Hello Christian, Thanks for your feedback!
On Thu, 10 Apr 2025 18:29:12 +0200 Christian König <christian.koe...@amd.com> wrote: > > Many UIO users performing DMA from their UIO device need to access the > > DMA addresses of the allocated buffers. There are out-of-tree drivers > > that allow to do it but nothing in the mainline. > > Well that basically disqualifies this patch set in the first paragraph. > > To justify some kernel change we always need an in kernel user of the > interface, since this is purely for out-of-tree drivers this is a > no-go to begin with. I'm not sure to understand your comment here. This patch series is about extending the UIO interface... which is a user-space interface. So obviously it has no "in-kernel user" because it's meant to be used from user-space. Could you clarify what you meant here? > What you could potentially do is to create an UIO driver which > imports DMA-bufs, pins them and then provide the DMA addresses to > userspace. > > But please be aware that DMA-fences are fundamentally incompatible > with UIO. So you won't be able to do any form of synchronization > which probably makes the implementation pretty limited. Could you clarify why DMA-fences would be needed here, and for what synchronization? The DMA buffers allocated here are DMA coherent buffers. So the user-space application that uses UIO will allocate such buffers once at application start, retrieve their DMA address, and then program DMA transfers as it sees fit using the memory-mapped registers accessible through UIO. I'm not sure which synchronization you are referring to. We are not "chaining" DMA transfers, with for example a camera interface feeding its DMA buffers to an ISP or something like that. The typical use case here is some IP block in an FPGA that does DMA transfers to transfer data to/from some sort of specialized I/O interface. We get an interrupt when the transfer is done, and we can re-use the buffer for the next transfer. If you could clarify here as well, it would definitely help in understanding the shortcomings/limitations. Thanks a lot! Thomas Petazzoni -- Thomas Petazzoni, co-owner and CEO, Bootlin Embedded Linux and Kernel engineering and training https://bootlin.com
