In accordance with [1, 2] the DW eDMA controller has been created to be
part of the DW PCIe Root Port and DW PCIe End-point controllers and to
offload the transferring of large blocks of data between application and
remote PCIe domains leaving the system CPU free for other tasks. In the
first case (eDMA being part of DW PCIe Root Port) the eDMA controller is
always accessible via the CPU DBI interface and never over the PCIe wire.
The latter case is more complex. Depending on the DW PCIe End-Point IP-core
synthesize parameters it's possible to have the eDMA registers accessible
not only from the application CPU side, but also via mapping the eDMA CSRs
over a dedicated endpoint BAR. So based on the specifics denoted above the
eDMA driver is supposed to support two types of the DMA controller setups:
1) eDMA embedded into the DW PCIe Root Port/End-point and accessible over
the local CPU from the application side.
2) eDMA embedded into the DW PCIe End-point and accessible via the PCIe
wire with MWr/MRd TLPs generated by the CPU PCIe host controller.
Since the CPU memory resides different sides in these cases the semantics
of the MEM_TO_DEV and DEV_TO_MEM operations is flipped with respect to the
Tx and Rx DMA channels. So MEM_TO_DEV/DEV_TO_MEM corresponds to the Tx/Rx
channels in setup 1) and to the Rx/Tx channels in case of setup 2).
The DW eDMA driver has supported the case 2) since e63d79d1ff
("dmaengine: Add Synopsys eDMA IP core driver") in the framework of the
drivers/dma/dw-edma/dw-edma-pcie.c driver.
The case 1) support was added later by bd96f1b2f4 ("dmaengine: dw-edma:
support local dma device transfer semantics"). Afterwards the driver was
supposed to cover the both possible eDMA setups, but the latter commit
turned out to be not fully correct.
The problem was that the commit together with the new functionality support
also changed the channel direction semantics so the eDMA Read-channel
(corresponding to the DMA_DEV_TO_MEM direction for case 1) now uses the
sgl/cyclic base addresses as the Source addresses of the DMA transfers and
dma_slave_config.dst_addr as the Destination address of the DMA transfers.
Similarly the eDMA Write-channel (corresponding to the DMA_MEM_TO_DEV
direction for case 1) now uses dma_slave_config.src_addr as a source
address of the DMA transfers and sgl/cyclic base address as the Destination
address of the DMA transfers. This contradicts the logic of the
DMA-interface, which implies that DEV side is supposed to belong to the
PCIe device memory and MEM - to the CPU/Application memory. Indeed it seems
irrational to have the SG-list defined in the PCIe bus space, while
expecting a contiguous buffer allocated in the CPU memory. Moreover the
passed SG-list and cyclic DMA buffers are supposed to be mapped in a way so
to be seen by the DW eDMA Application (CPU) interface.
So in order to have the correct DW eDMA interface we need to invert the
eDMA Rd/Wr-channels and DMA-slave directions semantics by selecting the
src/dst addresses based on the DMA transfer direction instead of using the
channel direction capability.
[1] DesignWare Cores PCI Express Controller Databook - DWC PCIe Root Port,
v.5.40a, March 2019, p.1092
[2] DesignWare Cores PCI Express Controller Databook - DWC PCIe Endpoint,
v.5.40a, March 2019, p.1189
Co-developed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Fixes: bd96f1b2f4 ("dmaengine: dw-edma: support local dma device transfer semantics")
Link: https://lore.kernel.org/r/20220524152159.2370739-7-Frank.Li@nxp.com
Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-By: Vinod Koul <vkoul@kernel.org>
c1e3397917