[PATCH v3 05/13] x86/dt: Parse the `enable-method` property of CPU nodes

Ricardo Neri
2025-05-03T19:10:15Z

Add functionality to parse and validate the `enable-method` property for
platforms that use alternative methods to wakeup secondary CPUs (e.g., a
wakeup mailbox).

Most x86 platforms boot secondary CPUs using INIT assert, de-assert
followed by a Start-Up IPI messages. These systems do no need to specify an
`enable-method` property in the cpu@N nodes of the DeviceTree.

Although it is possible to specify a different `enable-method` for each
secondary CPU, the existing functionality relies on using the
APIC wakeup_secondary_cpu{ (), _64()} callback to wake up all CPUs. Ensure
that either all CPUs specify the same `enable-method` or none at all.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
---
Changes since v2:
- Introduced this patch.

Changes since v1:
- N/A
---
arch/x86/kernel/devicetree.c | 88 +++++++++++++++++++++++++++++++++++-
1 file changed, 86 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/devicetree.c b/arch/x86/kernel/devicetree.c
index dd8748c45529..5835afc74acd 100644
--- a/arch/x86/kernel/devicetree.c
+++ b/arch/x86/kernel/devicetree.c
@@ -127,8 +127,59 @@ static void __init dtb_setup_hpet(void)

#ifdef CONFIG_X86_LOCAL_APIC

+#ifdef CONFIG_SMP
+static const char *dtb_supported_enable_methods[] __initconst = { };
+
+static bool __init dtb_enable_method_is_valid(const char *enable_method_a,
+ const char *enable_method_b)
+{
+ int i;
+
+ if (!enable_method_a && !enable_method_b)
+ return true;
+
+ if (strcmp(enable_method_a, enable_method_b))
+ return false;
+
+ for (i = 0; i < ARRAY_SIZE(dtb_supported_enable_methods); i++) {
+ if (!strcmp(enable_method_a, dtb_supported_enable_methods[i]))
+ return true;
+ }
+
+ return false;
+}
+
+static int __init dtb_configure_enable_method(const char *enable_method)
+{
+ /* Nothing to do for a missing enable-method or if the system has one CPU */
+ if (!enable_method || IS_ERR(enable_method))
+ return 0;
+
+ return -ENOTSUPP;
+}
+#else /* !CONFIG_SMP */
+static inline bool dtb_enable_method_is_valid(const char *enable_method_a,
+ const char *enable_method_b)
+{
+ /* No secondary CPUs. We do not care about the enable-method. */
+ return true;
+}
+
+static inline int dtb_configure_enable_method(const char *enable_method)
+{
+ return 0;
+}
+#endif /* CONFIG_SMP */
+
+static void __init dtb_register_apic_id(u32 apic_id, struct device_node *dn)
+{
+ topology_register_apic(apic_id, CPU_ACPIID_INVALID, true);
+ set_apicid_to_node(apic_id, of_node_to_nid(dn));
+}
+
static void __init dtb_cpu_setup(void)
{
+ const char *enable_method = ERR_PTR(-EINVAL), *this_em;
struct device_node *dn;
u32 apic_id;

@@ -138,9 +189,42 @@ static void __init dtb_cpu_setup(void)
pr_warn("%pOF: missing local APIC ID\n", dn);
continue;
}
- topology_register_apic(apic_id, CPU_ACPIID_INVALID, true);
- set_apicid_to_node(apic_id, of_node_to_nid(dn));
+
+ /*
+ * Also check the enable-method of the secondary CPUs, if present.
+ *
+ * Systems that use the INIT-!INIT-StartUp IPI sequence to boot
+ * secondary CPUs do not need to define an enable-method.
+ *
+ * All CPUs must have the same enable-method. The enable-method
+ * must be supported. If absent in one secondary CPU, it must be
+ * absent for all CPUs.
+ *
+ * Compare the first secondary CPU with the rest. We do not care
+ * about the boot CPU, as it is enabled already.
+ */
+
+ if (apic_id == boot_cpu_physical_apicid) {
+ dtb_register_apic_id(apic_id, dn);
+ continue;
+ }
+
+ this_em = of_get_property(dn, "enable-method", NULL);
+
+ if (IS_ERR(enable_method)) {
+ enable_method = this_em;
+ dtb_register_apic_id(apic_id, dn);
+ continue;
+ }
+
+ if (!dtb_enable_method_is_valid(enable_method, this_em))
+ continue;
+
+ dtb_register_apic_id(apic_id, dn);
}
+
+ if (dtb_configure_enable_method(enable_method))
+ pr_err("enable-method '%s' needed but not configured\n", enable_method);
}

static void __init dtb_lapic_setup(void)
--
2.43.0