RISC-V: Add a simple platform driver for RISC-V legacy perf

The old RISC-V perf implementation allowed counting of only cycle/instruction
counters using perf. Restore that feature by implementing a simple platform
driver under a separate config to provide backward compatibility. Any existing
software stack will continue to work as it is. However, it provides an easy
way out in future where we can remove the legacy driver.

Signed-off-by: Atish Patra <atish.patra@wdc.com>

drivers/perf/Kconfig

@@ -64,6 +64,15 @@ config RISCV_PMU
 	  Say y if you want to use CPU performance monitors on RISCV-based
 	  systems.
 
+config RISCV_PMU_LEGACY
+	depends on RISCV_PMU
+	bool "RISC-V legacy PMU implementation"
+	default y
+	help
+	  Say y if you want to use the legacy CPU performance monitor
+	  implementation on RISC-V based systems. This only allows counting
+	  of cycle/instruction counter and will be removed in future.
+
 config ARM_PMU_ACPI
 	depends on ARM_PMU && ACPI
 	def_bool y

drivers/perf/Makefile

@@ -11,6 +11,9 @@ obj-$(CONFIG_HISI_PMU) += hisilicon/
 obj-$(CONFIG_QCOM_L2_PMU)	+= qcom_l2_pmu.o
 obj-$(CONFIG_QCOM_L3_PMU) += qcom_l3_pmu.o
 obj-$(CONFIG_RISCV_PMU) += riscv_pmu.o
+ifeq ($(CONFIG_RISCV_PMU), y)
+obj-$(CONFIG_RISCV_PMU_LEGACY) += riscv_pmu_legacy.o
+endif
 obj-$(CONFIG_THUNDERX2_PMU) += thunderx2_pmu.o
 obj-$(CONFIG_XGENE_PMU) += xgene_pmu.o
 obj-$(CONFIG_ARM_SPE_PMU) += arm_spe_pmu.o

drivers/perf/riscv_pmu.c

@@ -355,6 +355,8 @@ static int riscv_pmu_device_probe(struct platform_device *pdev)
 	if (!pmu)
 		return -ENOMEM;
 
+	riscv_pmu_legacy_init(pmu);
+
 	cpuhp_setup_state(CPUHP_AP_PERF_RISCV_STARTING,
 			  "perf/riscv/pmu:starting",
 			  riscv_perf_starting_cpu, riscv_perf_dying_cpu);

drivers/perf/riscv_pmu_legacy.c

@@ -0,0 +1,86 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * RISC-V performance counter support.
+ *
+ * Copyright (C) 2021 Western Digital Corporation or its affiliates.
+ *
+ * This implementation is based on old RISC-V perf and ARM perf event code
+ * which are in turn based on sparc64 and x86 code.
+ *
+ */
+
+#include <linux/perf/riscv_pmu.h>
+
+#define RISCV_PMU_LEGACY_CYCLE		0
+#define RISCV_PMU_LEGACY_INSTRET	1
+#define RISCV_PMU_LEGACY_NUM_CTR	2
+
+static int pmu_legacy_get_ctr_idx(struct perf_event *event)
+{
+	struct perf_event_attr *attr = &event->attr;
+
+	if (event->attr.type != PERF_TYPE_HARDWARE)
+		return -EOPNOTSUPP;
+	if (attr->config == PERF_COUNT_HW_CPU_CYCLES)
+		return RISCV_PMU_LEGACY_CYCLE;
+	else if (attr->config == PERF_COUNT_HW_INSTRUCTIONS)
+		return RISCV_PMU_LEGACY_INSTRET;
+	else
+		return -EOPNOTSUPP;
+}
+
+/* For legacy config & counter index are same */
+static int pmu_legacy_map_event(struct perf_event *event, u64 *config)
+{
+	return pmu_legacy_get_ctr_idx(event);
+}
+
+static u64 pmu_legacy_read_ctr(struct perf_event *event)
+{
+	struct hw_perf_event *hwc = &event->hw;
+	int idx = hwc->idx;
+	u64 val;
+
+	if (idx == RISCV_PMU_LEGACY_CYCLE)
+		val = riscv_pmu_read_ctr_csr(CSR_CYCLE);
+	else if (idx == RISCV_PMU_LEGACY_INSTRET)
+		val = riscv_pmu_read_ctr_csr(CSR_INSTRET);
+	else
+		return 0;
+
+	return val;
+}
+
+static void pmu_legacy_start_ctr(struct perf_event *event, u64 ival)
+{
+	struct hw_perf_event *hwc = &event->hw;
+	u64 initial_val = pmu_legacy_read_ctr(event);
+
+	/**
+	 * The legacy method doesn't really have a start/stop method.
+	 * It also can not update the counter with a initial value.
+	 * But we still need to set the prev_count so that read() can compute
+	 * the delta. Just use the current counter value to set the prev_count.
+	 */
+	local64_set(&hwc->prev_count, initial_val);
+}
+
+/**
+ * This is just a simple implementation to allow legacy implementations
+ * compatible with new RISC-V PMU driver framework.
+ * This driver only allows reading two counters i.e CYCLE & INSTRET.
+ * However, it can not start or stop the counter. Thus, it is not very useful
+ * will be removed in future.
+ */
+void riscv_pmu_legacy_init(struct riscv_pmu *pmu)
+{
+
+	pmu->num_counters = RISCV_PMU_LEGACY_NUM_CTR;
+	pmu->start_ctr = pmu_legacy_start_ctr;
+	pmu->stop_ctr = NULL;
+	pmu->map_event = pmu_legacy_map_event;
+	pmu->get_ctr_idx = pmu_legacy_get_ctr_idx;
+	pmu->get_ctr_width = NULL;
+	pmu->clear_ctr_idx = NULL;
+	pmu->read_ctr = pmu_legacy_read_ctr;
+}

include/linux/perf/riscv_pmu.h

@@ -52,8 +52,11 @@ struct riscv_pmu {
 };
 
 #define to_riscv_pmu(p) (container_of(p, struct riscv_pmu, pmu))
+
 u64 riscv_pmu_read_ctr_csr(unsigned long csr);
 
+void riscv_pmu_legacy_init(struct riscv_pmu *pmu);
+
 #endif /* CONFIG_RISCV_PMU */
 
 #endif /* _ASM_RISCV_PERF_EVENT_H */