Re: [PATCH v3 06/13] dt-bindings: reserved-memory: Wakeup Mailbox for Intel processors

Ricardo Neri
2025-05-06T05:11:07Z

On Sun, May 04, 2025 at 06:51:17PM +0200, Krzysztof Kozlowski wrote:
> On Sat, May 03, 2025 at 12:15:08PM GMT, Ricardo Neri wrote:
> > Add DeviceTree bindings for the wakeup mailbox used on Intel processors.
> >
>
> Start using b4, so your cover letter will have proper lore links to
> previous versions.

Sure. Thanks for this suggestion I will follow it.

>
> > 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 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.
> >
> > Originally-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>
>
> What does this tag mean? Why you cannot use standard tags - SoB and
> co-developed?

Yunhong handed this work to me. We did not work in it concurrently. But
yes, SoB and Co-developed-by also looks appropriate as two individuals
worked on this.

>
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > ---
> > 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 | 87 +++++++++++++++++++
> > 1 file changed, 87 insertions(+)
> > create mode 100644 Documentation/devicetree/bindings/reserved-memory/intel,wakeup-mailbox.yaml
> >
> > 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..d97755b4673d
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/reserved-memory/intel,wakeup-mailbox.yaml
> > @@ -0,0 +1,87 @@
> > +# 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.
> > +
> > + Firmware must define the enable-method property in the CPU nodes as
> > + "intel,wakeup-mailbox" to use the mailbox.
> > +
> > + Firmware implements the wakeup mailbox as a 4KB-aligned memory region of size
> > + of 4KB. It is memory that the firmware reserves so that each secondary CPU can
> > + have the operating system send a single message to them. The firmware is
> > + responsible for putting the secondary CPUs in a state to check the mailbox.
> > +
> > + The structure of the mailbox is as follows:
> > +
> > + Field Byte Byte Description
> > + Length Offset
> > + ------------------------------------------------------------------------------
> > + Command 2 0 Command to wake up the secondary CPU:
> > + 0: Noop
> > + 1: Wakeup: Jump to the wakeup_vector
> > + 2-0xFFFF: Reserved:
> > + Reserved 2 2 Must be 0.
> > + APIC_ID 4 4 APIC ID of the secondary CPU to wake up.
> > + Wakeup_Vector 8 8 The wakeup address for the secondary CPU.
> > + ReservedForOs 2032 16 Reserved for OS use.
> > + ReservedForFW 2048 2048 Reserved for firmware use.
> > + ------------------------------------------------------------------------------
> > +
> > + To wake up a secondary CPU, the operating system 1) prepares the wakeup
> > + routine; 2) populates the address of the wakeup routine address into the
> > + Wakeup_Vector field; 3) populates the APIC_ID field with the APIC ID of the
> > + secondary CPU; 4) writes Wakeup in the Command field. Upon receiving the
> > + Wakeup command, the secondary CPU acknowledges the command by writing Noop in
> > + the Command field and jumps to the Wakeup_Vector. The operating system can
> > + send the next command only after the Command field is changed to Noop.
> > +
> > + The secondary CPU will no longer check the mailbox after waking up. The
> > + secondary CPU must ignore the command if its APIC_ID written in the mailbox
> > + does not match its own.
> > +
> > + When entering the Wakeup_Vector, interrupts must be disabled and 64-bit
> > + addressing mode must be enabled. Paging mode must be enabled. The virtual
> > + address of the Wakeup_Vector page must be equal to its physical address.
> > + Segment selectors are not used.
> > +
> > +maintainers:
> > + - Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > +
> > +allOf:
> > + - $ref: reserved-memory.yaml
> > +
> > +properties:
> > + compatible:
> > + const: intel,wakeup-mailbox
>
> If this is a device, then compatibles specific to devices. You do not
> get different rules than all other bindings... or this does not have to
> be binding at all. Why standard reserved-memory does not work for here?
>
> Why do you need compatible in the first place?

Are you suggesting something like this?

reserved-memory {
# address-cells = <2>;
# size-cells = <1>;

wakeup_mailbox: wakeupmb@fff000 {
reg = < 0x0 0xfff000 0x1000>
}

and then reference to the reserved memory using the wakeup_mailbox
phandle?

If this is the case, IIUC, this schema is not needed at all.

}
>
>
> > +
> > + alignment:
> > + description: The mailbox must be 4KB-aligned.
>
> Then drop it because it is implied by compatible.

Sure I can drop it.

>
> > + const: 0x1000
> > +
> > +required:
> > + - compatible
> > + - alignment
> > + - reg
> > +
> > +unevaluatedProperties: false
> > +
> > +examples:
> > + - |
> > + reserved-memory {
> > + #address-cells = <2>;
> > + #size-cells = <1>;
> > +
> > + wakeup-mailbox@12340000 {
>
> Please use real addresses from real DT.
>
> > + compatible = "intel,wakeup-mailbox";
> > + alignment = <0x1000>;
> > + reg = <0x0 0x12340000 0x1000>;
>
> reg is always the second property. See DTS coding style.

Sorry, I missed this convention.

Thanks and BR,
Ricardo