[RFC PATCH 04/32] drivers: base: Allow parts of GENERIC_CPU_DEVICES to be overridden

[James Morse <james.morse@arm.com>]

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>
---
 arch/ia64/include/asm/cpu.h      |  1 -
 arch/loongarch/include/asm/cpu.h |  1 -
 arch/x86/include/asm/cpu.h       |  1 -
 drivers/base/cpu.c               | 14 ++++++++++----
 include/linux/cpu.h              |  5 +++++
 5 files changed, 15 insertions(+), 7 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/loongarch/include/asm/cpu.h b/arch/loongarch/include/asm/cpu.h
index da79862ff1f3..1e2c7c61dbea 100644
--- a/arch/loongarch/include/asm/cpu.h
+++ b/arch/loongarch/include/asm/cpu.h
@@ -126,7 +126,6 @@ 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 78796b98a544..a0a62ac00e88 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 start_cpu0(void);
 #ifdef CONFIG_DEBUG_HOTPLUG_CPU0
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index cf6407c34ede..178936533d87 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -492,19 +492,25 @@ 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 314802f98b9d..86e79e702325 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -75,12 +75,17 @@ 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
-- 
2.30.2