Scatter-gather list constraints

[Inaky Perez-Gonzalez]

For WA, when we get a buffer to be sent from a URB, it has to be split
in chunks, each chunk has a header added. So we end up with a list of
chunks, most of them quite small. Each requires a single URB to send.
resources galore.
If we could queue all those, the overhead would be reduced to allocating
the headers (possibly in a continuous array) and the sg "descriptors"
to describe the whole thing.
However, the alignment stuff somebody mentioned in another email in this
thread might cause problems.
At the end it might not be all that doable (I might be missing some
subtle isssues), but it is well worth a look.

Note that usbcore already contains a scatter-gather library.
(Unfortunately the library is limited in usefulness because it needs to
run in process context.)

And the overhead of one URB per sg "node" kills it's usability for
WAs.

For this case (lots of small chunks making up a single URB), using a
bounce buffer might well be the easiest solution. It depends on the
size of the URB and the number and sizes of the small chunks. There
would be a lot less overhead -- only one URB -- and one large memory
allocation instead of lots of small ones.

That's what we have right now (if I rememeber correctly); the issue
is that you end up copying A LOT. I don't know, maybe I am just being
overperfectionist. The data chunks (segments) can be up to (digs)
3584 [from 512, in 512 increments] if I am reading the spec right
(WUSB1.0 4.5.1). Interleaving that with small chunks and change...
I don't know if that much copying will end up being that good, along
with the allocations it requires, etc.