[PATCH v7 13/16] arm64: arch_register_cpu() variant to check if an ACPI handle is now available.

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The ARM64 architecture does not support physical CPU HP today.
To avoid any possibility of a bug against such an architecture if defined
in future, check for the physical CPU HP case (not present) and
return an error on any such attempt.

On ARM64 virtual CPU Hotplug relies on the status value that can be
queried via the AML method _STA for the CPU object.

There are two conditions in which the CPU can be registered.
1) ACPI disabled.
2) ACPI enabled and the acpi_handle is available.
   _STA evaluates to the CPU is both enabled and present.
   (Note that in absence of the _STA method they are always in this

If neither of these conditions is met the CPU is not 'yet' ready
to be used and -EPROBE_DEFER is returned.

Success occurs in the early attempt to register the CPUs if we
are booting with DT (no concept yet of vCPU HP) if not it succeeds
for already enabled CPUs when the ACPI Processor driver attaches to
them.  Finally it may succeed via the CPU Hotplug code indicating that
the CPU is now enabled.

For ACPI if CONFIG_ACPI_PROCESSOR the only path to get to
arch_register_cpu() with that handle set is via
acpi_processor_hot_add_init() which is only called from an ACPI bus
scan in which _STA has already been queried there is no need to
repeat it here. Add a comment to remind us of this in the future.

Suggested-by: Rafael J. Wysocki <rafael@xxxxxxxxxx>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
v7: No change.
v6: Add protection again Physical CPU HP to the arch specific code
    and don't actually check _STA

Tested on arm64 with ACPI + DT build and DT only builds, booting
with ACPI and DT as appropriate.
 arch/arm64/kernel/smp.c | 53 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index dc0e0b3ec2d4..ccb6ad347df9 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -504,6 +504,59 @@ static int __init smp_cpu_setup(int cpu)
 static bool bootcpu_valid __initdata;
 static unsigned int cpu_count = 1;
+int arch_register_cpu(int cpu)
+	acpi_handle acpi_handle = acpi_get_processor_handle(cpu);
+	struct cpu *c = &per_cpu(cpu_devices, cpu);
+	if (!acpi_disabled && !acpi_handle &&
+		return -EPROBE_DEFER;
+	/* For now block anything that looks like physical CPU Hotplug */
+	if (invalid_logical_cpuid(cpu) || !cpu_present(cpu)) {
+		pr_err_once("Changing CPU present bit is not supported\n");
+		return -ENODEV;
+	}
+	/*
+	 * Availability of the acpi handle is sufficient to establish
+	 * that _STA has aleady been checked. No need to recheck here.
+	 */
+	c->hotpluggable = arch_cpu_is_hotpluggable(cpu);
+	return register_cpu(c, cpu);
+void arch_unregister_cpu(int cpu)
+	acpi_handle acpi_handle = acpi_get_processor_handle(cpu);
+	struct cpu *c = &per_cpu(cpu_devices, cpu);
+	acpi_status status;
+	unsigned long long sta;
+	if (!acpi_handle) {
+		pr_err_once("Removing a CPU without associated ACPI handle\n");
+		return;
+	}
+	status = acpi_evaluate_integer(acpi_handle, "_STA", NULL, &sta);
+	if (ACPI_FAILURE(status))
+		return;
+	/* For now do not allow anything that looks like physical CPU HP */
+	if (cpu_present(cpu) && !(sta & ACPI_STA_DEVICE_PRESENT)) {
+		pr_err_once("Changing CPU present bit is not supported\n");
+		return;
+	}
+	unregister_cpu(c);
 static struct acpi_madt_generic_interrupt cpu_madt_gicc[NR_CPUS];

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