[PATCH v4] MIPS: Expose prid and globalnumber to sysfs

[Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx>]

Some application would like to know precise model and rev of processor
to do errata workaround or optimization.

Expose them in sysfs as:
/sys/devices/system/cpu/cpuX/regs/identification/prid
/sys/devices/system/cpu/cpuX/regs/identification/globalnumber

Reusing AArch64 CPU registers directory.

Signed-off-by: Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx>
---
v2: Drop static qualifier for kobj (gregkh)
v3: Use kzalloc to allocate struct cpuregs.
    note: When Greg mentioned about static I was thinking about
    static qualifier of percpu variable. After reading documents
    again it turns out kobjs should be allocated at runtime. Arm64's
    cpuinfo kobj is also on a percpu variable... I guess that was a
    intentional use?
v4: Properly handle err of kobj creation. (gregkh)
---
 .../ABI/testing/sysfs-devices-system-cpu      |  11 ++
 arch/mips/kernel/topology.c                   | 103 ++++++++++++++++++
 2 files changed, 114 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 5bf61881f012..adf855e7bb9b 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -512,6 +512,17 @@ Description:	information about CPUs heterogeneity.
 
 		cpu_capacity: capacity of cpuX.
 
+What:		/sys/devices/system/cpu/cpuX/regs/
+		/sys/devices/system/cpu/cpuX/regs/identification/
+		/sys/devices/system/cpu/cpuX/regs/identification/prid
+		/sys/devices/system/cpu/cpuX/regs/identification/globalnumber
+Date:		Augest 2022
+Contact:	Linux MIPS Kernel Mailing list <linux-mips@xxxxxxxxxxxxxxx>
+Description:	MIPS CPU registers
+
+		'identification' directory exposes the Processor ID and Global Number
+		registers for identifying model and revision of the CPU.
+
 What:		/sys/devices/system/cpu/vulnerabilities
 		/sys/devices/system/cpu/vulnerabilities/meltdown
 		/sys/devices/system/cpu/vulnerabilities/spectre_v1
diff --git a/arch/mips/kernel/topology.c b/arch/mips/kernel/topology.c
index 9429d85a4703..45a15eda651a 100644
--- a/arch/mips/kernel/topology.c
+++ b/arch/mips/kernel/topology.c
@@ -5,6 +5,8 @@
 #include <linux/node.h>
 #include <linux/nodemask.h>
 #include <linux/percpu.h>
+#include <linux/seq_file.h>
+#include <linux/smp.h>
 
 static DEFINE_PER_CPU(struct cpu, cpu_devices);
 
@@ -26,3 +28,104 @@ static int __init topology_init(void)
 }
 
 subsys_initcall(topology_init);
+
+static struct kobj_type cpuregs_kobj_type = {
+	.sysfs_ops = &kobj_sysfs_ops,
+};
+
+struct cpureg {
+	struct kobject kobj;
+	struct cpuinfo_mips *info;
+};
+static DEFINE_PER_CPU(struct cpureg *, cpuregs);
+
+#define kobj_to_cpureg(kobj)	container_of(kobj, struct cpureg, kobj)
+#define CPUREGS_ATTR_RO(_name, _field)						\
+	static ssize_t _name##_show(struct kobject *kobj,			\
+			struct kobj_attribute *attr, char *buf)			\
+	{									\
+		struct cpuinfo_mips *info = kobj_to_cpureg(kobj)->info;		\
+										\
+		return sprintf(buf, "0x%08x\n", info->_field);	\
+	}									\
+	static struct kobj_attribute cpuregs_attr_##_name = __ATTR_RO(_name)
+
+CPUREGS_ATTR_RO(prid, processor_id);
+CPUREGS_ATTR_RO(globalnumber, globalnumber);
+
+static struct attribute *cpuregs_id_attrs[] = {
+	&cpuregs_attr_prid.attr,
+	&cpuregs_attr_globalnumber.attr,
+	NULL
+};
+
+static const struct attribute_group cpuregs_attr_group = {
+	.attrs = cpuregs_id_attrs,
+	.name = "identification"
+};
+
+static int cpuregs_cpu_online(unsigned int cpu)
+{
+	int rc;
+	struct device *dev;
+	struct cpureg *reg;
+
+	dev = get_cpu_device(cpu);
+	if (!dev) {
+		rc = -ENODEV;
+		goto out;
+	}
+	reg = kzalloc(sizeof(struct cpureg), GFP_KERNEL);
+	if (!reg) {
+		rc = -ENOMEM;
+		goto out;
+	}
+	rc = kobject_init_and_add(&reg->kobj, &cpuregs_kobj_type,
+					&dev->kobj, "regs");
+	if (rc)
+		goto out_kfree;
+	rc = sysfs_create_group(&reg->kobj, &cpuregs_attr_group);
+	if (rc)
+		goto out_kobj;
+
+	return 0;
+out_kobj:
+	kobject_put(&reg->kobj);
+out_kfree:
+	kfree(reg);
+out:
+	return rc;
+}
+
+static int cpuregs_cpu_offline(unsigned int cpu)
+{
+	struct device *dev;
+	struct cpureg *reg = per_cpu(cpuregs, cpu);
+
+	dev = get_cpu_device(cpu);
+	if (!dev || !reg)
+		return -ENODEV;
+	if (reg->kobj.parent) {
+		sysfs_remove_group(&reg->kobj, &cpuregs_attr_group);
+		kobject_put(&reg->kobj);
+	}
+	kfree(reg);
+
+	return 0;
+}
+
+static int __init cpuinfo_regs_init(void)
+{
+	int ret;
+
+
+	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "mips/topology:online",
+				cpuregs_cpu_online, cpuregs_cpu_offline);
+	if (ret < 0) {
+		pr_err("cpuinfo: failed to register hotplug callbacks.\n");
+		return ret;
+	}
+	return 0;
+}
+
+device_initcall(cpuinfo_regs_init);
-- 
2.34.1