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

[Niklas Cassel <cassel@kernel.org>]

Hello Sumit,

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@oss.qualcomm.com>

If you look at e.g.
drivers/pci/endpoint/functions/pci-epf-test.c:pci_epf_test_read()

you can see that it does call pci_epc_mem_map(), which will end up
as a call to dw_pcie_prog_outbound_atu() to setup an iATU mapping
on DWC based PCIe controllers.


If you configure DWC based controllers to bypass the iATU, this
call to pci_epc_mem_map() is completely useless.


Perhaps the DWC driver can set a DMA_SLAVE_SKIP_MEM_MAP flag
or similar when registering the eDMA, which pci-epf-test then could check,
and not call pci_epc_mem_map() if DMA_SLAVE_SKIP_MEM_MAP is set.


Because, while I can read in the databook that this is not needed
for DWC based controllers, I have no idea if the pci_epc_mem_map()
is still needed for e.g.:
drivers/pci/controller/cadence/pcie-cadence-ep.c
drivers/pci/controller/pcie-rcar-ep.c
drivers/pci/controller/pcie-rockchip-ep.c


Kind regards,
Niklas