Re: [PATCH v2] MIPS: Expose prid and globalnumber to sysfs

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@flygoat.com>
---
v2: Drop static qualifier for kobj (gregkh)
---
  .../ABI/testing/sysfs-devices-system-cpu      | 11 +++
  arch/mips/kernel/topology.c                   | 96 +++++++++++++++++++
  2 files changed, 107 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 [off-list ref]
+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..0e3730f3c00f 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 +28,97 @@ 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;
+};
+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);	\