linux-kernel - Re: [PATCH v7 0/5] dma-buf: heaps: Create a CMA heap for each CMA reserved region

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Message-ID: <20250911-didactic-authentic-cockle-e6d5fc@houat>
Date: Thu, 11 Sep 2025 09:01:24 +0200
From: Maxime Ripard <mripard@...nel.org>
To: "T.J. Mercier" <tjmercier@...gle.com>
Cc: Rob Herring <robh@...nel.org>, 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>, 
	Christian König <christian.koenig@....com>, Krzysztof Kozlowski <krzk+dt@...nel.org>, 
	Conor Dooley <conor+dt@...nel.org>, Marek Szyprowski <m.szyprowski@...sung.com>, 
	Robin Murphy <robin.murphy@....com>, Jonathan Corbet <corbet@....net>, 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, iommu@...ts.linux.dev, 
	linux-doc@...r.kernel.org
Subject: Re: [PATCH v7 0/5] dma-buf: heaps: Create a CMA heap for each CMA
 reserved region

Hi TJ,

On Wed, Sep 10, 2025 at 01:44:45PM -0700, T.J. Mercier wrote:
> On Wed, Sep 10, 2025 at 12:33 AM Maxime Ripard <mripard@...nel.org> wrote:
> >
> > On Tue, Aug 26, 2025 at 09:36:03AM +0200, Maxime Ripard wrote:
> > > Hi,
> > >
> > > On Mon, Jul 21, 2025 at 01:17:29PM +0200, Maxime Ripard wrote:
> > > > Here's another attempt at supporting user-space allocations from a
> > > > specific carved-out reserved memory region.
> > > >
> > > > The initial problem we were discussing was that I'm currently working on
> > > > a platform which has a memory layout with ECC enabled. However, enabling
> > > > the ECC has a number of drawbacks on that platform: lower performance,
> > > > increased memory usage, etc. So for things like framebuffers, the
> > > > trade-off isn't great and thus there's a memory region with ECC disabled
> > > > to allocate from for such use cases.
> > > >
> > > > After a suggestion from John, I chose to first start using heap
> > > > allocations flags to allow for userspace to ask for a particular ECC
> > > > setup. This is then backed by a new heap type that runs from reserved
> > > > memory chunks flagged as such, and the existing DT properties to specify
> > > > the ECC properties.
> > > >
> > > > After further discussion, it was considered that flags were not the
> > > > right solution, and relying on the names of the heaps would be enough to
> > > > let userspace know the kind of buffer it deals with.
> > > >
> > > > Thus, even though the uAPI part of it had been dropped in this second
> > > > version, we still needed a driver to create heaps out of carved-out memory
> > > > regions. In addition to the original usecase, a similar driver can be
> > > > found in BSPs from most vendors, so I believe it would be a useful
> > > > addition to the kernel.
> > > >
> > > > Some extra discussion with Rob Herring [1] came to the conclusion that
> > > > some specific compatible for this is not great either, and as such an
> > > > new driver probably isn't called for either.
> > > >
> > > > Some other discussions we had with John [2] also dropped some hints that
> > > > multiple CMA heaps might be a good idea, and some vendors seem to do
> > > > that too.
> > > >
> > > > So here's another attempt that doesn't affect the device tree at all and
> > > > will just create a heap for every CMA reserved memory region.
> > > >
> > > > It also falls nicely into the current plan we have to support cgroups in
> > > > DRM/KMS and v4l2, which is an additional benefit.
> > > >
> > > > Let me know what you think,
> > > > Maxime
> > >
> > > Any chance we can get this merged?
> >
> > Guys, can we move forward on this?
> >
> > Maxime
> 
> Hi Maxime,
> 
> Sorry I've been MIA the last couple of months.
> 
> The docs for the "reusable" property say, "device driver(s) owning the
> region need to be able to reclaim it back", but how can a driver
> reclaim memory backing a dmabuf, since pages allocated for a dmabuf
> aren't necessarily movable. Couldn't a user allocate all of it, and
> refuse to close those dmabufs?

I guess, but how is that any different than what we're doing on the
default allocator already?

It also has to be reusable, and will not be able to reclaim any memory
allocated through the heap.

> I backported this to 6.6 and ran it on a Pixel. While there are
> already similar out-of-tree dmabuf heap drivers that expose heaps for
> these reserved regions, they do more than just cma_alloc (multiple
> flavors of buffer securing, use case specific alignment and padding,
> and slightly different allocation strategies) so I don't think this
> series would allow us to completely drop the custom heap code, but
> it's a nice start.

Thanks for testing, and I totally expect more heaps coming for things
like protected memory, but it should indeed reduce the number of heap
drivers needed going forward.

> Does the cgroup part come in because the plan is to add charging in
> cma_heap.c?

Yes, and the system heap as well.

Maxime

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