[PATCH RFC v3 03/21] ACPI: processor: Register CPUs that are online, but not described in the DSDT

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From: James Morse <james.morse@xxxxxxx>

ACPI has two descriptions of CPUs, one in the MADT/APIC table, the other
in the DSDT. Both are required. (ACPI 6.5's 8.4 "Declaring Processors"
says "Each processor in the system must be declared in the ACPI
namespace"). Having two descriptions allows firmware authors to get
this wrong.

If CPUs are described in the MADT/APIC, they will be brought online
early during boot. Once the register_cpu() calls are moved to ACPI,
they will be based on the DSDT description of the CPUs. When CPUs are
missing from the DSDT description, they will end up online, but not

Add a helper that runs after acpi_init() has completed to register
CPUs that are online, but weren't found in the DSDT. Any CPU that
is registered by this code triggers a firmware-bug warning and kernel

Qemu TCG only describes the first CPU in the DSDT, unless cpu-hotplug
is configured.

Signed-off-by: James Morse <james.morse@xxxxxxx>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>
Tested-by: Miguel Luis <miguel.luis@xxxxxxxxxx>
Tested-by: Vishnu Pajjuri <vishnu@xxxxxxxxxxxxxxxxxxxxxx>
Tested-by: Jianyong Wu <jianyong.wu@xxxxxxx>
Signed-off-by: Russell King (Oracle) <rmk+kernel@xxxxxxxxxxxxxxx>
 drivers/acpi/acpi_processor.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
index 6a542e0ce396..0511f2bc10bc 100644
--- a/drivers/acpi/acpi_processor.c
+++ b/drivers/acpi/acpi_processor.c
@@ -791,6 +791,25 @@ void __init acpi_processor_init(void)
+static int __init acpi_processor_register_missing_cpus(void)
+	int cpu;
+	if (acpi_disabled)
+		return 0;
+	for_each_online_cpu(cpu) {
+		if (!get_cpu_device(cpu)) {
+			pr_err_once(FW_BUG "CPU %u has no ACPI namespace description!\n", cpu);
+			arch_register_cpu(cpu);
+		}
+	}
+	return 0;
  * acpi_processor_claim_cst_control - Request _CST control from the platform.

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