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

On Mon, Aug 9, 2021 at 3:45 AM Marc Zyngier [off-list ref] wrote:

quoted

On that note, I wonder how (if ever) we will be able to move away from
unnecessarily masking a 64 bit counter, i.e. a v8.6 or above
implementation. With this patch, one such counter would wrap after
36.56 years, short of the 40 year guarantee we have from the
architecture for < v8.6 implementations. Getting it to 64 bits would
squarely make it someone else's problem ~585 years from now :)

Hmmm. If you end-up with something that falls short of 40 years, then
I suspect something is wrong in the way you compute the required
width.

40 years @1GHz (which we shall call FY1G from now on) fits comfortably
in 61 bits, and I fear that your use of ilog2() gives you one less bit
than what it should be:

log2(FY1G) ~= 60.13

Right, this is round-down behavior was deliberate. Reading the ARM ARM
D11.1.2 'The system counter', I did not find any language that
suggested the counter saturates the register width before rolling
over. So, it may be paranoid, but I presumed it to be safer to wrap
within the guaranteed interval rather than assume the sanity of the
system counter implementation. That being said, fine with rounding up
instead, so long as we don't believe there's any chance of hardware
doing something crazy.

--
Thanks,
Oliver

Thanks,

        M.

--
Without deviation from the norm, progress is not possible.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel