From 57ca654bef6c43bbbccfb2d231fd245d3f67dd46 Mon Sep 17 00:00:00 2001 From: Russell King Date: Mon, 13 Apr 2015 10:36:04 +0100 Subject: 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 Signed-off-by: Russell King --- arch/arm/lib/delay.c | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'arch/arm/lib') diff --git a/arch/arm/lib/delay.c b/arch/arm/lib/delay.c index 312d43eb686a..8044591dca72 100644 --- a/arch/arm/lib/delay.c +++ b/arch/arm/lib/delay.c @@ -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; -- cgit v1.2.3