[PATCH v1 2/3] arm64: idle: Cache AMU counters before entering idle

[Vanshidhar Konda]

AMU counters do not increment while a CPU is in idle. Saving the value
of the core and constant counters prior to invoking WFI allows FIE to
compute the frequency of a CPU that is idle.

Signed-off-by: Vanshidhar Konda <vanshikonda at os.amperecomputing.com>
---
 arch/arm64/kernel/idle.c     | 10 ++++++++++
 arch/arm64/kernel/topology.c | 14 ++++++++------
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/idle.c b/arch/arm64/kernel/idle.c
index 05cfb347ec26..5ed2e57188a8 100644
--- a/arch/arm64/kernel/idle.c
+++ b/arch/arm64/kernel/idle.c
@@ -26,6 +26,16 @@ void __cpuidle cpu_do_idle(void)
 
 	arm_cpuidle_save_irq_context(&context);
 
+#ifdef CONFIG_ARM64_AMU_EXTN
+	/* Update the AMU counters before entering WFI. The cached AMU counter
+	 * value is used to determine CPU frequency while the CPU is idle
+	 * without needing to wake up the CPU.
+	 */
+
+	if (cpu_has_amu_feat(smp_processor_id()))
+		update_freq_counters_refs();
+#endif
+
 	dsb(sy);
 	wfi();
 
diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index db8d14525cf4..8905eb0c681f 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -240,13 +240,15 @@ unsigned int arch_freq_get_on_cpu(int cpu)
 	} while (read_seqcount_retry(&cpu_sample->seq, seq));
 
 	/*
-	 * Bail on invalid count and when the last update was too long ago,
-	 * which covers idle and NOHZ full CPUs.
+	 * Bail on invalid count and when the last update was too long ago.
+	 * This covers idle, NOHZ full and isolated CPUs.
+	 *
+	 * Idle CPUs don't need to be measured because AMU counters stop
+	 * incrementing during WFI/WFE.
 	 */
-	if (!delta_const_cnt || ((jiffies - last) > MAX_SAMPLE_AGE)) {
-		if (!(housekeeping_cpu(cpu, HK_TYPE_TICK) && idle_cpu(cpu)))
-			goto fallback;
-	}
+	if (!delta_const_cnt ||
+	    ((jiffies - last) > MAX_SAMPLE_AGE && !idle_cpu(cpu)))
+		goto fallback;
 
 	/*
 	 * CPU frequency = reference perf (in Hz) * (/\ delivered) / (/\ reference)
-- 
2.43.1