From 8c660065383976f09fbdae86c33448c8da643d4e Mon Sep 17 00:00:00 2001 From: Dave Johnson Date: Tue, 23 Oct 2007 22:37:22 +0200 Subject: [PATCH] x86: fix more TSC clock source calibration errors The previous patch wasn't correctly handling the 'count' variable. If a CPU gave bad results on the 1st or 2nd run but good results on the 3rd, it wouldn't do the correct thing. No idea if any such CPU exists, but the patch below handles that case by discarding the bad runs. If a bad result (too quick, or too slow) occurs on any of the 3 runs it will be discarded. Also updated some comments to explain what's going on. Signed-off-by: Dave Johnson Signed-off-by: Ingo Molnar Signed-off-by: Thomas Gleixner --- arch/x86/kernel/tsc_32.c | 34 ++++++++++++++++++++-------------- 1 file changed, 20 insertions(+), 14 deletions(-) diff --git a/arch/x86/kernel/tsc_32.c b/arch/x86/kernel/tsc_32.c index f04d08a7d022..9ebc0dab66b4 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); -- 2.20.1