linux-kernel - Re: [PATCH] dt-bindings: memory: Document linux,usable-memory property

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Message-ID: <CAL_JsqKU8hE=dXdQ+hO0WvU-GuXAjEBgCPJ4rnxef9851zyKMw@mail.gmail.com>
Date: Thu, 13 Mar 2025 13:57:30 -0500
From: Rob Herring <robh@...nel.org>
To: Abhishek Tiwari <abhitiwari@...ux.microsoft.com>
Cc: krzk+dt@...nel.org, conor+dt@...nel.org, kees@...nel.org, 
	tony.luck@...el.com, gpiccoli@...lia.com, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org, 
	abhitiwari@...rosoft.com
Subject: Re: [PATCH] dt-bindings: memory: Document linux,usable-memory property

On Thu, Mar 13, 2025 at 9:03 AM Abhishek Tiwari
<abhitiwari@...ux.microsoft.com> wrote:
>
> Add Documentation for linux,usable-memory
>
> Signed-off-by: Abhishek Tiwari <abhitiwari@...ux.microsoft.com>
> ---
>  .../bindings/linux,usable-memory.txt          | 32 +++++++++++++++++++
>  1 file changed, 32 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/linux,usable-memory.txt
>
> diff --git a/Documentation/devicetree/bindings/linux,usable-memory.txt b/Documentation/devicetree/bindings/linux,usable-memory.txt
> new file mode 100644
> index 000000000000..167054d2e9a2
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/linux,usable-memory.txt
> @@ -0,0 +1,32 @@
> +linux,usable-memory
> +===================

This belongs here:
https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/memory.yaml

> +
> +Description
> +-----------
> +The ``linux,usable-memory`` property can be used to restrict usable memory
> +region. This property holds a base address and size, Memory outside of this
> +range is not accessible by the kernel. This property is particularly useful
> +in specialized hardware platforms where certain memory regions must be
> +reserved for specific use.
> +
> +Common use cases include:
> +- Allocating ``ramoops`` region
> +- Reserving memory for hardware-specific needs
> +- Fake Protecting persistent memory (PMEM)

All these examples belong in /reserved-memory nodes, not
linux,usable-memory. Go see the ramoops binding for example.

This was really for the case where you already have 'reg' (in the
memory node), but need to limit memory while at the same time not
overwriting 'reg'. Basically, for kexec where you can keep booting
another kernel forever. If that's not your usecase, you shouldn't be
using this.

Rob