[PATCH v2] clocksource/arm_arch_timer: Fix masking for high freq counters

[Linus Walleij]

On Sat, Aug 7, 2021 at 9:14 PM Oliver Upton <oupton at google.com> wrote:

> Unfortunately, the architecture provides no means to determine the bit
> width of the system counter. However, we do know the following from the
> specification:
>
>  - the system counter is at least 56 bits wide
>  - Roll-over time of not less than 40 years
>
> To date, the arch timer driver has depended on the first property,
> assuming any system counter to be 56 bits wide and masking off the rest.
> However, combining a narrow clocksource mask with a high frequency
> counter could result in prematurely wrapping the system counter by a
> significant margin. For example, a 56 bit wide, 1GHz system counter
> would wrap in a mere 2.28 years!
>
> This is a problem for two reasons: v8.6+ implementations are required to
> provide a 64 bit, 1GHz system counter. Furthermore, before v8.6,
> implementers may select a counter frequency of their choosing.
>
> Fix the issue by deriving a valid clock mask based on the second
> property from above. Set the floor at 56 bits, since we know no system
> counter is narrower than that.
>
> Suggested-by: Marc Zyngier <maz at kernel.org>
> Signed-off-by: Oliver Upton <oupton at google.com>

This patch looks good to me:
Reviewed-by: Linus Walleij <linus.walleij at linaro.org>

Just a thought that crossed my mind: as this is real hardware we are
talking about mostly, how hard would it be for arch_counter_get_width()
to detect how wide it actually is if nbits > 56?

I would do something like this pseudocode:

nbits = 56;
while (nbits < 64)
    startval = GENMASK(nbits, 0);
    write_counter(startval);
    start_counter;
    nsleep(1);
    stop_counter;
    now = read_counter;
    if (now < startval)
         /* Ooops it wrapped */
         break;
    nbits++

pr_info("counter has %d bits\n", nbits);

Or did you folks already try this approach?

Yours,
Linus Walleij