[PATCH v1 3/3] ACPI: CPPC: Read CPC FFH counters in a single IPI

[Vanshidhar Konda <vanshikonda@os.amperecomputing.com>]

The CPPC driver reads delivered and reference counters using cpc_read
one at a time. This leads to inaccurate measurement of CPU frequency
discussed in [1]. If the firmware indicates that both the registers are
in the FFH interface the kernel can read the registers together in a
single IPI. This has two benefits:
1. Reduces the number of IPIs needed to read the two registers
2. The two registers will be read in close proximity resulting in more
   accurate CPU frequency measurement

[1]: https://lore.kernel.org/all/20230328193846.8757-1-yang@os.amperecomputing.com/

Signed-off-by: Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
---
 arch/arm64/kernel/topology.c | 37 ++++++++++++++++++++++++++++++++++++
 drivers/acpi/cppc_acpi.c     | 32 +++++++++++++++++++++++++++----
 include/acpi/cppc_acpi.h     | 13 +++++++++++++
 3 files changed, 78 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index 8905eb0c681f..8207565f43ee 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -421,6 +421,43 @@ int cpc_read_ffh(int cpu, struct cpc_reg *reg, u64 *val)
 	return ret;
 }
 
+static void cpc_update_freq_counters(void *info)
+{
+	update_freq_counters_refs();
+}
+
+int cpc_read_regs_ffh(int cpu, struct ffh_cpc_reg_values *ffh_regs)
+{
+	struct amu_counters *ctrs = per_cpu_ptr(&cpu_samples, cpu);
+	int idx;
+
+	if (!cpc_ffh_supported() || !freq_counters_valid(cpu))
+		return -EOPNOTSUPP;
+
+	if (WARN_ON_ONCE(irqs_disabled()))
+		return -EPERM;
+
+	if (!idle_cpu(cpu))
+		smp_call_function_single(cpu, cpc_update_freq_counters, NULL, 1);
+
+	for (idx = 0; idx < MAX_NUM_CPC_REGS_FFH; idx++) {
+
+		if (!ffh_regs->regs[idx].reg)
+			continue;
+
+		switch ((u64)(ffh_regs->regs[idx].reg->address)) {
+		case 0x0:
+			ffh_regs->regs[idx].value = ctrs->core_cnt;
+			break;
+		case 0x1:
+			ffh_regs->regs[idx].value = ctrs->const_cnt;
+			break;
+		}
+	}
+
+	return 0;
+}
+
 int cpc_write_ffh(int cpunum, struct cpc_reg *reg, u64 val)
 {
 	return -EOPNOTSUPP;
diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index d155a86a8614..55ffb1915e4f 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -113,6 +113,10 @@ static DEFINE_PER_CPU(struct cpc_desc *, cpc_desc_ptr);
 				(cpc)->cpc_entry.reg.space_id ==	\
 				ACPI_ADR_SPACE_SYSTEM_IO)
 
+#define CPC_IN_FFH(cpc) ((cpc)->type == ACPI_TYPE_BUFFER &&		\
+				(cpc)->cpc_entry.reg.space_id ==	\
+				ACPI_ADR_SPACE_FIXED_HARDWARE)
+
 /* Evaluates to True if reg is a NULL register descriptor */
 #define IS_NULL_REG(reg) ((reg)->space_id ==  ACPI_ADR_SPACE_SYSTEM_MEMORY && \
 				(reg)->address == 0 &&			\
@@ -974,6 +978,11 @@ int __weak cpc_write_ffh(int cpunum, struct cpc_reg *reg, u64 val)
 	return -ENOTSUPP;
 }
 
+int __weak cpc_read_regs_ffh(int cpu, struct ffh_cpc_reg_values *regs)
+{
+	return -ENOTSUPP;
+}
+
 /*
  * Since cpc_read and cpc_write are called while holding pcc_lock, it should be
  * as fast as possible. We have already mapped the PCC subspace during init, so
@@ -1317,7 +1326,7 @@ int cppc_get_perf_ctrs(int cpunum, struct cppc_perf_fb_ctrs *perf_fb_ctrs)
 	int pcc_ss_id = per_cpu(cpu_pcc_subspace_idx, cpunum);
 	struct cppc_pcc_data *pcc_ss_data = NULL;
 	u64 delivered, reference, ref_perf, ctr_wrap_time;
-	int ret = 0, regs_in_pcc = 0;
+	int ret = 0, regs_in_pcc = 0, regs_read_in_ffh = 0;
 
 	if (!cpc_desc) {
 		pr_debug("No CPC descriptor for CPU:%d\n", cpunum);
@@ -1353,8 +1362,23 @@ int cppc_get_perf_ctrs(int cpunum, struct cppc_perf_fb_ctrs *perf_fb_ctrs)
 		}
 	}
 
-	cpc_read(cpunum, delivered_reg, &delivered);
-	cpc_read(cpunum, reference_reg, &reference);
+	if (CPC_IN_FFH(delivered_reg) && CPC_IN_FFH(reference_reg)) {
+		struct ffh_cpc_reg_values ffh_regs;
+
+		ffh_regs.regs[0].reg = &(delivered_reg->cpc_entry.reg);
+		ffh_regs.regs[1].reg = &(reference_reg->cpc_entry.reg);
+		ret = cpc_read_regs_ffh(cpunum, &ffh_regs);
+		if (!ret) {
+			delivered = ffh_regs.regs[0].value;
+			reference = ffh_regs.regs[1].value;
+			regs_read_in_ffh = 1;
+		}
+	}
+
+	if (!regs_read_in_ffh) {
+		cpc_read(cpunum, delivered_reg, &delivered);
+		cpc_read(cpunum, reference_reg, &reference);
+	}
 	cpc_read(cpunum, ref_perf_reg, &ref_perf);
 
 	/*
@@ -1366,7 +1390,7 @@ int cppc_get_perf_ctrs(int cpunum, struct cppc_perf_fb_ctrs *perf_fb_ctrs)
 	if (CPC_SUPPORTED(ctr_wrap_reg))
 		cpc_read(cpunum, ctr_wrap_reg, &ctr_wrap_time);
 
-	if (!delivered || !reference ||	!ref_perf) {
+	if (!delivered || !reference || !ref_perf) {
 		ret = -EFAULT;
 		goto out_err;
 	}
diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
index 3a0995f8bce8..0da614a50edd 100644
--- a/include/acpi/cppc_acpi.h
+++ b/include/acpi/cppc_acpi.h
@@ -137,6 +137,18 @@ struct cppc_cpudata {
 };
 
 #ifdef CONFIG_ACPI_CPPC_LIB
+
+#define MAX_NUM_CPC_REGS_FFH 2
+
+struct ffh_cpc_reg {
+	struct cpc_reg *reg;
+	u64 value;
+};
+
+struct ffh_cpc_reg_values {
+	struct ffh_cpc_reg regs[MAX_NUM_CPC_REGS_FFH];
+};
+
 extern int cppc_get_desired_perf(int cpunum, u64 *desired_perf);
 extern int cppc_get_nominal_perf(int cpunum, u64 *nominal_perf);
 extern int cppc_get_perf_ctrs(int cpu, struct cppc_perf_fb_ctrs *perf_fb_ctrs);
@@ -153,6 +165,7 @@ extern unsigned int cppc_get_transition_latency(int cpu);
 extern bool cpc_ffh_supported(void);
 extern bool cpc_supported_by_cpu(void);
 extern int cpc_read_ffh(int cpunum, struct cpc_reg *reg, u64 *val);
+extern int cpc_read_regs_ffh(int cpu, struct ffh_cpc_reg_values *regs);
 extern int cpc_write_ffh(int cpunum, struct cpc_reg *reg, u64 val);
 extern int cppc_get_epp_perf(int cpunum, u64 *epp_perf);
 extern int cppc_set_epp_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls, bool enable);
-- 
2.43.1