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Diffstat (limited to 'arch/x86/kernel/tsc_32.c')
-rw-r--r--arch/x86/kernel/tsc_32.c34
1 files changed, 20 insertions, 14 deletions
diff --git a/arch/x86/kernel/tsc_32.c b/arch/x86/kernel/tsc_32.c
index f04d08a..9ebc0da 100644
--- a/arch/x86/kernel/tsc_32.c
+++ b/arch/x86/kernel/tsc_32.c
@@ -137,31 +137,37 @@ unsigned long native_calculate_cpu_khz(void)
local_irq_save(flags);
- /* run 3 times to ensure the cache is warm */
+ /* run 3 times to ensure the cache is warm and to get an accurate reading */
for (i = 0; i < 3; i++) {
mach_prepare_counter();
rdtscll(start);
mach_countup(&count);
rdtscll(end);
+
+ /*
+ * Error: ECTCNEVERSET
+ * The CTC wasn't reliable: we got a hit on the very first read,
+ * or the CPU was so fast/slow that the quotient wouldn't fit in
+ * 32 bits..
+ */
+ if (count <= 1)
+ continue;
+
+ /* cpu freq too slow: */
+ if ((end - start) <= CALIBRATE_TIME_MSEC)
+ continue;
+
+ /*
+ * We want the minimum time of all runs in case one of them
+ * is inaccurate due to SMI or other delay
+ */
delta64 = min(delta64, (end - start));
}
- /*
- * Error: ECTCNEVERSET
- * The CTC wasn't reliable: we got a hit on the very first read,
- * or the CPU was so fast/slow that the quotient wouldn't fit in
- * 32 bits..
- */
- if (count <= 1)
- goto err;
- /* cpu freq too fast: */
+ /* cpu freq too fast (or every run was bad): */
if (delta64 > (1ULL<<32))
goto err;
- /* cpu freq too slow: */
- if (delta64 <= CALIBRATE_TIME_MSEC)
- goto err;
-
delta64 += CALIBRATE_TIME_MSEC/2; /* round for do_div */
do_div(delta64,CALIBRATE_TIME_MSEC);