linux-kernel - Re: [PATCH v5 1/2] dt-bindings: reserved-memory: Introduce carved-out memory region binding

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Message-ID: <20250630-attentive-fortunate-turaco-2e36d2@houat>
Date: Mon, 30 Jun 2025 18:41:38 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Rob Herring <robh@...nel.org>
Cc: Saravana Kannan <saravanak@...gle.com>, 
	Sumit Semwal <sumit.semwal@...aro.org>, Benjamin Gaignard <benjamin.gaignard@...labora.com>, 
	Brian Starkey <Brian.Starkey@....com>, John Stultz <jstultz@...gle.com>, 
	"T.J. Mercier" <tjmercier@...gle.com>, Christian König <christian.koenig@....com>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, Andrew Davis <afd@...com>, 
	Jared Kangas <jkangas@...hat.com>, Mattijs Korpershoek <mkorpershoek@...nel.org>, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, linux-media@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v5 1/2] dt-bindings: reserved-memory: Introduce
 carved-out memory region binding

Hi Rob,

On Fri, Jun 27, 2025 at 02:31:32PM -0500, Rob Herring wrote:
> On Tue, Jun 17, 2025 at 02:25:40PM +0200, Maxime Ripard wrote:
> > Some parts of the memory can be dedicated to specific purposes and
> > exposed as a dedicated memory allocator.
> > 
> > This is especially useful if that particular region has a particular
> > properties the rest of the memory doesn't have. For example, some
> > platforms have their entire RAM covered by ECC but for a small area
> > meant to be used by applications that don't need ECC, and its associated
> > overhead.
> > 
> > Let's introduce a binding to describe such a region and allow the OS to
> > create a dedicated memory allocator for it.
> > 
> > Signed-off-by: Maxime Ripard <mripard@...nel.org>
> > ---
> >  .../bindings/reserved-memory/carved-out.yaml       | 49 ++++++++++++++++++++++
> >  1 file changed, 49 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/reserved-memory/carved-out.yaml b/Documentation/devicetree/bindings/reserved-memory/carved-out.yaml
> > new file mode 100644
> > index 0000000000000000000000000000000000000000..9ab5d1ebd9ebd9111b7c064fabe1c45e752da83b
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/reserved-memory/carved-out.yaml
> > @@ -0,0 +1,49 @@
> > +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/reserved-memory/carved-out.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Carved-out Memory Region
> > +
> > +description: |
> 
> Don't need '|'.
> 
> > +  Specifies that the reserved memory region has been carved out of the
> > +  main memory allocator, and is intended to be used by the OS as a
> > +  dedicated memory allocator.
> 
> Other than the commit msg, it is completely lost that this is for 
> ECC-less memory.

Because it's not. One of the first feedback I got was that the way to
identify what a heap provides was the heap name.

So, as far as the binding go, a heap just exposes a chunk of memory the
memory allocator wouldn't use. The actual semantics of that chunk of
memory don't matter.

> This description applies to CMA area as well. So what's the difference?

Yeah, I kind of agree, which is why I initially started with a property,
and you then asked for a compatible.

CMA (assuming you mean the allocator, not the CMA heap) is still more
though: it only covers some shared-dma-pool memory regions.

> > +
> > +maintainers:
> > +  - Maxime Ripard <mripard@...nel.org>
> > +
> > +properties:
> > +  compatible:
> > +    const: carved-out
> 
> Isn't everything in reserved-memory a carve out for some purpose. I'm 
> not sure if I'd add 'no ECC' or more along the lines of how this is 
> used. The latter might be useful on platforms which can't disable ECC or 
> don't have ECC at all.

I don't think we need any discriminant for ECC vs non-ECC. It's just a
carved-out memory region at some offset, and the system won't use it.

Maxime

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