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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Thu, 20 Sep 2018 08:45:57 -0700
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Subject: Re: [PATCH] clocksource/drivers/fttmr010: fix set_next_event handler
To:     taoren@fb.com, Joel Stanley <joel@jms.id.au>,
        Andrew Jeffery <andrew@aj.id.au>
Cc:     Daniel Lezcano <daniel.lezcano@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        OpenBMC Maillist <openbmc@lists.ozlabs.org>, cov@fb.com,
        tfang@fb.com
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On Wed, Sep 19, 2018 at 3:13 PM Tao Ren <taoren@fb.com> wrote:

> Currently, the aspeed MATCH1 register is updated to <current_count -
> cycles> in set_next_event handler, with the assumption that COUNT
> register value is preserved when the timer is disabled and it continues
> decrementing after the timer is enabled. But the assumption is wrong:
> RELOAD register is loaded into COUNT register when the aspeed timer is
> enabled, which means the next event may be delayed because timer
> interrupt won't be generated until <0xFFFFFFFF - current_count +
> cycles>.
>
> The problem can be fixed by updating RELOAD register to <cycles>, and
> COUNT register will be re-loaded when the timer is enabled and interrupt
> is generated when COUNT register overflows.
>
> The test result on Facebook Backpack-CMM BMC hardware (AST2500) shows
> the issue is fixed: without the patch, usleep(100) suspends the process
> for several milliseconds (and sometimes even over 40 milliseconds);
> after applying the fix, usleep(100) takes averagely 240 microseconds to
> return under the same workload level.
>
> Signed-off-by: Tao Ren <taoren@fb.com>

Actually this is much more intuitive too, it is the typical way to handle
a down-counting timer. Good catch!
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

Sorry for any cargo-cult programming on my part :/

Would be nice to get a nod from the AST2400 users that this works
for them too, so included them in the To: field.

I can't test it on up-counting hardware right now but I convinced
myself it is handled just like before so it shouldn't be an issue.

It actually would make kind of sense to restart the up-counting
timer from zero and set match to whatever value is passed in
as well, so I might send a patch for this. It's no regression though
so no hurry with that.

Yours,
Linus Walleij