Problem with non aligned DMA in usbnet on ARM

Am Freitag, 13. August 2010, 12:06:54 schrieb Martin Fuzzey:

So I think a policy needs to be defined to ensure this and enforced in
the code. I can see two possible methods:

1) Require that usb drivers submit buffers obtained from kmalloc() and
friends with no extra offsets. If they want some other alignment later
they can use memmove in the completion handler. Enforce this in the core
by checking the buffer pointers are aligned to ARCH_KMALLOC_MINALIGN

or

2) Require that usb_submit_urb() accept byte aligned buffers. Enforce
this by a new test in usbtest (which all HCDs are expected to pass).
Implement it either in individual HCDs that require it or by letting
HCDs inform the core of their requirements and have the core do the
alignment (as it already does the dma mapping). Of course HCDs that can
implement byte aligned transfers (either natively or using tricks such
as the one Russell suggested) should do so.

I think 2) is the better solution because:
a) Solution 1 will impose a runtime overhead even on platforms / HCDs
that don't need it (including the most common cases)
b) There are more drivers than HCDs

Yes. But it doesn't prevent you from publishing this information so
drivers can help the lower levels.
	Regards
		Oliver