Message-ID: <874d5908-f1db-412f-96a2-83fcebe8dd98@kernel.org>
Date: Fri, 20 Sep 2024 13:15:41 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Yunhong Jiang <yunhong.jiang@...ux.intel.com>
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, robh@...nel.org,
 krzk+dt@...nel.org, conor+dt@...nel.org, kys@...rosoft.com,
 haiyangz@...rosoft.com, wei.liu@...nel.org, decui@...rosoft.com,
 rafael@...nel.org, lenb@...nel.org, kirill.shutemov@...ux.intel.com,
 linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
 linux-hyperv@...r.kernel.org, linux-acpi@...r.kernel.org
Subject: Re: [PATCH v2 2/9] dt-bindings: x86: Add a binding for x86 wakeup
 mailbox

On 19/09/2024 21:19, Yunhong Jiang wrote:
> 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@...ux.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@...ux.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.

VM is purely a software construct, so it is not a device. But regardless
of terminology, you did not describe here VM or its part, either.

>  
>>
>> 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.

Most of board schemas, so for example in arm directory.

> 
> 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@...ux.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

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.


> 
> 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
>>
>>

Best regards,
Krzysztof

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