[PATCH v2 1/2] ACPI: CPPC: add paired FFH feedback-counter read hook

[Pengjie Zhang <zhangpengjie2@huawei.com>]

cppc_get_perf_ctrs() reads the delivered and reference performance
counters one at a time.

Allow architectures to provide both FFH feedback counters in one
operation when that either narrows the sampling window or avoids extra
cross-CPU reads. Add a small FFH-specific hook for that case and fall
back to the existing per-register reads when unsupported.

Tested-by: Sumit Gupta <sumitg@nvidia.com>
Reviewed-by: Sumit Gupta <sumitg@nvidia.com>
Tested-by: Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
Reviewed-by: Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
Signed-off-by: Pengjie Zhang <zhangpengjie2@huawei.com>
---
 drivers/acpi/cppc_acpi.c | 50 ++++++++++++++++++++++++++++++++++++----
 include/acpi/cppc_acpi.h |  7 ++++++
 2 files changed, 52 insertions(+), 5 deletions(-)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 9f572f481241..b6356476a46e 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -1004,6 +1004,22 @@ int __weak cpc_read_ffh(int cpunum, struct cpc_reg *reg, u64 *val)
 	return -ENOTSUPP;
 }
 
+/**
+ * cpc_read_ffh_fb_ctrs() - Read FFH feedback counters together
+ * @cpunum:	Target CPU
+ * @reg1:	first CPPC register information
+ * @val1:	place holder for first return value
+ * @reg2:	second CPPC register information
+ * @val2:	place holder for second return value
+ *
+ * Return: 0 on success, error code otherwise
+ */
+int __weak cpc_read_ffh_fb_ctrs(int cpunum, struct cpc_reg *reg1,
+				u64 *val1, struct cpc_reg *reg2, u64 *val2)
+{
+	return -EOPNOTSUPP;
+}
+
 /**
  * cpc_write_ffh() - Write FFH register
  * @cpunum:	CPU number to write
@@ -1496,6 +1512,33 @@ bool cppc_perf_ctrs_in_pcc_cpu(unsigned int cpu)
 }
 EXPORT_SYMBOL_GPL(cppc_perf_ctrs_in_pcc_cpu);
 
+static int cppc_read_fb_ctrs(int cpunum,
+			     struct cpc_register_resource *delivered_reg,
+			     struct cpc_register_resource *reference_reg,
+			     u64 *delivered, u64 *reference)
+{
+	int ret;
+
+	/*
+	 * For FFH feedback counters, try a paired read first to reduce
+	 * sampling skew between delivered and reference counters. Fall
+	 * back to the existing per-register reads if unsupported.
+	 */
+	if (CPC_IN_FFH(delivered_reg) && CPC_IN_FFH(reference_reg)) {
+		ret = cpc_read_ffh_fb_ctrs(cpunum,
+					&delivered_reg->cpc_entry.reg, delivered,
+					&reference_reg->cpc_entry.reg, reference);
+		if (ret != -EOPNOTSUPP)
+			return ret;
+	}
+
+	ret = cpc_read(cpunum, delivered_reg, delivered);
+	if (ret)
+		return ret;
+
+	return cpc_read(cpunum, reference_reg, reference);
+}
+
 /**
  * cppc_perf_ctrs_in_pcc - Check if any perf counters are in a PCC region.
  *
@@ -1561,11 +1604,8 @@ int cppc_get_perf_ctrs(int cpunum, struct cppc_perf_fb_ctrs *perf_fb_ctrs)
 		}
 	}
 
-	ret = cpc_read(cpunum, delivered_reg, &delivered);
-	if (ret)
-		goto out_err;
-
-	ret = cpc_read(cpunum, reference_reg, &reference);
+	ret = cppc_read_fb_ctrs(cpunum, delivered_reg, reference_reg,
+				&delivered, &reference);
 	if (ret)
 		goto out_err;
 
diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
index 8693890a7275..5acebe62feac 100644
--- a/include/acpi/cppc_acpi.h
+++ b/include/acpi/cppc_acpi.h
@@ -176,6 +176,8 @@ extern 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_ffh_fb_ctrs(int cpu, struct cpc_reg *reg1, u64 *val1,
+				struct cpc_reg *reg2, u64 *val2);
 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);
@@ -250,6 +252,11 @@ static inline int cpc_read_ffh(int cpunum, struct cpc_reg *reg, u64 *val)
 {
 	return -EOPNOTSUPP;
 }
+static inline int cpc_read_ffh_fb_ctrs(int cpu, struct cpc_reg *reg1, u64 *val1,
+				       struct cpc_reg *reg2, u64 *val2)
+{
+	return -EOPNOTSUPP;
+}
 static inline int cpc_write_ffh(int cpunum, struct cpc_reg *reg, u64 val)
 {
 	return -EOPNOTSUPP;
-- 
2.33.0