linux-kernel - [PATCH v4 04/10] x86/dt: Parse the Wakeup Mailbox for Intel processors

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Message-Id: <20250603-rneri-wakeup-mailbox-v4-4-d533272b7232@linux.intel.com>
Date: Tue, 03 Jun 2025 17:15:16 -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>, 
 "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Saurabh Sengar <ssengar@...ux.microsoft.com>, 
 Chris Oo <cho@...rosoft.com>, 
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
 linux-hyperv@...r.kernel.org, devicetree@...r.kernel.org, 
 linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org, 
 "Ravi V. Shankar" <ravi.v.shankar@...el.com>, 
 Ricardo Neri <ricardo.neri@...el.com>, 
 Yunhong Jiang <yunhong.jiang@...ux.intel.com>, 
 Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Subject: [PATCH v4 04/10] x86/dt: Parse the Wakeup Mailbox for Intel
 processors

The Wakeup Mailbox is a mechanism to boot secondary CPUs used on systems
that do not want or cannot use the INIT + StartUp IPI messages.

The platform firmware is expected to implement the mailbox as described in
the Multiprocessor Wakeup Structure of the ACPI specification. It is also
expected to publish the mailbox to the operating system as described in the
corresponding DeviceTree schema that accompanies the documentation of the
Linux kernel.

Reuse the existing functionality to set the memory location of the mailbox
and update the wakeup_secondary_cpu_64() APIC callback.

do_boot_cpu() uses wakeup_secondary_cpu_64() when set. If a wakeup mailbox
is found (enumerated via an ACPI table or a DeviceTree node) it will be
used unconditionally. For cases in which this behavior is not desired, this
APIC callback can be updated later during boot using platform-specific
hooks.

Co-developed-by: Yunhong Jiang <yunhong.jiang@...ux.intel.com>
Signed-off-by: Yunhong Jiang <yunhong.jiang@...ux.intel.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
---
Changes since v3:
 - Look for the wakeup mailbox unconditionally, regardless of whether
   cpu@N nodes have an `enable-method` property.
 - Add a reference to the ACPI specification. (Rafael)

Changes since v2:
 - Added extra sanity checks when parsing the mailbox node.
 - Probe the mailbox using its `compatible` property
 - Setup the Wakeup Mailbox if the `enable-method` is found in the CPU
   nodes.
 - Cleaned up unneeded ifdeffery.
 - Clarified the mechanisms used to override the wakeup_secondary_64()
   callback to not use the mailbox when not desired. (Michael)
 - Edited the commit message for clarity.

Changes since v1:
 - Disabled CPU offlining.
 - Modified dtb_parse_mp_wake() to return the address of the mailbox.
---
 arch/x86/kernel/devicetree.c | 47 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/arch/x86/kernel/devicetree.c b/arch/x86/kernel/devicetree.c
index dd8748c45529..494a560614a8 100644
--- a/arch/x86/kernel/devicetree.c
+++ b/arch/x86/kernel/devicetree.c
@@ -17,6 +17,7 @@
 #include <linux/pci.h>
 #include <linux/of_pci.h>
 #include <linux/initrd.h>
+#include <linux/smp.h>

 #include <asm/irqdomain.h>
 #include <asm/hpet.h>
@@ -125,6 +126,51 @@ static void __init dtb_setup_hpet(void)
 #endif
 }

+#if defined(CONFIG_X86_64) && defined(CONFIG_SMP)
+
+#define WAKEUP_MAILBOX_SIZE	0x1000
+#define WAKEUP_MAILBOX_ALIGN	0x1000
+
+/** dtb_wakeup_mailbox_setup() - Parse the wakeup mailbox from the device tree
+ *
+ * Look for the presence of a wakeup mailbox in the DeviceTree. The mailbox is
+ * expected to follow the structure and operation described in the Multiprocessor
+ * Wakeup Structure of the ACPI specification.
+ */
+static void __init dtb_wakeup_mailbox_setup(void)
+{
+	struct device_node *node;
+	struct resource res;
+
+	node = of_find_compatible_node(NULL, NULL, "intel,wakeup-mailbox");
+	if (!node)
+		return;
+
+	if (of_address_to_resource(node, 0, &res))
+		goto done;
+
+	/* The mailbox is a 4KB-aligned region.*/
+	if (res.start & (WAKEUP_MAILBOX_ALIGN - 1))
+		goto done;
+
+	/* The mailbox has a size of 4KB. */
+	if (res.end - res.start + 1 != WAKEUP_MAILBOX_SIZE)
+		goto done;
+
+	/* Not supported when the mailbox is used. */
+	cpu_hotplug_disable_offlining();
+
+	acpi_setup_mp_wakeup_mailbox(res.start);
+done:
+	of_node_put(node);
+}
+#else /* !CONFIG_X86_64 || !CONFIG_SMP */
+static inline int dtb_wakeup_mailbox_setup(void)
+{
+	return -EOPNOTSUPP;
+}
+#endif /* CONFIG_X86_64 && CONFIG_SMP */
+
 #ifdef CONFIG_X86_LOCAL_APIC

 static void __init dtb_cpu_setup(void)
@@ -287,6 +333,7 @@ static void __init x86_dtb_parse_smp_config(void)

 	dtb_setup_hpet();
 	dtb_apic_setup();
+	dtb_wakeup_mailbox_setup();
 }

 void __init x86_flattree_get_config(void)

-- 
2.43.0