On Wed, Nov 30, 2022 at 08:23:45AM +0000, hinxx wrote: > > > > > > Sent with Proton Mail secure email. > > ------- Original Message ------- > On Wednesday, November 30th, 2022 at 8:05 AM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > > > On Tue, Nov 29, 2022 at 09:15:24PM +0000, hinxx wrote: > > > > > I'm looking to use a sendfile(2) with a Xilinx XDMA kernel driver in order to move data from a PCIe board with Xilinx FPGA to the network card with "zero-copy". > > > > > > Currently I'm getting EINVAL return status from sendfile(2) when providing opened XDMA device file descriptor as input fd. > > > > > > The device driver provides a character device that can be mmap'ed. > > > > > > There seem to be other restrictions. Can anyone provide insight on what would be needed to make this work? > > > > > > Please contact the authors of your kernel driver, they can answer this > > best. > > > > That would make sense, sadly they are MIA on their github repo engagement. Have a link to that repo? Again, they are the only ones that can resolve this, or you can modify the code to support this. > But in general, if I write a PCIe, DMA capable char device, what is the general guidance on what it takes to support splice/sendfile from a char device driver? Maybe there is an existing one in the kernel I could look at? > > Is it even something worth pursuing? This line make me think it might not work for char devices https://elixir.bootlin.com/linux/v5.19.17/source/fs/splice.c#L827. Is going for block device the only way that such device driver can work with the sendfile? Step back and ask yourself why you want to try to do this first? Are you sure the slowdown of a char driver is the copying of the data? Usually the hardware is the bottleneck, not the kernel code. Anyway, this is way off-topic for the stable@xxxxxxxxxxxxxxx mailing list, please work on this on a subsystem-specific list. good luck! greg k-h
