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Date:   Wed, 16 Jun 2021 17:09:29 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@kernel.org>
To:     Claire Chang <tientzu@chromium.org>
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Subject: Re: [PATCH v12 11/12] dt-bindings: of: Add restricted DMA pool
In-Reply-To: <20210616062157.953777-12-tientzu@chromium.org>
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References: <20210616062157.953777-1-tientzu@chromium.org> <20210616062157.953777-12-tientzu@chromium.org>
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On Wed, 16 Jun 2021, 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 reserved-memory node.
> 
> Signed-off-by: Claire Chang <tientzu@chromium.org>
> ---
>  .../reserved-memory/reserved-memory.txt       | 36 +++++++++++++++++--
>  1 file changed, 33 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> index e8d3096d922c..46804f24df05 100644
> --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> +++ b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> @@ -51,6 +51,23 @@ 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 lock down
> +          the memory access, e.g., MPU. Note that since coherent allocation
> +          needs remapping, one must set up another device coherent pool by
> +          shared-dma-pool and use dma_alloc_from_dev_coherent instead for atomic
> +          coherent allocation.
>          - vendor specific string in the form <vendor>,[<device>-]<usage>
>  no-map (optional) - empty property
>      - Indicates the operating system must not create a virtual mapping
> @@ -85,10 +102,11 @@ memory-region-names (optional) - a list of names, one for each corresponding
>  
>  Example
>  -------
> -This example defines 3 contiguous regions are defined for Linux kernel:
> +This example defines 4 contiguous regions for Linux kernel:
>  one default of all device drivers (named linux,cma@72000000 and 64MiB in size),
> -one dedicated to the framebuffer device (named framebuffer@78000000, 8MiB), and
> -one for multimedia processing (named multimedia-memory@77000000, 64MiB).
> +one dedicated to the framebuffer device (named framebuffer@78000000, 8MiB),
> +one for multimedia processing (named multimedia-memory@77000000, 64MiB), and
> +one for restricted dma pool (named restricted_dma_reserved@0x50000000, 64MiB).
>  
>  / {
>  	#address-cells = <1>;
> @@ -120,6 +138,11 @@ one for multimedia processing (named multimedia-memory@77000000, 64MiB).
>  			compatible = "acme,multimedia-memory";
>  			reg = <0x77000000 0x4000000>;
>  		};
> +
> +		restricted_dma_reserved: restricted_dma_reserved {
> +			compatible = "restricted-dma-pool";
> +			reg = <0x50000000 0x4000000>;
> +		};
>  	};
>  
>  	/* ... */
> @@ -138,4 +161,11 @@ one for multimedia processing (named multimedia-memory@77000000, 64MiB).
>  		memory-region = <&multimedia_reserved>;
>  		/* ... */
>  	};
> +
> +	pcie_device: pcie_device@0,0 {
> +		reg = <0x83010000 0x0 0x00000000 0x0 0x00100000
> +		       0x83010000 0x0 0x00100000 0x0 0x00100000>;
> +		memory-region = <&restricted_dma_mem_reserved>;

Shouldn't it be &restricted_dma_reserved ?