From 57ca654bef6c43bbbccfb2d231fd245d3f67dd46 Mon Sep 17 00:00:00 2001
From: Russell King <rmk+kernel@arm.linux.org.uk>
Date: Mon, 13 Apr 2015 10:36:04 +0100
Subject: [PATCH] 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>
---
arch/arm/lib/delay.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/arch/arm/lib/delay.c b/arch/arm/lib/delay.c
index 312d43eb686a0..8044591dca729 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;
--
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