[CPUFREQ] Handle P4 TSC scaling.

Currently, the TSC cpufreq notifiers does almost nothing on P4s, as we
assumed the TSC to be constant independent of _all_ frequency transitions.
Extensive testing by Karol Kozimor has shown, though, that only _throttling_
does not affect the TSC, but _scaling_ does.
                                                                                
So:
- pass the CPUFREQ_CONST_LOOPS flags (to be exact, all flags) to cpufreq
  transition notifiers
- skip TSC value changes if this flag is set
- set this flag for P4 / P4-Ms only in p4-clockmod [On Pentium-M banias
  the TSC _is_ affected by p4-clock modulation

arch/i386/kernel/cpu/cpufreq/p4-clockmod.c

@@ -51,6 +51,7 @@ enum {
 
 static int has_N44_O17_errata[NR_CPUS];
 static unsigned int stock_freq;
+static struct cpufreq_driver p4clockmod_driver;
 
 static int cpufreq_p4_setdc(unsigned int cpu, unsigned int newstate)
 {
@@ -193,6 +194,10 @@ static unsigned int cpufreq_p4_get_frequency(struct cpuinfo_x86 *c)
 		return 0;
 	}
 
+	/* on P-4s, the TSC runs with constant frequency independent wether
+	 * throttling is active or not. */
+	p4clockmod_driver.flags |= CPUFREQ_CONST_LOOPS;
+
 	if (speedstep_detect_processor() == SPEEDSTEP_PROCESSOR_P4M) {
 		printk(KERN_WARNING PFX "Warning: Pentium 4-M detected. "
 		       "The speedstep-ich or acpi cpufreq modules offer "

arch/i386/kernel/timers/timer_tsc.c

@@ -353,14 +353,11 @@ static inline void cpufreq_delayed_get(void)
 }
 
 /* If the CPU frequency is scaled, TSC-based delays will need a different
- * loops_per_jiffy value to function properly. An exception to this
- * are modern Intel Pentium 4 processors, where the TSC runs at a constant
- * speed independent of frequency scaling. 
+ * loops_per_jiffy value to function properly.
  */
 
 static unsigned int  ref_freq = 0;
 static unsigned long loops_per_jiffy_ref = 0;
-static unsigned int  variable_tsc = 1;
 
 #ifndef CONFIG_SMP
 static unsigned long fast_gettimeoffset_ref = 0;
@@ -386,13 +383,13 @@ time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
 	if ((val == CPUFREQ_PRECHANGE  && freq->old < freq->new) ||
 	    (val == CPUFREQ_POSTCHANGE && freq->old > freq->new) ||
 	    (val == CPUFREQ_RESUMECHANGE)) {
-		if (variable_tsc)
+		if (!freq->flags & CPUFREQ_CONST_LOOPS)
 			cpu_data[freq->cpu].loops_per_jiffy = cpufreq_scale(loops_per_jiffy_ref, ref_freq, freq->new);
 #ifndef CONFIG_SMP
 		if (cpu_khz)
 			cpu_khz = cpufreq_scale(cpu_khz_ref, ref_freq, freq->new);
 		if (use_tsc) {
-			if (variable_tsc) {
+			if (!freq->flags & CPUFREQ_CONST_LOOPS) {
 				fast_gettimeoffset_quotient = cpufreq_scale(fast_gettimeoffset_ref, freq->new, ref_freq);
 				set_cyc2ns_scale(cpu_khz/1000);
 			}
@@ -411,13 +408,12 @@ static struct notifier_block time_cpufreq_notifier_block = {
 
 static int __init cpufreq_tsc(void)
 {
+	int ret;
 	INIT_WORK(&cpufreq_delayed_get_work, handle_cpufreq_delayed_get, NULL);
+	ret = cpufreq_register_notifier(&time_cpufreq_notifier_block, CPUFREQ_TRANSITION_NOTIFIER);
+	if (!ret)
 		cpufreq_init = 1;
-	/* P4 and above CPU TSC freq doesn't change when CPU frequency changes*/
-	if ((boot_cpu_data.x86 >= 15) && (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL))
-		variable_tsc = 0;
-
-	return cpufreq_register_notifier(&time_cpufreq_notifier_block, CPUFREQ_TRANSITION_NOTIFIER);
+	return ret;
 }
 core_initcall(cpufreq_tsc);
 

drivers/cpufreq/cpufreq.c

@@ -1018,7 +1018,7 @@ static unsigned int  l_p_j_ref_freq;
 
 static inline void adjust_jiffies(unsigned long val, struct cpufreq_freqs *ci)
 {
-	if (cpufreq_driver->flags & CPUFREQ_CONST_LOOPS)
+	if (ci->flags & CPUFREQ_CONST_LOOPS)
 		return;
 
 	if (!l_p_j_ref_freq) {
@@ -1045,6 +1045,8 @@ void cpufreq_notify_transition(struct cpufreq_freqs *freqs, unsigned int state)
 {
 	BUG_ON(irqs_disabled());
 
+	freqs->flags = cpufreq_driver->flags;
+
 	down_read(&cpufreq_notifier_rwsem);
 	switch (state) {
 	case CPUFREQ_PRECHANGE:

include/linux/cpufreq.h

@@ -106,6 +106,7 @@ struct cpufreq_freqs {
 	unsigned int cpu;	/* cpu nr */
 	unsigned int old;
 	unsigned int new;
+	u8 flags;		/* flags of cpufreq_driver, see below. */
 };