I/O USERPTR for videobuf2-dma-sg

["Matthias Wächter" <matthias.waechter@tttech.com>]

Hi everyone,

I’m in the process of providing a driver for a camera input and have 
just finished SG DMA operation to work in hardware and driver. Now I’m 
quite surprised that videobuf2-dma-sg is lacking basic support for user 
pointers to I/O memory (i.e., to graphics card). However, 
videobuf2-dma-contig does have support for it, at least from reading the 
code I see that vb2_dc_get_userptr() can tell I/O memory from RAM and as 
such do the right thing for I/O. OTOH, vb2_dma_sg_get_userptr() just 
does plain get_user_pages() which is not returning any page information 
for I/O memory.

Is this missing just because no-one has bothered to do it, or is there a 
known problem ? Right now it seems that only one driver in the kernel 
tree is actually using videobuf2-dma-sg, so maybe it hasn’t been worth 
the effort. Similarly, DMABUF has not been implemented in videobuf2-dma-sg.

Would a patch for adding I/O USERPTR functionality similar to 
vb2_dc_get/put_userptr() be accepted? Any known problems with this task 
I’m going to face?

Thanks,
– Matthias