Commit 57ca654b authored Apr 13, 2015 by Russell King

ARM: ensure delay timer has sufficient accuracy for delays

We have recently had an example of someone wanting to use a 90kHz timer
for the software delay loop.

udelay() needs to have at least microsecond resolution to allow drivers
access to a delay mechanism with a reasonable chance of delaying the
period they requested within at least a 50% marging of error, especially
for small delays.

Discussion about the udelay() accuracy can be found at:
	https://lkml.org/lkml/2011/1/9/37



Reject timers which are unable to supply this level of resolution.

Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

parent 37463be8

arch/arm/lib/delay.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -83,6 +83,12 @@ void __init register_current_timer_delay(const struct delay_timer *timer)
		NSEC_PER_SEC, 3600);
		res = cyc_to_ns(1ULL, new_mult, new_shift);

		if (res > 1000) {
		pr_err("Ignoring delay timer %ps, which has insufficient resolution of %lluns\n",
		timer, res);
		return;
		}

		if (!delay_calibrated && (!delay_res \|\| (res < delay_res))) {
		pr_info("Switching to timer-based delay loop, resolution %lluns\n", res);
		delay_timer = timer;

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