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From:   John Stultz <john.stultz@linaro.org>
Date:   Tue, 17 Nov 2020 19:00:54 -0800
Message-ID: <CALAqxLWqDLHpOHNEayvhDjJeXjEk_uneH2=d9fy8M87EjKfReA@mail.gmail.com>
Subject: Re: [PATCH 4/4] dma-heap: Devicetree binding for chunk heap
To:     Minchan Kim <minchan@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, Hyesoo Yu <hyesoo.yu@samsung.com>,
        Matthew Wilcox <willy@infradead.org>, david@redhat.com,
        iamjoonsoo.kim@lge.com, vbabka@suse.cz,
        Suren Baghdasaryan <surenb@google.com>,
        KyongHo Cho <pullip.cho@samsung.com>,
        John Dias <joaodias@google.com>,
        Hridya Valsaraju <hridya@google.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Brian Starkey <Brian.Starkey@arm.com>,
        linux-media <linux-media@vger.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>, Rob Herring <robh@kernel.org>,
        Christian Koenig <christian.koenig@amd.com>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>
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On Tue, Nov 17, 2020 at 10:19 AM Minchan Kim <minchan@kernel.org> wrote:
>
> From: Hyesoo Yu <hyesoo.yu@samsung.com>
>
> Document devicetree binding for chunk heap on dma heap framework
>
> Signed-off-by: Hyesoo Yu <hyesoo.yu@samsung.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  .../bindings/dma-buf/chunk_heap.yaml          | 52 +++++++++++++++++++
>  1 file changed, 52 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/dma-buf/chunk_heap.yaml
>
> diff --git a/Documentation/devicetree/bindings/dma-buf/chunk_heap.yaml b/Documentation/devicetree/bindings/dma-buf/chunk_heap.yaml
> new file mode 100644
> index 000000000000..f382bee02778
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/dma-buf/chunk_heap.yaml
> @@ -0,0 +1,52 @@
> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/dma-buf/chunk_heap.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: Device tree binding for chunk heap on DMA HEAP FRAMEWORK
> +
> +maintainers:
> +  - Sumit Semwal <sumit.semwal@linaro.org>
> +
> +description: |
> +  The chunk heap is backed by the Contiguous Memory Allocator (CMA) and
> +  allocates the buffers that are made up to a list of fixed size chunks
> +  taken from CMA. Chunk sizes are configurated when the heaps are created.
> +
> +properties:
> +  compatible:
> +    enum:
> +      - dma_heap,chunk
> +
> +  memory-region:
> +    maxItems: 1
> +
> +  alignment:
> +    maxItems: 1
> +
> +required:
> +  - compatible
> +  - memory-region
> +  - alignment
> +
> +additionalProperties: false
> +
> +examples:
> +  - |
> +    reserved-memory {
> +        #address-cells = <2>;
> +        #size-cells = <1>;
> +
> +        chunk_memory: chunk_memory {
> +            compatible = "shared-dma-pool";
> +            reusable;
> +            size = <0x10000000>;
> +        };
> +    };
> +
> +    chunk_default_heap: chunk_default_heap {
> +        compatible = "dma_heap,chunk";
> +        memory-region = <&chunk_memory>;
> +        alignment = <0x10000>;
> +    };


So I suspect Rob will push back on this as he has for other dt
bindings related to ion/dmabuf heaps (I tried to push a similar
solution to exporting multiple CMA areas via dmabuf heaps).

The proposal he seemed to like best was having an in-kernel function
that a driver would call to initialize the heap (associated with the
CMA region the driver is interested in). Similar to Kunihiko Hayashi's
patch here:
  - https://lore.kernel.org/lkml/1594948208-4739-1-git-send-email-hayashi.kunihiko@socionext.com/

The one sticking point for that patch (which I think is a good one),
is that we don't have any in-tree users, so it couldn't be merged yet.

A similar approach might be good here, but again we probably need to
have at least one in-tree user which could call such a registration
function.

thanks
-john