Re: [RFC PATCH V1 1/6] dt-bindings: mt8183: Add binding for FD shared memory

[Rob Herring <robh@kernel.org>]

On Tue, Apr 23, 2019 at 06:45:00PM +0800, Jerry-ch Chen wrote:
> From: Jerry-ch Chen <jerry-ch.chen@mediatek.com>
> 
> This patch adds the binding for describing the shared memory
> used to exchange meta data between the co-processor and Face
> Detection (FD) unit of the camera system on Mediatek SoCs.
> 
> Signed-off-by: Jerry-ch Chen <jerry-ch.chen@mediatek.com>
> ---
>  .../mediatek,reserve-memory-fd_smem.txt       | 44 +++++++++++++++++++
>  1 file changed, 44 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/reserved-memory/mediatek,reserve-memory-fd_smem.txt
> 
> diff --git a/Documentation/devicetree/bindings/reserved-memory/mediatek,reserve-memory-fd_smem.txt b/Documentation/devicetree/bindings/reserved-memory/mediatek,reserve-memory-fd_smem.txt
> new file mode 100644
> index 000000000000..52ae5071e238
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/reserved-memory/mediatek,reserve-memory-fd_smem.txt
> @@ -0,0 +1,44 @@
> +Mediatek FD Shared Memory binding
> +
> +This binding describes the shared memory, which serves the purpose of
> +describing the shared memory region used to exchange data between Face
> +Detection hardware (FD) and co-processor in Mediatek SoCs.
> +
> +The co-processor doesn't have the iommu so we need to use the physical
> +address to access the shared buffer in the firmware.
> +
> +The Face Detection hardware (FD) can access memory through mt8183 IOMMU so
> +it can use dma address to access the memory region.
> +(See iommu/mediatek,iommu.txt for the detailed description of Mediatek IOMMU)
> +
> +
> +Required properties:
> +
> +- compatible: must be "mediatek,reserve-memory-fd_smem"
> +
> +- reg: required for static allocation (see reserved-memory.txt for
> +  the detailed usage)
> +
> +- alloc-range: required for dynamic allocation. The range must
> +  between 0x00000400 and 0x100000000 due to the co-processer's
> +  addressing limitation
> +
> +- size: required for dynamic allocation. The unit is bytes.
> +  for Face Detection Unit, you need 1 MB at least.

What's the most?

I don't think you really need reserved memory here if you don't need a 
fixed address. The size is not that big that a contiguous allocation 
shouldn't be a problem (though with IOMMU you don't even need that). 
'dma-ranges' can be used to set the kernel's dma mask and ensure a 
range below 0x10000000.

Rob