED_CAPACITY_SHIFT);
+ ratio = div64_u64(ratio, max_freq_hz);
+ if (!ratio) {
+ pr_err("System timer frequency too low.\n");
+ return -EINVAL;
+ }
+

[1] https://lore.kernel.org/lkml/89339501-5ee4-e871-3076-c8b02c6fbf6e@arm.com/

Thanks,
Ionela.

> Thanks,
>
> tglx

\
 
 \ /
  Last update: 2020-02-14 16:46    [W:0.127 / U:2.548 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site