cpufreq: Allow arch_freq_get_on_cpu to return an error

Allow arch_freq_get_on_cpu to return an error for cases when retrieving
current CPU frequency is not possible, whether that being due to lack of
required arch support or due to other circumstances when the current
frequency cannot be determined at given point of time.

Signed-off-by: Beata Michalska <beata.michalska@arm.com>
Reviewed-by: Prasanna Kumar T S M <ptsm@linux.microsoft.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20250131162439.3843071-2-beata.michalska@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/x86/kernel/cpu/aperfmperf.c

@@ -498,7 +498,7 @@ void arch_scale_freq_tick(void)
  */
 #define MAX_SAMPLE_AGE	((unsigned long)HZ / 50)
 
-unsigned int arch_freq_get_on_cpu(int cpu)
+int arch_freq_get_on_cpu(int cpu)
 {
 	struct aperfmperf *s = per_cpu_ptr(&cpu_samples, cpu);
 	unsigned int seq, freq;

arch/x86/kernel/cpu/proc.c

@@ -86,9 +86,12 @@ static int show_cpuinfo(struct seq_file *m, void *v)
 		seq_printf(m, "microcode\t: 0x%x\n", c->microcode);
 
 	if (cpu_has(c, X86_FEATURE_TSC)) {
-		unsigned int freq = arch_freq_get_on_cpu(cpu);
+		int freq = arch_freq_get_on_cpu(cpu);
 
-		seq_printf(m, "cpu MHz\t\t: %u.%03u\n", freq / 1000, (freq % 1000));
+		if (freq < 0)
+			seq_puts(m, "cpu MHz\t\t: Unknown\n");
+		else
+			seq_printf(m, "cpu MHz\t\t: %u.%03u\n", freq / 1000, (freq % 1000));
 	}
 
 	/* Cache size */

drivers/cpufreq/cpufreq.c

@@ -729,18 +729,18 @@ show_one(cpuinfo_transition_latency, cpuinfo.transition_latency);
 show_one(scaling_min_freq, min);
 show_one(scaling_max_freq, max);
 
-__weak unsigned int arch_freq_get_on_cpu(int cpu)
+__weak int arch_freq_get_on_cpu(int cpu)
 {
-	return 0;
+	return -EOPNOTSUPP;
 }
 
 static ssize_t show_scaling_cur_freq(struct cpufreq_policy *policy, char *buf)
 {
 	ssize_t ret;
-	unsigned int freq;
+	int freq;
 
 	freq = arch_freq_get_on_cpu(policy->cpu);
-	if (freq)
+	if (freq > 0)
 		ret = sysfs_emit(buf, "%u\n", freq);
 	else if (cpufreq_driver->setpolicy && cpufreq_driver->get)
 		ret = sysfs_emit(buf, "%u\n", cpufreq_driver->get(policy->cpu));

include/linux/cpufreq.h

@@ -1184,7 +1184,7 @@ static inline int of_perf_domain_get_sharing_cpumask(int pcpu, const char *list_
 }
 #endif
 
-extern unsigned int arch_freq_get_on_cpu(int cpu);
+extern int arch_freq_get_on_cpu(int cpu);
 
 #ifndef arch_set_freq_scale
 static __always_inline