[RFC PATCH v2 16/35] ACPI: processor: Register CPUs that are online, but not described in the DSDT

[James Morse]

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
registered.

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
taint.

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

Signed-off-by: James Morse <james.morse at arm.com>
---
 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 b4bde78121bb..a01e315aa16a 100644
--- a/drivers/acpi/acpi_processor.c
+++ b/drivers/acpi/acpi_processor.c
@@ -790,6 +790,25 @@ void __init acpi_processor_init(void)
 	acpi_pcc_cpufreq_init();
 }
 
+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);
+			add_taint(TAINT_FIRMWARE_WORKAROUND, LOCKDEP_STILL_OK);
+			arch_register_cpu(cpu);
+		}
+	}
+
+	return 0;
+}
+subsys_initcall_sync(acpi_processor_register_missing_cpus);
+
 #ifdef CONFIG_ACPI_PROCESSOR_CSTATE
 /**
  * acpi_processor_claim_cst_control - Request _CST control from the platform.
-- 
2.39.2