[BUG] mt76x0u: Probing issues on Raspberry Pi 3 B+

On Tue, Feb 19, 2019 at 10:40:47AM -0500, Alan Stern wrote:

On Tue, 19 Feb 2019, Stanislaw Gruszka wrote:

quoted

It would be interesting why urb->num_sgs = 0 & urb->sg cause
the troubles. This is how usb_sg_init() submit urbs for sg_tablesize = 0
controllers. So either are there are some requirement on urb->sg
mapped via dma_map_page() (which mt76usb does not meet) not needed
for urb->transfer_buffer mapped via dma_map_single() or there
is something wrong in dwc2 with sg and this driver will not
work with urb_sg_init() as well. I don't have hardware to investigate
this and don't want to bother you with more patches.

urb->sg != NULL and urb->num_sgs == 0 means that the transfer buffer 
must fit into a single page, which is pointed to by the first element 
of the scatterlist.

I asked about that in other thread 
https://lore.kernel.org/linux-wireless/2cc5674a-a3a0-d8fe-65f5-4357da9b85d3@arm.com/ (local)

the answer was it is weird but valid. However I think do dma_map_page()
on buffer not fit in the single page is asking for troubles. I just posted
patch that should fix this for mt76usb.

But now that I look at the code in usb_sg_init(), it seems odd that the
!use_sg case doesn't increment sg during each loop iteration.  I don't
see how that could possibly work -- it looks like a bug!

Looks for me that this is done via for_each_sg(sg, sg, io->entries, i) loop.

Stanislaw