Re: [PATCH v2 2/9] dt-bindings: x86: Add a binding for x86 wakeup mailbox

On Tue, Mar 11, 2025 at 11:01:28AM +0100, Krzysztof Kozlowski wrote:

On 03/03/2025 23:21, Ricardo Neri wrote:

quoted

On Fri, Sep 20, 2024 at 01:15:41PM +0200, Krzysztof Kozlowski wrote:

[...]
 

quoted

enable-method is part of CPUs, so you probably should match the CPUs...
I am not sure, I don't have the big picture here.

Maybe if companies want to push more of bindings for purely virtual
systems, then they should first get involved more, instead of relying on
us. Provide reviews for your virtual stuff, provide guidance. There is
resistance in accepting bindings for such cases for a reason - I don't
even know what exactly is this and judging/reviewing based on my
practices will no be accurate.

Hi Krzysztof,

I am taking over this work from Yunhong.

First of all, I apologize for the late reply. I will make sure
communications are timely in the future.

Our goal is to describe in the device tree a mechanism or artifact to boot
secondary CPUs.

In our setup, the firmware puts secondary CPUs to monitor a memory location
(i.e., the wakeup mailbox) while spinning. From the boot CPU, the OS writes
in the mailbox the wakeup vector and the ID of the secondary CPU it wants
to boot. When a secondary CPU sees its own ID it will jump to the wakeup
vector.

This is similar to the spin-table described in the Device Tree
specification. The key difference is that with the spin-table CPUs spin
until a non-zero value is written in `cpu-release-addr`. The wakeup mailbox
uses CPU IDs.

You raised the issue of the lack of a `compatible` property, and the fact
that we are not describing an actual device.

I took your suggestion of matching by node and I came up with the binding
below. I see these advantages in this approach:

  * I define a new node with a `compatible` property.
  * There is precedent: the psci node. In the `cpus` node, each cpu@n has

Thanks for your feedback!

psci is a standard. If you are documenting here a standard, clearly
express it and provide reference to the specification.

It is not really a standard, but this mailbox behaves indentically to the
wakeup mailbox described in the ACPI spec [1]. I am happy reference the
spec in the documentation of the binding... or describe in full the
mechanism of mailbox without referring to ACPI. You had indicated you don't
care about what ACPI does [2].

In a nutshell, the wakeup mailbox is similar to the spin table used in ARM
boards: it is reserved memory region that secondary CPUs monitor while
spinning.

quoted

    an `enable-method` property that specify `psci`.
  * The mailbox is a device as it is located in a reserved memory region.
    This true regardless of the device tree describing bare-metal or
    virtualized machines.

Thanks in advance for your feedback!

Best,
Ricardo

(only the relevant sections of the binding are shown for brevity)

properties:
  $nodename:
    const: wakeup-mailbox

  compatible:
    const: x86,wakeup-mailbox

You need vendor prefix for this particular device. If I pointed out lack
of device and specific compatible, then adding random compatible does
not solve it. I understand it solves for you, but not from the bindings
point of view.

I see. Platform firmware will implement the mailbox. It would not be any
specific hardware from Intel. Perhaps `intel,wakeup-mailbox`?

quoted

  mailbox-addr:
    $ref: /schemas/types.yaml#/definitions/uint64

So is this some sort of reserved memory?

Yes, the mailbox is located in reserved memory.

Mailbox needs mbox-cells, so
maybe that's not mailbox.

Your comment got me to look under Documentation/devicetree/bindings/mailbox.
Maybe I am ovethinking this and I can describe the mailbox as others
vendors do (see below).

Thanks and BR,
Ricardo

[1]. https://uefi.org/htmlspecs/ACPI_Spec_6_4_html/05_ACPI_Software_Programming_Model/ACPI_Software_Programming_Model.html#multiprocessor-wakeup-structure
if that is OK.
[2]. https://lore.kernel.org/lkml/624e1985-7dd2-4abe-a918-78cb43556967@kernel.oirg

description: |
  [Removed for brevity]

properties:
  compatible:
    const: intel,wakeup-mailbox

  reg:
    maxItems: 1

  '#mbox-cells':
    const: 1

required:
  - compatible
  - reg

additionalProperties: false

examples:
  - |
    wakeupmailbox: mailbox@1c000500 {
      compatible = "intel,wakeup-mailbox";
      reg = <0x1c000500 0x100>;
      #mbox-cells = <1>;
    };

    cpus {
        #address-cells = <1>;
        #size-cells = <0>;

        cpu@0 {
            device_type = "cpu";
            reg = <0x00>;
            enable-method = "wakeup-mailbox";
        };
    };
...