Re: [PATCH v12 11/12] dt-bindings: of: Add restricted DMA pool

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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@xxxxxxxxxxxx>
> ---
>  .../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 ?