On Fri, 27 Jun 2025 20:35:09 -0700, 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 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 v4: > - 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 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 | 50 ++++++++++++++++++++++ > 1 file changed, 50 insertions(+) > Reviewed-by: Rob Herring (Arm) <robh@xxxxxxxxxx>
