On Wed, Oct 23, 2024 at 11:51:41AM +0900, Damien Le Moal wrote: (snip) > For an endpoint initiated transfer, typically, the PCI address is obtained from > some "command" received through a BAR or through DMA. The command has the PCI > addresses to use for transfering data to/from the host (e.g. an nvme rw command > uses PRPs or SGLs to specify the PCI address segments for the data buffer of a > command). For this case, the EPF driver calls calls pci_epc_mem_map() for the > command buffer, does the transfer (memcpy_toio/fromio()) and unmaps with > pci_epc_mem_unmap(). Note though that here, if an eDMA channel is used for the > transfer, the DMA engine will do the mapping automatically and the epf does not > need to call pci_epc_mem_map()/pci_epc_mem_unmap(). There is still an issue in > this area which is that it is *not* clear if the DMA channel used can actually > do the mapping automatically or not. E.g. the generic DMA channel (mem copy > offload engine) will not. So there is still some API improvement needed to > abstract more HW dependent things here. FWIW, in my final reply here: https://lore.kernel.org/lkml/ZiYuIaX7ZV0exKMt@ryzen/ " I did suggest that DWC-based drivers could set a DMA_SLAVE_SKIP_MEM_MAP flag or similar when registering the eDMA, which pci-epf-test then could check, but I got no response if anyone else thought that this was a good idea. " For DMA_SLAVE (private tx/rx DMA channels): For DWC-based controllers, we can definitely set DMA_SLAVE_SKIP_MEM_MAP when registering the eDMA (e.g. in dw_pcie_edma_detect()). However, I don't know how the DMA hardware (if any) in: drivers/pci/controller/cadence/pcie-cadence-ep.c drivers/pci/controller/pcie-rcar-ep.c drivers/pci/controller/pcie-rockchip-ep.c works, so I'm not sure if those drivers can set DMA_SLAVE_SKIP_MEM_MAP (the safest thing is to not set that flag, until we know how they work). For DMA_MEMCPY: We know that we need to perform pci_epc_mem_map() when using DMA API + "dummy" memcpy dma-channel (DMA_MEMCPY). I could imagine that some embedded DMA controllers also provide DMA_MEMCPY capabilities (in addition to DMA_SLAVE). So I guess the safest thing is to call the flag something like: PCI_EPC_DMA_SKIP_MEM_MAP (rather than PCI_EPC_DMA_SLAVE_SKIP_MEM_MAP). (since the embedded DMA controller might provide both DMA_SLAVE and DMA_MEMCPY). And let the EPC driver (e.g. dw_pcie_edma_detect()), or possibly the DMA driver itself to provide/set this flag. Kind regards, Niklas
