drivers: base: Allow parts of GENERIC_CPU_DEVICES to be overridden

architectures often have extra per-cpu work that needs doing
before a CPU is registered, often to determine if a CPU is
hotpluggable.

To allow more architectures to use GENERIC_CPU_DEVICES, wrap the call
as a __weak arch_register_cpu(). This aligns with the way x86, ia64
and loongarch register hotplug CPUs when they become present.

ACPI's acpi_processor.c also has a __weak version of this symbol
because arm64 doesn't define one. The duplicate __weak definitions
are only a problem if arm64 selects GENERIC_CPU_DEVICES without
defining one. This gets fixed up in later patches.

Signed-off-by: James Morse <james.morse@arm.com>
---
Changes since RFC:
 * Dropped __init from x86/ia64 arch_register_cpu()

fixup for prev

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

7 files changed, 17 insertions, 9 deletions

diff --git a/arch/ia64/include/asm/cpu.h b/arch/ia64/include/asm/cpu.h
index db125df9e088..a3e690e685e5 100644
--- a/arch/ia64/include/asm/cpu.h
+++ b/arch/ia64/include/asm/cpu.h
@@ -16,7 +16,6 @@ DECLARE_PER_CPU(struct ia64_cpu, cpu_devices);
 DECLARE_PER_CPU(int, cpu_state);
 
 #ifdef CONFIG_HOTPLUG_CPU
-extern int arch_register_cpu(int num);
 extern void arch_unregister_cpu(int);
 #endif
 
diff --git a/arch/ia64/kernel/topology.c b/arch/ia64/kernel/topology.c
index 94a848b06f15..741863a187a6 100644
--- a/arch/ia64/kernel/topology.c
+++ b/arch/ia64/kernel/topology.c
@@ -59,7 +59,7 @@ void __ref arch_unregister_cpu(int num)
 }
 EXPORT_SYMBOL(arch_unregister_cpu);
 #else
-static int __init arch_register_cpu(int num)
+int __init arch_register_cpu(int num)
 {
 	return register_cpu(&sysfs_cpus[num].cpu, num);
 }
diff --git a/arch/loongarch/include/asm/cpu.h b/arch/loongarch/include/asm/cpu.h
index 9be65d86aba6..3d977732538b 100644
--- a/arch/loongarch/include/asm/cpu.h
+++ b/arch/loongarch/include/asm/cpu.h
@@ -130,7 +130,6 @@ enum cpu_type_enum {
 
 #if !defined(__ASSEMBLY__)
 #ifdef CONFIG_HOTPLUG_CPU
-extern int arch_register_cpu(int num);
 extern void arch_unregister_cpu(int);
 #endif
 #endif /* ! __ASSEMBLY__ */
diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index 3a233ebff712..96dc4665e87d 100644
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -28,7 +28,6 @@ struct x86_cpu {
 };
 
 #ifdef CONFIG_HOTPLUG_CPU
-extern int arch_register_cpu(int num);
 extern void arch_unregister_cpu(int);
 extern void soft_restart_cpu(void);
 #endif
diff --git a/arch/x86/kernel/topology.c b/arch/x86/kernel/topology.c
index ca004e2e4469..0bab03130033 100644
--- a/arch/x86/kernel/topology.c
+++ b/arch/x86/kernel/topology.c
@@ -54,7 +54,7 @@ void arch_unregister_cpu(int num)
 EXPORT_SYMBOL(arch_unregister_cpu);
 #else /* CONFIG_HOTPLUG_CPU */
 
-static int __init arch_register_cpu(int num)
+int __init arch_register_cpu(int num)
 {
 	return register_cpu(&per_cpu(cpu_devices, num).cpu, num);
 }
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 4a65a728c8c4..eb0d28a38b61 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -493,19 +493,25 @@ bool cpu_is_hotpluggable(unsigned int cpu)
 EXPORT_SYMBOL_GPL(cpu_is_hotpluggable);
 
 #ifdef CONFIG_GENERIC_CPU_DEVICES
-static DEFINE_PER_CPU(struct cpu, cpu_devices);
+DEFINE_PER_CPU(struct cpu, cpu_devices);
+
+int __weak arch_register_cpu(int cpu)
+{
+	return register_cpu(&per_cpu(cpu_devices, cpu), cpu);
+}
 #endif
 
 static void __init cpu_dev_register_generic(void)
 {
-#ifdef CONFIG_GENERIC_CPU_DEVICES
 	int i;
 
+	if (!IS_ENABLED(CONFIG_GENERIC_CPU_DEVICES))
+		return;
+
 	for_each_present_cpu(i) {
-		if (register_cpu(&per_cpu(cpu_devices, i), i))
+		if (arch_register_cpu(i))
 			panic("Failed to register CPU device");
 	}
-#endif
 }
 
 #ifdef CONFIG_GENERIC_CPU_VULNERABILITIES
diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index e006c719182b..7efa3adc65a2 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -79,12 +79,17 @@ extern __printf(4, 5)
 struct device *cpu_device_create(struct device *parent, void *drvdata,
 				 const struct attribute_group **groups,
 				 const char *fmt, ...);
+extern int arch_register_cpu(int cpu);
 #ifdef CONFIG_HOTPLUG_CPU
 extern void unregister_cpu(struct cpu *cpu);
 extern ssize_t arch_cpu_probe(const char *, size_t);
 extern ssize_t arch_cpu_release(const char *, size_t);
 #endif
 
+#ifdef CONFIG_GENERIC_CPU_DEVICES
+DECLARE_PER_CPU(struct cpu, cpu_devices);
+#endif
+
 /*
  * These states are not related to the core CPU hotplug mechanism. They are
  * used by various (sub)architectures to track internal state