Hi Rob, On Tue, Jul 01, 2025 at 09:12:18AM +0200, Maxime Ripard wrote: > On Mon, Jun 30, 2025 at 05:08:19PM -0500, Rob Herring wrote: > > On Mon, Jun 30, 2025 at 06:41:38PM +0200, Maxime Ripard wrote: > > > 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 <mrip...@kernel.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. > > > > But they do because you use one carve out for one thing and another > > carve out for another purpose and they probably aren't interchangeable. > > That was also my initial thought, but it was then discussed that the > name of the region is enough of a discriminant. And it makes sense too, > it's a sufficient discriminant for the device tree to uniquely identify > a given memory region on a given platform already, so we don't really > need anything else. > > > For the most part, everything in /reserved-memory is a carve out from > > regular memory though we failed to enforce that. > > > > > > 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. > > > > My issues with properties is we have to support N factorial cases for > > combinations of N properties. It's already fragile. Whereas a compatible > > is (hopefully) well defined as to what's needed and is only 1 more case > > to support. > > I think that's also what John especially wanted to avoid. If we have a > generic compatible, but the attributes/properties/whatever of the > buffers allocated from that region differ (like ecc vs non-ecc, > protected vs non-protected, etc.) we will need properties in the device > tree to describe them too.
I thought about it some more over the weekend, but if a compatible isn't a great solution either, would it make sense to just create a heap for each shared-dma-pool region, and thus we just wouldn't need any extra compatible or property? Maxime
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