Message-ID: <CAO_48GGD8sCoQt_qWKqcbg6v7Cyi5U9QsxsvNOcqfkLRqHS7_w@mail.gmail.com>
Date: Wed, 15 Oct 2025 13:53:41 +0530
From: Sumit Semwal <sumit.semwal@...aro.org>
To: Maxime Ripard <mripard@...nel.org>
Cc: Benjamin Gaignard <benjamin.gaignard@...labora.com>, Brian Starkey <Brian.Starkey@....com>, 
	John Stultz <jstultz@...gle.com>, "T.J. Mercier" <tjmercier@...gle.com>, 
	Jonathan Corbet <corbet@....net>, Christian König <christian.koenig@....com>, 
	Marek Szyprowski <m.szyprowski@...sung.com>, Robin Murphy <robin.murphy@....com>, 
	Andrew Davis <afd@...com>, Jared Kangas <jkangas@...hat.com>, 
	Mattijs Korpershoek <mkorpershoek@...nel.org>, linux-media@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	iommu@...ts.linux.dev
Subject: Re: [PATCH v8 0/5] dma-buf: heaps: Create a CMA heap for each CMA
 reserved region

Hi Maxime,

On Mon, 13 Oct 2025 at 14:05, Maxime Ripard <mripard@...nel.org> wrote:
>
> Hi,
>
> Here's another attempt at supporting user-space allocations from a
> specific carved-out reserved memory region.


Thank you for the series - I think it looks pretty decent, and with
Marek's Acked-by for the cma patch [1], I'm inclined to merge it.

I'll wait till today evening, in case there are any more comments, and
then go ahead and merge it.

Best,
Sumit.

>
> 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
>
> 1: https://lore.kernel.org/all/20250707-cobalt-dingo-of-serenity-dbf92c@houat/
> 2: https://lore.kernel.org/all/CANDhNCroe6ZBtN_o=c71kzFFaWK-fF5rCdnr9P5h1sgPOWSGSw@mail.gmail.com/
>
> Let me know what you think,
> Maxime
>
> Signed-off-by: Maxime Ripard <mripard@...nel.org>
> ---
> Changes in v8:
> - Rebased on top of 6.18-rc1
> - Added TJ R-b
> - Link to v7: https://lore.kernel.org/r/20250721-dma-buf-ecc-heap-v7-0-031836e1a942@kernel.org
>
> Changes in v7:
> - Invert the logic and register CMA heap from the reserved memory /
>   dma contiguous code, instead of iterating over them from the CMA heap.
> - Link to v6: https://lore.kernel.org/r/20250709-dma-buf-ecc-heap-v6-0-dac9bf80f35d@kernel.org
>
> Changes in v6:
> - Drop the new driver and allocate a CMA heap for each region now
> - Dropped the binding
> - Rebased on 6.16-rc5
> - Link to v5: https://lore.kernel.org/r/20250617-dma-buf-ecc-heap-v5-0-0abdc5863a4f@kernel.org
>
> Changes in v5:
> - Rebased on 6.16-rc2
> - Switch from property to dedicated binding
> - Link to v4: https://lore.kernel.org/r/20250520-dma-buf-ecc-heap-v4-1-bd2e1f1bb42c@kernel.org
>
> Changes in v4:
> - Rebased on 6.15-rc7
> - Map buffers only when map is actually called, not at allocation time
> - Deal with restricted-dma-pool and shared-dma-pool
> - Reword Kconfig options
> - Properly report dma_map_sgtable failures
> - Link to v3: https://lore.kernel.org/r/20250407-dma-buf-ecc-heap-v3-0-97cdd36a5f29@kernel.org
>
> Changes in v3:
> - Reworked global variable patch
> - Link to v2: https://lore.kernel.org/r/20250401-dma-buf-ecc-heap-v2-0-043fd006a1af@kernel.org
>
> Changes in v2:
> - Add vmap/vunmap operations
> - Drop ECC flags uapi
> - Rebase on top of 6.14
> - Link to v1: https://lore.kernel.org/r/20240515-dma-buf-ecc-heap-v1-0-54cbbd049511@kernel.org
>
> ---
> Maxime Ripard (5):
>       doc: dma-buf: List the heaps by name
>       dma-buf: heaps: cma: Register list of CMA regions at boot
>       dma: contiguous: Register reusable CMA regions at boot
>       dma: contiguous: Reserve default CMA heap
>       dma-buf: heaps: cma: Create CMA heap for each CMA reserved region
>
>  Documentation/userspace-api/dma-buf-heaps.rst | 24 ++++++++------
>  MAINTAINERS                                   |  1 +
>  drivers/dma-buf/heaps/Kconfig                 | 10 ------
>  drivers/dma-buf/heaps/cma_heap.c              | 47 +++++++++++++++++----------
>  include/linux/dma-buf/heaps/cma.h             | 16 +++++++++
>  kernel/dma/contiguous.c                       | 11 +++++++
>  6 files changed, 72 insertions(+), 37 deletions(-)
> ---
> base-commit: 47633099a672fc7bfe604ef454e4f116e2c954b1
> change-id: 20240515-dma-buf-ecc-heap-28a311d2c94e
> prerequisite-message-id: <20250610131231.1724627-1-jkangas@...hat.com>
> prerequisite-patch-id: bc44be5968feb187f2bc1b8074af7209462b18e7
> prerequisite-patch-id: f02a91b723e5ec01fbfedf3c3905218b43d432da
> prerequisite-patch-id: e944d0a3e22f2cdf4d3b3906e5603af934696deb
>
> Best regards,
> --
> Maxime Ripard <mripard@...nel.org>
>


-- 
Thanks and regards,

Sumit Semwal (he / him)
Senior Tech Lead - Android, Platforms and Virtualisation
Linaro.org │ Arm Solutions at Light Speed

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