On Wed, Jun 4, 2025 at 2:18 AM Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx> 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 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 to specify a separate `cpu-release-addr` for each > secondary CPU. > > The operation and structure of the mailbox is 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. > > Add a `compatible` property that the operating system can use to discover > the mailbox. Nodes wanting 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. > > Co-developed-by: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx> > Signed-off-by: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx> > --- > Changes since v3: > - 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 since v2: > - 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 since v1: > - 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 | 48 ++++++++++++++++++++++ > 1 file changed, 48 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..f18643805866 > --- /dev/null > +++ b/Documentation/devicetree/bindings/reserved-memory/intel,wakeup-mailbox.yaml > @@ -0,0 +1,48 @@ > +# 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. Please make this more specific: Which specification version and what section. You may as well add a URL here too. > + > + The implementation of the mailbox in platform firmware is described in the > + Intel TDX Virtual Firmware Design Guide section 4.3.5. > + > + See https://www.intel.com/content/www/us/en/content-details/733585/intel-tdx-virtual-firmware-design-guide.html > + > +maintainers: > + - Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx> > + > +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>; > + }; > + }; > > --
