Subject: Re: [RFC PATCH v3 5/6] dt-bindings: of: Add restricted DMA pool

From: Rob Herring <[email-masked]>
Date: Wed, 20 Jan 2021 10:53:48 -0600

On Wed, Jan 06, 2021 at 11:41:23AM +0800, Claire Chang wrote:
> Introduce the new compatible string, restricted-dma-pool, for restricted
> DMA. One can specify the address and length of the restricted DMA memory
> region by restricted-dma-pool in the device tree.

If this goes into DT, I think we should be able to use dma-ranges for 
this purpose instead. Normally, 'dma-ranges' is for physical bus 
restrictions, but there's no reason it can't be used for policy or to 
express restrictions the firmware has enabled.

> Signed-off-by: Claire Chang <[email-masked]>
> ---
>  .../reserved-memory/reserved-memory.txt       | 24 +++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> index e8d3096d922c..44975e2a1fd2 100644
> --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> +++ b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> @@ -51,6 +51,20 @@ 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.
> +        - restricted-dma-pool: This indicates a region of memory meant to be
> +          used as a pool of restricted DMA buffers for a set of devices. The
> +          memory region would be the only region accessible to those devices.
> +          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. 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 way to restrict
> +          the DMA to a predefined memory region.
>          - vendor specific string in the form <vendor>,[<device>-]<usage>
>  no-map (optional) - empty property
>      - Indicates the operating system must not create a virtual mapping
> @@ -120,6 +134,11 @@ one for multimedia processing (named multimedia-memory@77000000, 64MiB).
>  			compatible = "acme,multimedia-memory";
>  			reg = <0x77000000 0x4000000>;
>  		};
> +
> +		restricted_dma_mem_reserved: restricted_dma_mem_reserved {
> +			compatible = "restricted-dma-pool";
> +			reg = <0x50000000 0x400000>;
> +		};
>  	};
>  
>  	/* ... */
> @@ -138,4 +157,9 @@ one for multimedia processing (named multimedia-memory@77000000, 64MiB).
>  		memory-region = <&multimedia_reserved>;
>  		/* ... */
>  	};
> +
> +	pcie_device: pcie_device@0,0 {
> +		memory-region = <&restricted_dma_mem_reserved>;

PCI hosts often have inbound window configurations that limit the 
address range and translate PCI to bus addresses. Those windows happen 
to be configured by dma-ranges. In any case, wouldn't you want to put 
the configuration in the PCI host node? Is there a usecase of 
restricting one PCIe device and not another? 

Rob