linux-kernel - Re: [PATCH v3 06/13] dt-bindings: reserved-memory: Wakeup Mailbox for Intel processors

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Message-ID: <20250506055054.GG25533@ranerica-svr.sc.intel.com>
Date: Mon, 5 May 2025 22:50:54 -0700
From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: x86@...nel.org, Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, Rob Herring <robh@...nel.org>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	Michael Kelley <mhklinux@...look.com>, devicetree@...r.kernel.org,
	Saurabh Sengar <ssengar@...ux.microsoft.com>,
	Chris Oo <cho@...rosoft.com>, linux-hyperv@...r.kernel.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Ricardo Neri <ricardo.neri@...el.com>
Subject: Re: [PATCH v3 06/13] dt-bindings: reserved-memory: Wakeup Mailbox
 for Intel processors

On Mon, May 05, 2025 at 03:07:43PM +0200, Rafael J. Wysocki wrote:
> On Sat, May 3, 2025 at 9:10 PM Ricardo Neri
> <ricardo.neri-calderon@...ux.intel.com> wrote:
> >
> > Add DeviceTree bindings for the wakeup mailbox used on Intel processors.
> >
> > x86 platforms commonly boot secondary CPUs using an INIT assert, de-assert
> > followed by Start-Up IPI messages. The wakeup mailbox can be used when this
> > mechanism unavailable.
> >
> > The wakeup mailbox offers more control to the operating system to boot
> > secondary CPUs than a spin-table. It allows the reuse of same wakeup vector
> > for all CPUs while maintaining control over which CPUs to boot and when.
> > While it is possible to achieve the same level of control using a spin-
> > table, it would require to specify a separate cpu-release-addr for each
> > secondary CPU.
> >
> > Originally-by: Yunhong Jiang <yunhong.jiang@...ux.intel.com>
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> > ---
> > Changes since v2:
> >  - Implemented the mailbox as a reserved-memory node. Add to it a
> >    `compatible` property. (Krzysztof)
> >  - Explained the relationship between the mailbox and the `enable-mehod`
> >    property of the CPU nodes.
> >  - Expanded the documentation of the binding.
> >
> > Changes since v1:
> >  - Added more details to the description of the binding.
> >  - Added requirement a new requirement for cpu@N nodes to add an
> >    `enable-method`.
> > ---
> >  .../reserved-memory/intel,wakeup-mailbox.yaml | 87 +++++++++++++++++++
> >  1 file changed, 87 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/reserved-memory/intel,wakeup-mailbox.yaml
> >
> > diff --git a/Documentation/devicetree/bindings/reserved-memory/intel,wakeup-mailbox.yaml b/Documentation/devicetree/bindings/reserved-memory/intel,wakeup-mailbox.yaml
> > new file mode 100644
> > index 000000000000..d97755b4673d
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/reserved-memory/intel,wakeup-mailbox.yaml
> > @@ -0,0 +1,87 @@
> > +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/reserved-memory/intel,wakeup-mailbox.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Wakeup Mailbox for Intel processors
> > +
> > +description: |
> > +  The Wakeup Mailbox provides a mechanism for the operating system to wake up
> > +  secondary CPUs on Intel processors. It is an alternative to the INIT-!INIT-
> > +  SIPI sequence used on most x86 systems.
> > +
> > +  Firmware must define the enable-method property in the CPU nodes as
> > +  "intel,wakeup-mailbox" to use the mailbox.
> > +
> > +  Firmware implements the wakeup mailbox as a 4KB-aligned memory region of size
> > +  of 4KB. It is memory that the firmware reserves so that each secondary CPU can
> > +  have the operating system send a single message to them. The firmware is
> > +  responsible for putting the secondary CPUs in a state to check the mailbox.
> > +
> > +  The structure of the mailbox is as follows:
> > +
> > +  Field           Byte   Byte  Description
> > +                 Length Offset
> > +  ------------------------------------------------------------------------------
> > +  Command          2      0    Command to wake up the secondary CPU:
> > +                                        0: Noop
> > +                                        1: Wakeup: Jump to the wakeup_vector
> > +                                        2-0xFFFF: Reserved:
> > +  Reserved         2      2    Must be 0.
> > +  APIC_ID          4      4    APIC ID of the secondary CPU to wake up.
> > +  Wakeup_Vector    8      8    The wakeup address for the secondary CPU.
> > +  ReservedForOs 2032     16    Reserved for OS use.
> > +  ReservedForFW 2048   2048    Reserved for firmware use.
> > +  ------------------------------------------------------------------------------
> > +
> > +  To wake up a secondary CPU, the operating system 1) prepares the wakeup
> > +  routine; 2) populates the address of the wakeup routine address into the
> > +  Wakeup_Vector field; 3) populates the APIC_ID field with the APIC ID of the
> > +  secondary CPU; 4) writes Wakeup in the Command field. Upon receiving the
> > +  Wakeup command, the secondary CPU acknowledges the command by writing Noop in
> > +  the Command field and jumps to the Wakeup_Vector. The operating system can
> > +  send the next command only after the Command field is changed to Noop.
> > +
> > +  The secondary CPU will no longer check the mailbox after waking up. The
> > +  secondary CPU must ignore the command if its APIC_ID written in the mailbox
> > +  does not match its own.
> > +
> > +  When entering the Wakeup_Vector, interrupts must be disabled and 64-bit
> > +  addressing mode must be enabled. Paging mode must be enabled. The virtual
> > +  address of the Wakeup_Vector page must be equal to its physical address.
> > +  Segment selectors are not used.
> 
> This interface is defined in the ACPI specification and all of the
> above information is present there.
> 
> Why are you copying it without acknowledging the source of it instead
> of just saying where this interface is defined and pointing to its
> definition?

There was a discussion in the past about preferring a full description of
the mailbox instead of references to ACPI [1]. I am happy to acknowledge
the source in the changeset description. I explicitly acknowledge the ACPI
specification in the cover letter.

[1]. https://lore.kernel.org/all/20240809232928.GB25056@yjiang5-mobl.amr.corp.intel.com/

> 
> > +
> > +maintainers:
> > +  - Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> > +
> > +allOf:
> > +  - $ref: reserved-memory.yaml
> > +
> > +properties:
> > +  compatible:
> > +    const: intel,wakeup-mailbox
> > +
> > +  alignment:
> > +    description: The mailbox must be 4KB-aligned.
> > +    const: 0x1000
> > +
> > +required:
> > +  - compatible
> > +  - alignment
> 
> Why do you need the "alignment" property if the alignment is always the same?

I want to enforce a 4KB alignment. It can also be inferred from the
address of the mailbox.

Thanks and BR,
Ricardo