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-rw-r--r--arch/x86/kernel/tsc.c29
1 files changed, 22 insertions, 7 deletions
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 6e1a368d21d4..71f4368b357e 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -275,15 +275,20 @@ static unsigned long pit_calibrate_tsc(u32 latch, unsigned long ms, int loopmin)
275 * use the TSC value at the transitions to calculate a pretty 275 * use the TSC value at the transitions to calculate a pretty
276 * good value for the TSC frequencty. 276 * good value for the TSC frequencty.
277 */ 277 */
278static inline int pit_verify_msb(unsigned char val)
279{
280 /* Ignore LSB */
281 inb(0x42);
282 return inb(0x42) == val;
283}
284
278static inline int pit_expect_msb(unsigned char val, u64 *tscp, unsigned long *deltap) 285static inline int pit_expect_msb(unsigned char val, u64 *tscp, unsigned long *deltap)
279{ 286{
280 int count; 287 int count;
281 u64 tsc = 0; 288 u64 tsc = 0;
282 289
283 for (count = 0; count < 50000; count++) { 290 for (count = 0; count < 50000; count++) {
284 /* Ignore LSB */ 291 if (!pit_verify_msb(val))
285 inb(0x42);
286 if (inb(0x42) != val)
287 break; 292 break;
288 tsc = get_cycles(); 293 tsc = get_cycles();
289 } 294 }
@@ -336,8 +341,7 @@ static unsigned long quick_pit_calibrate(void)
336 * to do that is to just read back the 16-bit counter 341 * to do that is to just read back the 16-bit counter
337 * once from the PIT. 342 * once from the PIT.
338 */ 343 */
339 inb(0x42); 344 pit_verify_msb(0);
340 inb(0x42);
341 345
342 if (pit_expect_msb(0xff, &tsc, &d1)) { 346 if (pit_expect_msb(0xff, &tsc, &d1)) {
343 for (i = 1; i <= MAX_QUICK_PIT_ITERATIONS; i++) { 347 for (i = 1; i <= MAX_QUICK_PIT_ITERATIONS; i++) {
@@ -348,8 +352,19 @@ static unsigned long quick_pit_calibrate(void)
348 * Iterate until the error is less than 500 ppm 352 * Iterate until the error is less than 500 ppm
349 */ 353 */
350 delta -= tsc; 354 delta -= tsc;
351 if (d1+d2 < delta >> 11) 355 if (d1+d2 >= delta >> 11)
352 goto success; 356 continue;
357
358 /*
359 * Check the PIT one more time to verify that
360 * all TSC reads were stable wrt the PIT.
361 *
362 * This also guarantees serialization of the
363 * last cycle read ('d2') in pit_expect_msb.
364 */
365 if (!pit_verify_msb(0xfe - i))
366 break;
367 goto success;
353 } 368 }
354 } 369 }
355 printk("Fast TSC calibration failed\n"); 370 printk("Fast TSC calibration failed\n");