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

[Manivannan Sadhasivam]

On Tue, Sep 08, 2026 at 05:08:26PM +0200, Niklas Cassel wrote:
> 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@xxxxxxxxxxxxxxxx>
> 
> 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.
> 

pci_epc_mem_map() is already useless in the eDMA path today. So this patch
doesn't change that.

> 
> 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
> 

Fair point. But this should be done as a separate improvement, not related to
this patch.

- Mani

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