New patch to review for coreboot: b285a0f x86: harden tsc udelay() function

[Aaron Durbin]

Aaron Durbin ([e-mail]) just uploaded a new patch set to gerrit, which you can find at http://review.coreboot.org/3171

-gerrit

commit b285a0fb0f72b6d4b8e7e39050c44e72d741ca96
Author: Aaron Durbin 
Date:   Wed May 1 15:55:14 2013 -0500

x86: harden tsc udelay() function

Since the TSC udelay() fucntion can be used in SMM that means the
    TSC can count up to whatever value. The current loop was not handling
    TSC rollover properly. In most cases this should not matter as the TSC
    typically starts ticking at value 0, and it would take a very long time
    to roll it over. However, it is my understanding that this behavior is
    not guaranteed. Theoretically the TSC could start or be be written to
    with a large value that would cause the rollover.

Change-Id: I2f11a5bc4f27d5543e74f8224811fa91e4a55484
    Signed-off-by: Aaron Durbin 
---
 src/cpu/x86/tsc/delay_tsc.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/src/cpu/x86/tsc/delay_tsc.c b/src/cpu/x86/tsc/delay_tsc.c
index 0540496..0e2a9c0 100644
--- a/src/cpu/x86/tsc/delay_tsc.c
+++ b/src/cpu/x86/tsc/delay_tsc.c
@@ -172,18 +172,18 @@ static inline unsigned long get_clocks_per_usec(void)

void udelay(unsigned us)
 {
-        unsigned long long count;
-        unsigned long long stop;
-        unsigned long long clocks;
+	unsigned long long start;
+	unsigned long long current;
+	unsigned long long clocks;

+	start = rdtscll();
    clocks = us;
    clocks *= get_clocks_per_usec();
-        count = rdtscll();
-        stop = clocks + count;
-        while(stop > count) {
+	current = rdtscll();
+	while((current - start) < clocks) {
    	cpu_relax();
-		count = rdtscll();
-        }
+		current = rdtscll();
+	}
 }

#if CONFIG_TSC_MONOTONIC_TIMER && !defined(__PRE_RAM__) && !defined(__SMM__)