[tip:x86/urgent] x86/tsc: Read all ratio bits from MSR_PLATFORM_INFO

[tip-bot for Chen Yu]

Commit-ID:  886123fb3a8656699dff40afa0573df359abeb18
Gitweb:     http://git.kernel.org/tip/886123fb3a8656699dff40afa0573df359abeb18
Author:     Chen Yu <yu.c.chen@xxxxxxxxx>
AuthorDate: Fri, 6 May 2016 11:33:39 +0800
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Fri, 6 May 2016 11:50:50 +0200

x86/tsc: Read all ratio bits from MSR_PLATFORM_INFO

Currently we read the tsc radio: ratio = (MSR_PLATFORM_INFO >> 8) & 0x1f;

Thus we get bit 8-12 of MSR_PLATFORM_INFO, however according to the SDM
(35.5), the ratio bits are bit 8-15.

Ignoring the upper bits can result in an incorrect tsc ratio, which causes the
TSC calibration and the Local APIC timer frequency to be incorrect.

Fix this problem by masking 0xff instead.

[ tglx: Massaged changelog ]

Fixes: 7da7c1561366 "x86, tsc: Add static (MSR) TSC calibration on Intel Atom SoCs"
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: Bin Gao <bin.gao@xxxxxxxxx>
Cc: Len Brown <lenb@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/1462505619-5516-1-git-send-email-yu.c.chen@xxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

---
 arch/x86/kernel/tsc_msr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tsc_msr.c b/arch/x86/kernel/tsc_msr.c
index 92ae6ac..6aa0f4d 100644
--- a/arch/x86/kernel/tsc_msr.c
+++ b/arch/x86/kernel/tsc_msr.c
@@ -92,7 +92,7 @@ unsigned long try_msr_calibrate_tsc(void)
 
 	if (freq_desc_tables[cpu_index].msr_plat) {
 		rdmsr(MSR_PLATFORM_INFO, lo, hi);
-		ratio = (lo >> 8) & 0x1f;
+		ratio = (lo >> 8) & 0xff;
 	} else {
 		rdmsr(MSR_IA32_PERF_STATUS, lo, hi);
 		ratio = (hi >> 8) & 0x1f;