[PATCH v2 1/2] arm64: Provide an AMU-based version of arch_freq_get_on_cpu

[Beata Michalska <beata.michalska@arm.com>]

With the Frequency Invariance Engine (FIE) being already wired up with
sched tick and making use of relevant (core counter and constant
counter) AMU counters, getting the current frequency for a given CPU
on supported platforms, can be achieved by utilizing the frequency scale
factor which reflects an average CPU frequency for the last tick period
length.

Suggested-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Beata Michalska <beata.michalska@arm.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
---

Notes:
    Due to [1], if merged, there might be a need to modify the patch to
    accommodate changes [1] introduces:
    
    	freq = cpufreq_get_hw_max_freq(cpu) >> SCHED_CAPACITY_SHIFT
    	to
    	freq = per_cpu(capacity_freq_ref, cpu); >> SCHED_CAPACITY_SHIFT
    [1]
    https://lore.kernel.org/linux-arm-kernel/20231121154349.GA1938@willie-the-truck/T/#mcb018d076dbce6f60ed2779634a9b6ffe622641e

 arch/arm64/kernel/topology.c | 39 ++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index 615c1a20129f..ae2445f6e7da 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -17,6 +17,7 @@
 #include <linux/cpufreq.h>
 #include <linux/init.h>
 #include <linux/percpu.h>
+#include <linux/sched/isolation.h>
 
 #include <asm/cpu.h>
 #include <asm/cputype.h>
@@ -186,6 +187,44 @@ static void amu_scale_freq_tick(void)
 	this_cpu_write(arch_freq_scale, (unsigned long)scale);
 }
 
+unsigned int arch_freq_get_on_cpu(int cpu)
+{
+	unsigned int freq;
+	u64 scale;
+
+	if (!cpumask_test_cpu(cpu, amu_fie_cpus))
+		return 0;
+
+	/*
+	 * For those CPUs that are in full dynticks mode, try an alternative
+	 * source for the counters (and thus freq scale),
+	 * if available for given policy
+	 */
+	if (!housekeeping_cpu(cpu, HK_TYPE_TICK)) {
+		struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
+		int ref_cpu = nr_cpu_ids;
+
+		if (cpumask_intersects(housekeeping_cpumask(HK_TYPE_TICK),
+				       policy->cpus))
+			ref_cpu = cpumask_nth_and(cpu, policy->cpus,
+						  housekeeping_cpumask(HK_TYPE_TICK));
+		cpufreq_cpu_put(policy);
+		if (ref_cpu >= nr_cpu_ids)
+			return 0;
+		cpu = ref_cpu;
+	}
+
+	/*
+	 * Reversed computation to the one used to determine
+	 * the arch_freq_scale value
+	 * (see amu_scale_freq_tick for details)
+	 */
+	scale = per_cpu(arch_freq_scale, cpu);
+	freq = cpufreq_get_hw_max_freq(cpu) >> SCHED_CAPACITY_SHIFT;
+	freq *= scale;
+	return freq;
+}
+
 static struct scale_freq_data amu_sfd = {
 	.source = SCALE_FREQ_SOURCE_ARCH,
 	.set_freq_scale = amu_scale_freq_tick,
-- 
2.25.1