Re: [PATCH v2] pci: designware: Set DMA_BYPASS bit in outbound IATU CTRL2 register

[Bjorn Helgaas]

On Tue, Sep 08, 2026 at 11:48:22AM +0530, Sumit Kumar wrote:
> An outbound iATU region matches any local (AXI/application-side)
> address that falls within its Base/Limit window and rewrites it to
> the PCIe-side address programmed in that region's Target Address
> Registers before the transaction goes out on the link. This matching
> does not distinguish CPU-initiated transactions from ones issued by
> the internal DMA engine (eDMA/HDMA).
> 
> Example: outbound region 0 is programmed with Base/Limit covering
> local addresses 0x8000_0000-0x8000_0FFF and Target 0x1_0000_0000, for
> CPU-initiated accesses. The DMA engine is later given a descriptor
> whose destination is the host address 0x8000_0080, which happens to
> fall inside region 0's Base/Limit range. The iATU matches the DMA
> transaction against region 0 too and rewrites it to 0x1_0000_0080
> instead of passing 0x8000_0080 through unchanged. The transaction
> then lands on the wrong host address, causing data corruption or an
> IOMMU fault when the rewritten address has no valid mapping.
> 
> Set the DMA_BYPASS bit (bit 27) in PCIE_ATU_REGION_CTRL2 when
> programming outbound iATU regions so only CPU/AXI-initiated
> transactions are matched against the region; DMA engine transactions
> bypass the translation and go out on the PCIe link with their
> original address.
> 
> Signed-off-by: Sumit Kumar <sumit.kumar@xxxxxxxxxxxxxxxx>

Thanks for the fix.  A data corruption is pretty disastrous.

Is there a Fixes: tag we can add here?

Probably also a stable tag?

What is the connection with ver 460A?  It would be nice to have
something meaningful here, e.g., "has_eDMA" or something that connects
the test with a controller property.

> ---
> pci: designware: Set DMA_BYPASS in outbound IATU to prevent HDMA
> interception
> ---
> Changes in v2:
> - Rewrote the commit message according to suggestion(Mani).
> - Link to v1: https://lore.kernel.org/r/20260826-dma_bypass-v1-1-3a53dc748ce2@xxxxxxxxxxxxxxxx
> ---
>  drivers/pci/controller/dwc/pcie-designware.c | 2 ++
>  drivers/pci/controller/dwc/pcie-designware.h | 1 +
>  2 files changed, 3 insertions(+)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-designware.c b/drivers/pci/controller/dwc/pcie-designware.c
> index 593388f29bdd48381cb04710f445042aaf2be29f..9297d0e607e3a1caaeb585b23742f9a29607b7f2 100644
> --- a/drivers/pci/controller/dwc/pcie-designware.c
> +++ b/drivers/pci/controller/dwc/pcie-designware.c
> @@ -577,6 +577,8 @@ int dw_pcie_prog_outbound_atu(struct dw_pcie *pci,
>  		/* The data-less messages only for now */
>  		val |= PCIE_ATU_INHIBIT_PAYLOAD | atu->code;
>  	}
> +	if (dw_pcie_ver_is_ge(pci, 460A))
> +		val |= PCIE_ATU_DMA_BYPASS;
>  	dw_pcie_writel_atu_ob(pci, atu->index, PCIE_ATU_REGION_CTRL2, val);
>  
>  	/*
> diff --git a/drivers/pci/controller/dwc/pcie-designware.h b/drivers/pci/controller/dwc/pcie-designware.h
> index 0735ae94092404bc2976af67a58f6842cc5e5c50..668a5ba1c6f59aef1aaf15566b77a649bd282ce0 100644
> --- a/drivers/pci/controller/dwc/pcie-designware.h
> +++ b/drivers/pci/controller/dwc/pcie-designware.h
> @@ -201,6 +201,7 @@
>  #define PCIE_ATU_ENABLE			BIT(31)
>  #define PCIE_ATU_BAR_MODE_ENABLE	BIT(30)
>  #define PCIE_ATU_CFG_SHIFT_MODE_ENABLE	BIT(28)
> +#define PCIE_ATU_DMA_BYPASS		BIT(27)
>  #define PCIE_ATU_INHIBIT_PAYLOAD	BIT(22)
>  #define PCIE_ATU_FUNC_NUM_MATCH_EN      BIT(19)
>  #define PCIE_ATU_LOWER_BASE		0x008
> 
> ---
> base-commit: 45c13f3f9e3bb15fd89ff2864c6f627a3b4b4229
> change-id: 20260826-dma_bypass-bbbf8a0c9af7
> 
> Best regards,
> -- 
> Sumit Kumar <sumit.kumar@xxxxxxxxxxxxxxxx>
>