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

[Ricardo Neri <ricardo.neri-calderon@linux.intel.com>]

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

[...]
 
> 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
    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

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

required:
  - compatible
  - mailbox-addr

additionalProperties: false

examples:
  - |
    wakeup-mailbox {
      compatible = "x86,wakeup-mailbox";
      mailbox-addr = <0 0x1c000500>;
    };

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

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