Re: [PATCH v4 3/4] dt-bindings: reserved-memory: Make DMA-BUF CMA heap DT-configurable

[Rob Herring <robh@kernel.org>]

On Thu, Jan 21, 2021 at 09:55:01AM -0800, Minchan Kim wrote:
> From: Hyesoo Yu <hyesoo.yu@samsung.com>
> 
> Document devicetree binding for chunk cma heap on dma heap framework.
> 
> The DMA chunk heap supports the bulk allocation of higher order pages.
> 
> The chunk heap's allocator allocates from the CMA area. It is optimized
> to perform bulk allocation of higher order pages in an efficient manner.
> For this purpose, the heap needs an exclusive CMA area that will only be
> used for allocation by the heap. This is the reason why we need to use
> the DT to create and configure a reserved memory region for use by the
> chunk CMA heap driver. Since all allocation from DMA-BUF heaps happen
> from the user-space, there is no other appropriate device-driver that
> we can use to register the chunk CMA heap and configure the reserved
> memory region for its use.

LWN tells me we don't need carve outs any more[1]: "CMA now relies on 
compaction and no longer uses a carved-out memory region."

So why do we need this?

[1] https://lwn.net/Articles/839216/

> 
> Signed-off-by: Hyesoo Yu <hyesoo.yu@samsung.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> Signed-off-by: Hridya Valsaraju <hridya@google.com>
> ---
>  .../reserved-memory/dma_heap_chunk.yaml       | 56 +++++++++++++++++++
>  1 file changed, 56 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/reserved-memory/dma_heap_chunk.yaml
> 
> diff --git a/Documentation/devicetree/bindings/reserved-memory/dma_heap_chunk.yaml b/Documentation/devicetree/bindings/reserved-memory/dma_heap_chunk.yaml
> new file mode 100644
> index 000000000000..00db0ae6af61
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/reserved-memory/dma_heap_chunk.yaml
> @@ -0,0 +1,56 @@
> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/reserved-memory/dma_heap_chunk.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: Device tree binding for chunk heap on DMA HEAP FRAMEWORK
> +
> +description: |
> +  The DMA chunk heap is backed by the Contiguous Memory Allocator (CMA) and
> +  supports bulk allocation of fixed size pages.
> +
> +maintainers:
> +  - Hyesoo Yu <hyesoo.yu@samsung.com>
> +  - John Stultz <john.stultz@linaro.org>
> +  - Minchan Kim <minchan@kernel.org>
> +  - Hridya Valsaraju<hridya@google.com>
> +
> +
> +properties:
> +  compatible:
> +    enum:
> +      - dma_heap,chunk

Convention is vendor,thing and 'dma_heap' is not a vendor. Also, '-' is 
preferred over '_'.

> +
> +  chunk-order:
> +    description: |
> +            order of pages that will get allocated from the chunk DMA heap.

Page size depends on Linux configuration. And 'order' is very much a 
Linuxism.

> +    maxItems: 1
> +
> +  size:
> +    maxItems: 1
> +
> +  alignment:
> +    maxItems: 1
> +
> +required:
> +  - compatible
> +  - size
> +  - alignment
> +  - chunk-order
> +
> +additionalProperties: false
> +
> +examples:
> +  - |
> +    reserved-memory {
> +        #address-cells = <2>;
> +        #size-cells = <1>;
> +
> +        chunk_memory: chunk_memory {
> +            compatible = "dma_heap,chunk";
> +            size = <0x3000000>;
> +            alignment = <0x0 0x00010000>;
> +            chunk-order = <4>;
> +        };
> +    };
> -- 
> 2.30.0.296.g2bfb1c46d8-goog
>