Message-ID: <49e3fa834aadb37452112bb704a1a1593c1fd0b8.camel@ndufresne.ca>
Date: Wed, 09 Jul 2025 09:10:02 -0400
From: Nicolas Dufresne <nicolas@...fresne.ca>
To: Maxime Ripard <mripard@...nel.org>, 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>, "T.J. Mercier" <tjmercier@...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>
Cc: 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
Subject: Re: [PATCH v6 0/2] dma-buf: heaps: Create a CMA heap for each CMA
 reserved region

Hi Maxime,

Le mercredi 09 juillet 2025 à 14:44 +0200, Maxime Ripard a écrit :
> Hi,
> 
> 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.

Does that means that if we carve-out memory for a co-processor operating system,
that memory region is now available to userspace to allocate from ? Or is there
a nuance to that ?

For other carveout, such as RK3588 HDMI receiver, that is clearly a win, giving
user the ability to allocate using externally supplied constraints rather then
having to convince the v4l2 driver to match these. While keeping the safety that
this carveout will yield valid addresses for the IP.

Will there be a generic way to find out which driver/device this carveout
belongs to ? In V4L2, only complex cameras have userspace drivers, everything
else is generic code.

Nicolas

> 
> 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 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 (2):
>       dma/contiguous: Add helper to test reserved memory type
>       dma-buf: heaps: cma: Create CMA heap for each CMA reserved region
> 
>  drivers/dma-buf/heaps/cma_heap.c | 52
> +++++++++++++++++++++++++++++++++++++++-
>  include/linux/dma-map-ops.h      | 13 ++++++++++
>  kernel/dma/contiguous.c          |  7 ++++++
>  3 files changed, 71 insertions(+), 1 deletion(-)
> ---
> 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,

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