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

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Message-Id: <20250503191515.24041-6-ricardo.neri-calderon@linux.intel.com>
Date: Sat,  3 May 2025 12:15:07 -0700
From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To: x86@...nel.org,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Rob Herring <robh@...nel.org>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>,
	Dexuan Cui <decui@...rosoft.com>,
	Michael Kelley <mhklinux@...look.com>
Cc: devicetree@...r.kernel.org,
	Saurabh Sengar <ssengar@...ux.microsoft.com>,
	Chris Oo <cho@...rosoft.com>,
	linux-hyperv@...r.kernel.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-acpi@...r.kernel.org ,
	linux-kernel@...r.kernel.org,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Ricardo Neri <ricardo.neri@...el.com>
Subject: [PATCH v3 05/13] x86/dt: Parse the `enable-method` property of CPU nodes

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@...ux.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