svn commit: r259197 - head/sys/x86/cpufreq

[Alexander Motin]

Author: mav
Date: Tue Dec 10 20:25:43 2013
New Revision: 259197
URL: http://svnweb.freebsd.org/changeset/base/259197

Log:
  Do not DELAY() for P-state transition unless we want to see the result.
  
  Intel manual says: "If a transition is already in progress, transition to
  a new value will subsequently take effect. Reads of IA32_PERF_CTL determine
  the last targeted operating point."  So seems it should be fine to just
  trigger wanted transition and go.  Linux does the same.
  
  MFC after:	1 month

Modified:
  head/sys/x86/cpufreq/est.c

Modified: head/sys/x86/cpufreq/est.c
==============================================================================
--- head/sys/x86/cpufreq/est.c	Tue Dec 10 20:09:49 2013	(r259196)
+++ head/sys/x86/cpufreq/est.c	Tue Dec 10 20:25:43 2013	(r259197)
@@ -1288,10 +1288,9 @@ est_set_id16(device_t dev, uint16_t id16
 	msr = (msr & ~0xffff) | id16;
 	wrmsr(MSR_PERF_CTL, msr);
 
-	/* Wait a short while for the new setting.  XXX Is this necessary? */
-	DELAY(EST_TRANS_LAT);
-
 	if  (need_check) {
+		/* Wait a short while and read the new status. */
+		DELAY(EST_TRANS_LAT);
 		est_get_id16(&new_id16);
 		if (new_id16 != id16) {
 			if (bootverbose)