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From:   Oliver Upton <oupton@google.com>
Date:   Sun, 8 Aug 2021 12:01:10 -0700
Message-ID: <CAOQ_Qsi-r0=YubmaTezB7Cq-RhzU3njWm+P5w-RWZcUz4kMfDA@mail.gmail.com>
Subject: Re: [PATCH v2] clocksource/arm_arch_timer: Fix masking for high freq counters
To:     Marc Zyngier <maz@kernel.org>
Cc:     Linus Walleij <linus.walleij@linaro.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Shier <pshier@google.com>,
        Raghavendra Rao Ananta <rananta@google.com>,
        Ricardo Koller <ricarkol@google.com>
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On Sun, Aug 8, 2021 at 3:40 AM Marc Zyngier <maz@kernel.org> wrote:
>
> On Sun, 08 Aug 2021 02:14:35 +0100,
> Oliver Upton <oupton@google.com> wrote:
>
> > The only other sane idea that I could come up with is providing this
> > information to the kernel through DT, although that would leave ACPI
> > systems behind.
>
> It also has the disadvantage that a large number of DT timer nodes are
> a mess of cargo-culted, copy-pasted idioms, and that adding another
> property would only make it worse.

Agreed, this does seem like the best solution, short of the
architecture actually providing something to determine the counter
width.

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 :)

--
Thanks,
Oliver