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

[Yunhong Jiang <yunhong.jiang@linux.intel.com>]

On Mon, Sep 16, 2024 at 10:56:38AM +0200, Krzysztof Kozlowski wrote:
> On 10/09/2024 08:13, Yunhong Jiang wrote:
> > On Tue, Aug 27, 2024 at 01:45:49PM -0700, Yunhong Jiang wrote:
> >> On Sun, Aug 25, 2024 at 09:10:01AM +0200, Krzysztof Kozlowski wrote:
> >>> On Fri, Aug 23, 2024 at 04:23:20PM -0700, Yunhong Jiang wrote:
> >>>> Add the binding to use mailbox wakeup mechanism to bringup APs.
> >>>>
> >>>> Signed-off-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>
> >>>> ---
> >>>>  .../devicetree/bindings/x86/wakeup.yaml       | 64 +++++++++++++++++++
> >>>>  1 file changed, 64 insertions(+)
> >>>>  create mode 100644 Documentation/devicetree/bindings/x86/wakeup.yaml
> >>>>
> >>>> diff --git a/Documentation/devicetree/bindings/x86/wakeup.yaml b/Documentation/devicetree/bindings/x86/wakeup.yaml
> >>>> new file mode 100644
> >>>> index 000000000000..cb84e2756bca
> >>>> --- /dev/null
> >>>> +++ b/Documentation/devicetree/bindings/x86/wakeup.yaml
> >>>> @@ -0,0 +1,64 @@
> >>>> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> >>>> +# Copyright (C) 2024 Intel Corporation
> >>>> +%YAML 1.2
> >>>> +---
> >>>> +$id: http://devicetree.org/schemas/x86/wakeup.yaml#
> >>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> >>>> +
> >>>> +title: x86 mailbox wakeup
> >>>> +maintainers:
> >>>> +  - Yunhong Jiang <yunhong.jiang@linux.intel.com>
> >>>> +
> >>>> +description: |
> >>>> +  The x86 mailbox wakeup mechanism defines a mechanism to let the bootstrap
> >>>> +  processor (BSP) to wake up application processors (APs) through a wakeup
> >>>> +  mailbox.
> >>>> +
> >>>> +  The "wakeup-mailbox-addr" property specifies the wakeup mailbox address. The
> >>>> +  wakeup mailbox is a 4K-aligned 4K-size memory block allocated in the reserved
> >>>> +  memory.
> >>>> +
> >>>> +  The wakeup mailbox structure is defined as follows.
> >>>> +
> >>>> +    uint16_t command;
> >>>> +    uint16_t reserved;
> >>>> +    uint32_t apic_id;
> >>>> +    uint64_t wakeup_vector;
> >>>> +    uint8_t  reservedForOs[2032];
> >>>> +
> >>>> +  The memory after reservedForOs field is reserved and OS should not touch it.
> >>>> +
> >>>> +  To wakes up a AP, the BSP prepares the wakeup routine, fills the wakeup
> >>>> +  routine's address into the wakeup_vector field, fill the apic_id field with
> >>>> +  the target AP's APIC_ID, and write 1 to the command field. After receiving the
> >>>> +  wakeup command, the target AP will jump to the wakeup routine.
> >>>> +
> >>>> +  For each AP, the mailbox can be used only once for the wakeup command. After
> >>>> +  the AP jumps to the wakeup routine, the mailbox will no longer be checked by
> >>>> +  this AP.
> >>>> +
> >>>> +  The wakeup mailbox structure and the wakeup process is the same as
> >>>> +  the Multiprocessor Wakeup Mailbox Structure defined in ACPI spec version 6.5,
> >>>> +  section 5.2.12.19 [1].
> >>>> +
> >>>> +  References:
> >>>> +
> >>>> +  [1] https://uefi.org/specs/ACPI/6.5/05_ACPI_Software_Programming_Model.html
> >>>> +
> >>>> +select: false
> >>>
> >>> This schema is still a no-op because of this false.
> >>>
> >>> What is the point of defining one property if it is not placed anywhere?
> >>> Every device node can have it? Seems wrong...
> >>>
> >>> You need to come with proper schema. Lack of an example is another thing
> >>> - this cannot be even validated by the tools. 
> >>>
> >>> Best regards,
> >>> Krzysztof
> > 
> > Hi, Krzysztof, I'm working to address your comments and have some questions.
> > Hope to get help/guide from your side.
> > 
> > For the select, the writing-schema.rst describes it as "A json-schema used to
> > match nodes for applying the schema" but I'm a bit confused. In my case, should
> > it be "cpus" node? Is there any code/tools that uses this property, so that I
> > can have a better understanding?
> 
> Usually we expect matching by compatible, but it does not seem suitable
> here because it is not related to any specific device, right? That is
> the problem with all this DT-reuse-for-virtual-stuff work. It just does
> not follow usual expectations and guidelines - you do not describe a device.

Thank you for the reply.

I'm a bit confused on your "do not describe a device".
I think VM is also a device, it's just a virtual device, but I don't see much
difference of the virtual and physical device from DT point of view, possibly I
missed some point.
 
> 
> You can still match by nodes. See all top-level bindings.

After checking the code at
https://github.com/devicetree-org/dt-schema/blob/main/dtschema/validator.py,
seems the 'select' is translated to 'if'/'then'.

Do you have any example of "top-level bindings"? I tried to check binding for
enable-methods like arm/cpu-enable-method/nuvoton,npcm750-smp or
cpu/idle-states.yaml, but they are either not schema file, or quite different.

I have been struggling on this device binding document for a while. I
reconsidered what this binding is for. This binding means, if the cpus node has
"enable-method" as "acpi-wakeup-mailbox", then the device should have property
"wakeup-mailbox-addr" with uint64 type.

In that case, I'm considering to set the "select" to be true so that it will
apply to any potential device, and add if/then keyword to check the
enable-method. But seems it does not work and I'm still trying to figure out the
reason (I'm new to the json/json schema and is still learning).

I received followed error:
cpus: '#address-cells', '#size-cells', 'cpu@0', 'enable-method' do not match any of the regexes: 'pinctrl-[0-9]+'
        from schema $id: http://devicetree.org/schemas/x86/wakeup.yaml#
cpu@0: 'device_type', 'reg' do not match any of the regexes: 'pinctrl-[0-9]+'
        from schema $id: http://devicetree.org/schemas/x86/wakeup.yaml#

With the followed yaml file (I delete some description).

$ cat Documentation/devicetree/bindings/x86/wakeup.yaml
# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
# Copyright (C) 2024 Intel Corporation
%YAML 1.2
---
$id: http://devicetree.org/schemas/x86/wakeup.yaml#
$schema: http://devicetree.org/meta-schemas/core.yaml#

title: x86 mailbox wakeup
maintainers:
  - Yunhong Jiang <yunhong.jiang@linux.intel.com>

description: |
  ......
  Removed to save space.

properties:
  wakeup-mailbox-addr:
    $ref: /schemas/types.yaml#/definitions/uint32
    description: |
      ......
      Removed to save space.

select: true

if:
  properties:
    enable-method:
      contains:
        const: acpi-wakeup-mailbox
  required:
    - enable-method

then:
  required:
    - wakeup-mailbox-addr

additionalProperties: false

examples:
  - |
    cpus {
      #address-cells = <1>;
      #size-cells = <0>;
      enable-method = "acpi-wakeup-mailbox";
      wakeup-mailbox-addr = <0x1c000500>;
      cpu@0 {
        device_type = "cpu";
        reg = <0x1>;
      };
    };
...

> 
> > 
> > For your "validated by the tools", can you please share the tools you used to
> > validate the schema? I used "make dt_binding_check" per the
> > submitting-patches.rst but I think your comments is about another tool.
> 
> See writing-schema document.
Yes, I figured out in the end that the validate tools means the dt-schema tools.

Thank you
--jyh

> 
> 
> Best regards,
> Krzysztof
> 
>