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

From: Lukasz Luba
Date: Wed Feb 12 2020 - 06:43:21 EST



On 2/12/20 11:10 AM, Marc Zyngier wrote:
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@xxxxxxx>

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@xxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Marc Zyngier <maz@xxxxxxxxxx>
---
 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)
ÂÂÂÂÂ return 0;
 }
 +static int validate_timer_rate(void)
+{
+ÂÂÂ if (!arch_timer_rate)
+ÂÂÂÂÂÂÂ return -EINVAL;
+
+ÂÂÂ /* Arch timer frequency < 1MHz can cause trouble */
+ÂÂÂ 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:

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

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.

Fair enough. As I said I don't know details of that code.



[...]

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?

What I was saying is that patch 6/7 has the code which checks the rate
and reacts, so it does not need this patch. In case of helping with
debugging, the patch 6/7 also prints error
"System timer frequency too low" and bails out.
The commit message could have better emphasize it.

Regards,
Lukasz