Re: [RFC v2 4/5] dt-bindings: of: Add plumbing for restricted DMA pool

[Rob Herring <robh@kernel.org>]

On Tue, Jul 28, 2020 at 01:01:39PM +0800, Claire Chang wrote:
> Introduce the new compatible string, device-swiotlb-pool, for restricted
> DMA. One can specify the address and length of the device swiotlb memory
> region by device-swiotlb-pool in the device tree.
> 
> Signed-off-by: Claire Chang <tientzu@chromium.org>
> ---
>  .../reserved-memory/reserved-memory.txt       | 35 +++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> index 4dd20de6977f..78850896e1d0 100644
> --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> +++ b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> @@ -51,6 +51,24 @@ compatible (optional) - standard definition
>            used as a shared pool of DMA buffers for a set of devices. It can
>            be used by an operating system to instantiate the necessary pool
>            management subsystem if necessary.
> +        - device-swiotlb-pool: This indicates a region of memory meant to be

swiotlb is a Linux thing. The binding should be independent.

> +          used as a pool of device swiotlb buffers for a given device. When
> +          using this, the no-map and reusable properties must not be set, so the
> +          operating system can create a virtual mapping that will be used for
> +          synchronization. Also, there must be a restricted-dma property in the
> +          device node to specify the indexes of reserved-memory nodes. One can
> +          specify two reserved-memory nodes in the device tree. One with
> +          shared-dma-pool to handle the coherent DMA buffer allocation, and
> +          another one with device-swiotlb-pool for regular DMA to/from system
> +          memory, which would be subject to bouncing. The main purpose for
> +          restricted DMA is to mitigate the lack of DMA access control on
> +          systems without an IOMMU, which could result in the DMA accessing the
> +          system memory at unexpected times and/or unexpected addresses,
> +          possibly leading to data leakage or corruption. The feature on its own
> +          provides a basic level of protection against the DMA overwriting buffer
> +          contents at unexpected times. However, to protect against general data
> +          leakage and system memory corruption, the system needs to provide a
> +          way to restrict the DMA to a predefined memory region.

I'm pretty sure we already support per device carveouts and I don't 
understand how this is different.

What is the last sentence supposed to imply? You need an IOMMU?

>          - vendor specific string in the form <vendor>,[<device>-]<usage>
>  no-map (optional) - empty property
>      - Indicates the operating system must not create a virtual mapping
> @@ -117,6 +135,16 @@ one for multimedia processing (named multimedia-memory@77000000, 64MiB).
>  			compatible = "acme,multimedia-memory";
>  			reg = <0x77000000 0x4000000>;
>  		};
> +
> +		wifi_coherent_mem_region: wifi_coherent_mem_region {
> +			compatible = "shared-dma-pool";
> +			reg = <0x50000000 0x400000>;
> +		};
> +
> +		wifi_device_swiotlb_region: wifi_device_swiotlb_region {
> +			compatible = "device-swiotlb-pool";
> +			reg = <0x50400000 0x4000000>;
> +		};
>  	};
>  
>  	/* ... */
> @@ -135,4 +163,11 @@ one for multimedia processing (named multimedia-memory@77000000, 64MiB).
>  		memory-region = <&multimedia_reserved>;
>  		/* ... */
>  	};
> +
> +	pcie_wifi: pcie_wifi@0,0 {
> +		memory-region = <&wifi_coherent_mem_region>,
> +			 <&wifi_device_swiotlb_region>;
> +		restricted-dma = <0>, <1>;
> +		/* ... */
> +	};
>  };
> -- 
> 2.28.0.rc0.142.g3c755180ce-goog
>