On Tue, Dec 17, 2024 at 10:57:02AM +0530, Vinod Koul wrote: > On 16-12-24, 11:12, Damien Le Moal wrote: > > On 2024/12/16 8:35, Vinod Koul wrote: > > > Hi Niklas, > > > > > > On 13-12-24, 17:59, Niklas Cassel wrote: > > >> Hello Vinod, > > >> > > >> I am a bit confused about the usage of the dmaengine API, and I hope that you > > >> could help make me slightly less confused :) > > > > > > Sure thing! > > > > > >> If you look at the nvmet_pciep_epf_dma_transfer() function below, it takes a > > >> mutex around the dmaengine_slave_config(), dmaengine_prep_slave_single(), > > >> dmaengine_submit(), dma_sync_wait(), and dmaengine_terminate_sync() calls. > > >> > > >> I really wish that we would remove this mutex, to get better performance. > > >> > > >> > > >> If I look at e.g. the drivers/dma/dw-edma/dw-edma-core.c driver, I can see > > >> that dmaengine_prep_slave_single() (which will call > > >> device_prep_slave_sg(.., .., 1, .., .., ..)) allocates a new > > >> dma_async_tx_descriptor for each function call. > > >> > > >> I can see that device_prep_slave_sg() (dw_edma_device_prep_slave_sg()) will > > >> call dw_edma_device_transfer() which will call vchan_tx_prep(), which adds > > >> the descriptor to the tail of a list. > > >> > > >> I can also see that dw_edma_done_interrupt() will automatically start the > > >> transfer of the next descriptor (using vchan_next_desc()). > > >> > > >> So this looks like it is supposed to be asynchronous... however, if we simply > > >> remove the mutex, we get IOMMU errors, most likely because the DMA writes to > > >> an incorrect address. > > >> > > >> It looks like this is because dmaengine_prep_slave_single() really requires > > >> dmaengine_slave_config() for each transfer. (Since we are supplying a src_addr > > >> in the sconf that we are supplying to dmaengine_slave_config().) > > >> > > >> (i.e. we can't call dmaengine_slave_config() while a DMA transfer is active.) > > >> > > >> So while this API is supposed to be async, to me it looks like it can only > > >> be used in a synchronous manner... But that seems like a really weird design. > > >> > > >> Am I missing something obvious here? > > > > > > Yes, I feel nvme being treated as slave transfer, which it might not be. > > > This API was designed for peripherals like i2c/spi etc where we have a > > > hardware address to read/write to. So the dma_slave_config would pass on > > > the transfer details for the peripheral like address, width of fifo, > > > depth etc and these are setup config, so call once for a channel and then > > > prepare the descriptor, submit... and repeat of prepare and submit ... > > > > > > I suspect since you are passing an address which keep changing in the > > > dma_slave_config, you need to guard that and prep_slave_single() call, > > > as while preparing the descriptor driver would lookup what was setup for > > > the configuration. > > > > > > I suggest then use the prep_memcpy() API instead and pass on source and > > > destination, no need to lock the calls... > > > > Vinod, > > > > Thank you for the information. However, I think we can use this only if the DMA > > controller driver implements the device_prep_dma_memcpy operation, no ? > > In our case, the DWC EDMA driver does not seem to implement this. > > It should be added in that case. > > Before that, the bigger question is, should nvme be slave transfer or > memcpy.. Was driver support the reason why the slave transfer was used here...? > > As i said, slave is for peripherals which have a static fifo to > send/receive data from, nvme sounds like a memory transfer to me, is > that a right assumption? > My understanding is that DMA_MEMCPY is for local DDR transfer i.e., src and dst are local addresses. And DMA_SLAVE is for transfer between remote and local addresses. I haven't looked into the NVMe EPF driver yet, but it should do the transfer between remote and local addresses. This is similar to MHI EPF driver as well. - Mani -- மணிவண்ணன் சதாசிவம்
