Re: [PATCH v3 0/2] dma-buf: heaps: Support carved-out heaps

[Maxime Ripard <mripard@kernel.org>]

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Hi,

On Mon, Apr 07, 2025 at 06:29:06PM +0200, Maxime Ripard wrote:
> Hi,
> 
> This series is the follow-up of the discussion that John and I had some
> time ago here:
> 
> https://lore.kernel.org/all/CANDhNCquJn6bH3KxKf65BWiTYLVqSd9892-xtFDHHqqyrroCMQ@mail.gmail.com/
> 
> 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 has been dropped in this second
> version, we still need 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.
> 
> I submitted a draft PR to the DT schema for the bindings used in this
> PR:
> https://github.com/devicetree-org/dt-schema/pull/138

One thing the discussion about the CMA heap naming[1] with John made me
realize is that if we have both a region that is exported as a
carved-out heap, and with devices pointing to it with reserved-memory,
we wouldn't use the same allocator in both cases.

It looks like we have four cases:

  - We have a shared-dma-pool region with the reusable property: the
    region is registered as a CMA area, the devices will allocate from
    that.

  - We have a shared-dma-pool region without the reusable property: the
    region is registered as a coherent DMA area, and the devices will
    allocate from that pool.

  - We have a restricted-dma-pool region, devices will allocate from
    swiotlb

  - We have any other region, we can do whatever.

So, the driver only supports the fourth case, and should be
significantly more complicated.

I'll work on that.

Maxime

1: https://lore.kernel.org/dri-devel/20250422191939.555963-1-jkangas@redhat.com/

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