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 [off-list ref] wrote:

quoted

On Tue, Aug 26, 2025 at 09:36:03AM +0200, Maxime Ripard wrote:

quoted

Hi,

On Mon, Jul 21, 2025 at 01:17:29PM +0200, Maxime Ripard wrote:

quoted

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