Re: [PATCH] dt-bindings: PCI: mediatek-gen3: Allow memory-region for restricted DMA buffer

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, May 08, 2026 at 02:36:32PM +0800, Chen-Yu Tsai wrote:
> On some SoCs without an IOMMU behind the PCIe controller, the PCIe
> controller memory access could be limited to a small region by the
> firmware configuring a memory protection unit. This memory region
> must be assigned to the PCIe controller so that the OS knows to
> use that region. Otherwise PCIe devices would not work properly.
> 
> Allow the memory-region property with one item pointing to a
> restricted DMA buffer.
> 
> Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
> ---
> This patch compliments another patch that moved the memory-region from
> the PCIe device to the PCIe controller [1].
> 
> [1] https://lore.kernel.org/all/20260430120725.241779-1-wenst@chromium.org/
> 
>  Documentation/devicetree/bindings/pci/mediatek-pcie-gen3.yaml | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/pci/mediatek-pcie-gen3.yaml b/Documentation/devicetree/bindings/pci/mediatek-pcie-gen3.yaml
> index 4db700fc36ba..4a9e41d01628 100644
> --- a/Documentation/devicetree/bindings/pci/mediatek-pcie-gen3.yaml
> +++ b/Documentation/devicetree/bindings/pci/mediatek-pcie-gen3.yaml
> @@ -115,6 +115,10 @@ properties:
>    power-domains:
>      maxItems: 1
>  
> +  memory-region:
> +    maxItems: 1
> +    description: phandle to restricted DMA buffer

I guess this is similar to
https://lore.kernel.org/linux-pci/20250716053950.199079-1-huaqian.li@siemens.com/
and uses
https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/reserved-memory/shared-dma-pool.yaml?

Looks like those keystone changes were never merged; I don't know what
happened to them.  But it will be good if everybody does it the same
way.

I wish there were a simple way to grep for this restricted DMA
concept.  Maybe there is and I just haven't found it :)

>    mediatek,pbus-csr:
>      $ref: /schemas/types.yaml#/definitions/phandle-array
>      items:
> -- 
> 2.54.0.563.g4f69b47b94-goog
>