streaming driver just overflows if more data is coming in

[hmaarrfk]

I don't think that it actually overwrites your data, just that it consumed all the traffic from the streaming interface

I guess they are putting it in a temporary buffer and copying it again to the userland buffer? that is silly....

[hmaarrfk]

Right, so first they transfer it all to a kernel buffer, THEN they copy it to a user buffer.

[Sat Feb 16 20:05:20 2019] xdma:complete_cyclic: 0-C2H0-ST, result[0].status = 0x52b40001 length = 0x200.
[Sat Feb 16 20:05:20 2019] xdma:complete_cyclic: 0-C2H0-ST, pkt_length=512 (with EOP)
[Sat Feb 16 20:05:20 2019] xdma:copy_cyclic_to_user: 0-C2H0-ST, pkt_len 512, head 0, user buf idx 0.
[Sat Feb 16 20:05:20 2019] xdma:copy_cyclic_to_user: 0-C2H0-ST sg 0, 0x00000000f3eaca70, copy 256 to user 0.

what a waste of memory bandwidth.

[hmaarrfk]

I think there are 2 ways forward:

1. Increase the size of the circular buffer using alloc_pages instead of alloc_page. The problem with this is that it still requires a copy from kernel memory to userland memory. Not ideal. Second, alloc_pages creates contiguous pages which may be difficult to handle in many applications.

2. We could try and get the userland pointer to the memory in kernel space. Apparently that is a bad idea and we should use mmap instead. https://stackoverflow.com/questions/46181090/how-to-avoid-copy-from-user-and-copy-to-user-in-linux-device-driver

For now, lets start with option 1, and see what happens.

[hmaarrfk]

 *
 * Iterate over the userspace buffer, taking at most 255 * PAGE_SIZE bytes for
 * each DMA transfer.

[hmaarrfk]

https://gitlab.com/WZab/Artix-DMA1