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

Ricardo Neri
2025-05-07T11:43:52Z

On Tue, May 06, 2025 at 04:00:56PM +0200, Rafael J. Wysocki wrote:
> On Tue, May 6, 2025 at 7:46 AM Ricardo Neri
> <ricardo.neri-calderon@linux.intel.com> wrote:
> >
> > On Mon, May 05, 2025 at 03:07:43PM +0200, Rafael J. Wysocki wrote:
> > > On Sat, May 3, 2025 at 9:10 PM Ricardo Neri
> > > <ricardo.neri-calderon@linux.intel.com> wrote:
> > > >
> > > > Add DeviceTree bindings for the wakeup mailbox used on 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 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>
> > > > 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.
> > >
> > > This interface is defined in the ACPI specification and all of the
> > > above information is present there.
> > >
> > > Why are you copying it without acknowledging the source of it instead
> > > of just saying where this interface is defined and pointing to its
> > > definition?
> >
> > There was a discussion in the past about preferring a full description of
> > the mailbox instead of references to ACPI [1]. I am happy to acknowledge
> > the source in the changeset description. I explicitly acknowledge the ACPI
> > specification in the cover letter.
> >
> > [1]. https://lore.kernel.org/all/20240809232928.GB25056@yjiang5-mobl.amr.corp.intel.com/
>
> In which I clearly was not involved.
>
> Acknowledgement in the cover letter is fine, but insufficient.

I see.

>
> Also, there is the question regarding what happens when the ASWG
> decides to update this part of the ACPI specification. Is the DT
> binding going to be updated too?

That was my plan. The ACPI Multiprocessor Wakeup Structure has a version
field that I could have used.

But, it looks like these bindings will not be needed after all.

I will drop this patch.

Thanks and BR,
Ricardo