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

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Message-ID: <1d0ba3fc-1504-4af3-a0bc-fba86abe41e8@kernel.org>
Date: Mon, 16 Sep 2024 10:56:38 +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 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.

You can still match by nodes. See all top-level bindings.

> 
> 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.


Best regards,
Krzysztof