Add DeviceTree bindings to enumerate the wakeup mailbox used in platform
firmware for Intel processors.

x86 platforms commonly boot secondary CPUs using an INIT assert, de-assert
followed by Start-Up IPI messages. The wakeup mailbox can be used when this
mechanism is unavailable.

The wakeup mailbox offers more control to the operating system to boot
secondary CPUs than a spin-table. It allows the reuse of the same wakeup
vector for all CPUs while maintaining control over which CPUs to boot and
when. While it is possible to achieve the same level of control using a
spin-table, it would require specifying a separate `cpu-release-addr` for
each secondary CPU.

The operation and structure of the mailbox are described in the
Multiprocessor Wakeup Structure defined in the ACPI specification. Note
that this structure does not specify how to publish the mailbox to the
operating system (ACPI-based platform firmware uses a separate table). No
ACPI table is needed in DeviceTree-based firmware to enumerate the mailbox.

Nodes that want to refer to the reserved memory usually define
a `memory-region` property. /cpus/cpu* nodes would want to refer to the
mailbox, but they do not have such property defined in the DeviceTree
specification. Moreover, it would imply that there is a memory region per
CPU. Instead, add a `compatible` property that the operating system can use
to discover the mailbox.

Reviewed-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Rafael J. Wysocki (Intel) <rafael.j.wysocki@intel.com>
Co-developed-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>
Signed-off-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
---
Changes in v8:
 - None

Changes in v7:
 - Fixed Acked-by tag from Rafael to include the "(Intel)" suffix.

Changes in v6:
 - Reworded the changelog for clarity.
 - Added Acked-by tag from Rafael. Thanks!
 - Added Reviewed-by tag from Rob. Thanks!
 - Added Reviewed-by tag from Dexuan. Thanks!

Changes in v5:
 - Specified the version and section of the ACPI spec in which the
   wakeup mailbox is defined. (Rafael)
 - Fixed a warning from yamllint about line lengths of URLs.

Changes in v4:
 - Removed redefinitions of the mailbox and instead referred to ACPI
   specification as per discussion on LKML.
 - Clarified that DeviceTree-based firmware do not require the use of
   ACPI tables to enumerate the mailbox. (Rob)
 - Described the need of using a `compatible` property.
 - Dropped the `alignment` property. (Krzysztof, Rafael)
 - Used a real address for the mailbox node. (Krzysztof)

Changes in v3:
 - Implemented the mailbox as a reserved-memory node. Add to it a
   `compatible` property. (Krzysztof)
 - Explained the relationship between the mailbox and the `enable-mehod`
   property of the CPU nodes.
 - Expanded the documentation of the binding.

Changes in v2:
 - Added more details to the description of the binding.
 - Added requirement a new requirement for cpu@N nodes to add an
   `enable-method`.
---
 .../reserved-memory/intel,wakeup-mailbox.yaml      | 50 ++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/Documentation/devicetree/bindings/reserved-memory/intel,wakeup-mailbox.yaml b/Documentation/devicetree/bindings/reserved-memory/intel,wakeup-mailbox.yaml
new file mode 100644
index 000000000000..a80d3bac44c2
--- /dev/null
+++ b/Documentation/devicetree/bindings/reserved-memory/intel,wakeup-mailbox.yaml
@@ -0,0 +1,50 @@
+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/reserved-memory/intel,wakeup-mailbox.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Wakeup Mailbox for Intel processors
+
+description: |
+  The Wakeup Mailbox provides a mechanism for the operating system to wake up
+  secondary CPUs on Intel processors. It is an alternative to the INIT-!INIT-
+  SIPI sequence used on most x86 systems.
+
+  The structure and operation of the mailbox is described in the Multiprocessor
+  Wakeup Structure of the ACPI specification version 6.6 section 5.2.12.19 [1].
+
+  The implementation of the mailbox in platform firmware is described in the
+  Intel TDX Virtual Firmware Design Guide section 4.3.5 [2].
+
+  1: https://uefi.org/specs/ACPI/6.6/05_ACPI_Software_Programming_Model.html#multiprocessor-wakeup-structure
+  2: https://www.intel.com/content/www/us/en/content-details/733585/intel-tdx-virtual-firmware-design-guide.html
+
+
+maintainers:
+  - Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
+
+allOf:
+  - $ref: reserved-memory.yaml
+
+properties:
+  compatible:
+    const: intel,wakeup-mailbox
+
+required:
+  - compatible
+  - reg
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    reserved-memory {
+        #address-cells = <2>;
+        #size-cells = <1>;
+
+        wakeup-mailbox@ffff0000 {
+            compatible = "intel,wakeup-mailbox";
+            reg = <0x0 0xffff0000 0x1000>;
+        };
+    };

-- 
2.43.0

On Wed, 07 Jan 2026 13:44:39 -0800, Ricardo Neri wrote:
> Add DeviceTree bindings to enumerate the wakeup mailbox used in platform
> firmware for Intel processors.
> 
> x86 platforms commonly boot secondary CPUs using an INIT assert, de-assert
> followed by Start-Up IPI messages. The wakeup mailbox can be used when this
> mechanism is unavailable.
> 
> The wakeup mailbox offers more control to the operating system to boot
> secondary CPUs than a spin-table. It allows the reuse of the same wakeup
> vector for all CPUs while maintaining control over which CPUs to boot and
> when. While it is possible to achieve the same level of control using a
> spin-table, it would require specifying a separate `cpu-release-addr` for
> each secondary CPU.
> 
> The operation and structure of the mailbox are described in the
> Multiprocessor Wakeup Structure defined in the ACPI specification. Note
> that this structure does not specify how to publish the mailbox to the
> operating system (ACPI-based platform firmware uses a separate table). No
> ACPI table is needed in DeviceTree-based firmware to enumerate the mailbox.
> 
> Nodes that want to refer to the reserved memory usually define
> a `memory-region` property. /cpus/cpu* nodes would want to refer to the
> mailbox, but they do not have such property defined in the DeviceTree
> specification. Moreover, it would imply that there is a memory region per
> CPU. Instead, add a `compatible` property that the operating system can use
> to discover the mailbox.
> 
> Reviewed-by: Dexuan Cui <decui@microsoft.com>
> Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
> Acked-by: Rafael J. Wysocki (Intel) <rafael.j.wysocki@intel.com>
> Co-developed-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>
> Signed-off-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> Changes in v8:
>  - None
> 
> Changes in v7:
>  - Fixed Acked-by tag from Rafael to include the "(Intel)" suffix.
> 
> Changes in v6:
>  - Reworded the changelog for clarity.
>  - Added Acked-by tag from Rafael. Thanks!
>  - Added Reviewed-by tag from Rob. Thanks!
>  - Added Reviewed-by tag from Dexuan. Thanks!
> 
> Changes in v5:
>  - Specified the version and section of the ACPI spec in which the
>    wakeup mailbox is defined. (Rafael)
>  - Fixed a warning from yamllint about line lengths of URLs.
> 
> Changes in v4:
>  - Removed redefinitions of the mailbox and instead referred to ACPI
>    specification as per discussion on LKML.
>  - Clarified that DeviceTree-based firmware do not require the use of
>    ACPI tables to enumerate the mailbox. (Rob)
>  - Described the need of using a `compatible` property.
>  - Dropped the `alignment` property. (Krzysztof, Rafael)
>  - Used a real address for the mailbox node. (Krzysztof)
> 
> Changes in v3:
>  - Implemented the mailbox as a reserved-memory node. Add to it a
>    `compatible` property. (Krzysztof)
>  - Explained the relationship between the mailbox and the `enable-mehod`
>    property of the CPU nodes.
>  - Expanded the documentation of the binding.
> 
> Changes in v2:
>  - Added more details to the description of the binding.
>  - Added requirement a new requirement for cpu@N nodes to add an
>    `enable-method`.
> ---
>  .../reserved-memory/intel,wakeup-mailbox.yaml      | 50 ++++++++++++++++++++++
>  1 file changed, 50 insertions(+)
> 

My bot found errors running 'make dt_binding_check' on your patch:

yamllint warnings/errors:
./Documentation/devicetree/bindings/reserved-memory/intel,wakeup-mailbox.yaml:23:1: [warning] too many blank lines (2 > 1) (empty-lines)

dtschema/dtc warnings/errors:

doc reference errors (make refcheckdocs):

See https://patchwork.kernel.org/project/devicetree/patch/20260107-rneri-wakeup-mailbox-v8-3-2f5b6785f2f5@linux.intel.com

The base for the series is generally the latest rc1. A different dependency
should be noted in *this* patch.

If you already ran 'make dt_binding_check' and didn't see the above
error(s), then make sure 'yamllint' is installed and dt-schema is up to
date:

pip3 install dtschema --upgrade

Please check and re-submit after running the above command yourself. Note
that DT_SCHEMA_FILES can be set to your schema file to speed up checking
your schema. However, it must be unset to test all examples with your schema.

On Wed, Jan 07, 2026 at 05:23:51PM -0600, Rob Herring (Arm) wrote:
> 
> On Wed, 07 Jan 2026 13:44:39 -0800, Ricardo Neri wrote:
> > Add DeviceTree bindings to enumerate the wakeup mailbox used in platform
> > firmware for Intel processors.
> > 
> > x86 platforms commonly boot secondary CPUs using an INIT assert, de-assert
> > followed by Start-Up IPI messages. The wakeup mailbox can be used when this
> > mechanism is unavailable.
> > 
> > The wakeup mailbox offers more control to the operating system to boot
> > secondary CPUs than a spin-table. It allows the reuse of the same wakeup
> > vector for all CPUs while maintaining control over which CPUs to boot and
> > when. While it is possible to achieve the same level of control using a
> > spin-table, it would require specifying a separate `cpu-release-addr` for
> > each secondary CPU.
> > 
> > The operation and structure of the mailbox are described in the
> > Multiprocessor Wakeup Structure defined in the ACPI specification. Note
> > that this structure does not specify how to publish the mailbox to the
> > operating system (ACPI-based platform firmware uses a separate table). No
> > ACPI table is needed in DeviceTree-based firmware to enumerate the mailbox.
> > 
> > Nodes that want to refer to the reserved memory usually define
> > a `memory-region` property. /cpus/cpu* nodes would want to refer to the
> > mailbox, but they do not have such property defined in the DeviceTree
> > specification. Moreover, it would imply that there is a memory region per
> > CPU. Instead, add a `compatible` property that the operating system can use
> > to discover the mailbox.
> > 
> > Reviewed-by: Dexuan Cui <decui@microsoft.com>
> > Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
> > Acked-by: Rafael J. Wysocki (Intel) <rafael.j.wysocki@intel.com>
> > Co-developed-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>
> > Signed-off-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > ---
> > Changes in v8:
> >  - None
> > 
> > Changes in v7:
> >  - Fixed Acked-by tag from Rafael to include the "(Intel)" suffix.
> > 
> > Changes in v6:
> >  - Reworded the changelog for clarity.
> >  - Added Acked-by tag from Rafael. Thanks!
> >  - Added Reviewed-by tag from Rob. Thanks!
> >  - Added Reviewed-by tag from Dexuan. Thanks!
> > 
> > Changes in v5:
> >  - Specified the version and section of the ACPI spec in which the
> >    wakeup mailbox is defined. (Rafael)
> >  - Fixed a warning from yamllint about line lengths of URLs.
> > 
> > Changes in v4:
> >  - Removed redefinitions of the mailbox and instead referred to ACPI
> >    specification as per discussion on LKML.
> >  - Clarified that DeviceTree-based firmware do not require the use of
> >    ACPI tables to enumerate the mailbox. (Rob)
> >  - Described the need of using a `compatible` property.
> >  - Dropped the `alignment` property. (Krzysztof, Rafael)
> >  - Used a real address for the mailbox node. (Krzysztof)
> > 
> > Changes in v3:
> >  - Implemented the mailbox as a reserved-memory node. Add to it a
> >    `compatible` property. (Krzysztof)
> >  - Explained the relationship between the mailbox and the `enable-mehod`
> >    property of the CPU nodes.
> >  - Expanded the documentation of the binding.
> > 
> > Changes in v2:
> >  - Added more details to the description of the binding.
> >  - Added requirement a new requirement for cpu@N nodes to add an
> >    `enable-method`.
> > ---
> >  .../reserved-memory/intel,wakeup-mailbox.yaml      | 50 ++++++++++++++++++++++
> >  1 file changed, 50 insertions(+)
> > 
> 
> My bot found errors running 'make dt_binding_check' on your patch:
> 
> yamllint warnings/errors:
> ./Documentation/devicetree/bindings/reserved-memory/intel,wakeup-mailbox.yaml:23:1: [warning] too many blank lines (2 > 1) (empty-lines)

This got triggered by an empty line in an patch already reviewed by the DT
binding maintainers. Previous versions did not trigger this warning since
the check to allow at most 1 empty line is rather recent.

Would it be possible for the review proceed? I am happy to post a new version
fixing this after the rest of the patches have been reviewed.

Thanks and BR,
Ricardo