Re: [PATCH v7 14/15] dt-bindings: of: Add restricted DMA pool

Hi Claire,

On Tue, May 18, 2021 at 02:42:14PM +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 reserved-memory node.

Signed-off-by: Claire Chang tientzu@chromium.org

.../reserved-memory/reserved-memory.txt | 27 +++++++++++++++++++ 1 file changed, 27 insertions(+)

diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt index e8d3096d922c..284aea659015 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 @@ -120,6 +137,11 @@ 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>;
  

• };
  

nit: You need to update the old text that states "This example defines 3 contiguous regions ...".

};

/* ... */ @@ -138,4 +160,9 @@ 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>;
  

• /* ... */
  

• };

I still don't understand how this works for individual PCIe devices -- how is dev->of_node set to point at the node you have above?

I tried adding the memory-region to the host controller instead, and then I see it crop up in dmesg:

| pci-host-generic 40000000.pci: assigned reserved memory node restricted_dma_mem_reserved

but none of the actual PCI devices end up with 'dma_io_tlb_mem' set, and so the restricted DMA area is not used. In fact, swiotlb isn't used at all.

What am I missing to make this work with PCIe devices?

Thanks,

Will