linux-kernel - Re: [PATCH v5 03/10] dt-bindings: reserved-memory: Wakeup Mailbox for Intel processors

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Message-ID: <175197849289.389672.9119321529069936382.robh@kernel.org>
Date: Tue, 8 Jul 2025 07:41:33 -0500
From: "Rob Herring (Arm)" <robh@...nel.org>
To: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc: Haiyang Zhang <haiyangz@...rosoft.com>,
	Yunhong Jiang <yunhong.jiang@...ux.intel.com>,
	Conor Dooley <conor+dt@...nel.org>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	Chris Oo <cho@...rosoft.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Michael Kelley <mhklinux@...look.com>,
	Ricardo Neri <ricardo.neri@...el.com>,
	Saurabh Sengar <ssengar@...ux.microsoft.com>,
	devicetree@...r.kernel.org, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	linux-hyperv@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v5 03/10] dt-bindings: reserved-memory: Wakeup Mailbox
 for Intel processors


On Fri, 27 Jun 2025 20:35:09 -0700, Ricardo Neri wrote:
> Add DeviceTree bindings to enumerate the wakeup mailbox used in platform
> firmware for 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 is 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.
> 
> The operation and structure of the mailbox is described in the
> Multiprocessor Wakeup Structure defined in the ACPI specification. Note
> that this structure does not specify how to publish the mailbox to the
> operating system (ACPI-based platform firmware uses a separate table). No
> ACPI table is needed in DeviceTree-based firmware to enumerate the mailbox.
> 
> Add a `compatible` property that the operating system can use to discover
> the mailbox. Nodes wanting to refer to the reserved memory usually define a
> `memory-region` property. /cpus/cpu* nodes would want to refer to the
> mailbox, but they do not have such property defined in the DeviceTree
> specification. Moreover, it would imply that there is a memory region per
> CPU.
> 
> Co-developed-by: Yunhong Jiang <yunhong.jiang@...ux.intel.com>
> Signed-off-by: Yunhong Jiang <yunhong.jiang@...ux.intel.com>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> ---
> Changes since v4:
>  - Specified the version and section of the ACPI spec in which the
>    wakeup mailbox is defined. (Rafael)
>  - Fixed a warning from yamllint about line lengths of URLs.
> 
> Changes since v3:
>  - Removed redefinitions of the mailbox and instead referred to ACPI
>    specification as per discussion on LKML.
>  - Clarified that DeviceTree-based firmware do not require the use of
>    ACPI tables to enumerate the mailbox. (Rob)
>  - Described the need of using a `compatible` property.
>  - Dropped the `alignment` property. (Krzysztof, Rafael)
>  - Used a real address for the mailbox node. (Krzysztof)
> 
> 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      | 50 ++++++++++++++++++++++
>  1 file changed, 50 insertions(+)
> 

Reviewed-by: Rob Herring (Arm) <robh@...nel.org>