^^^^^^^^^^^^^^^^^So all I/O buffers _must_ be allocated using kmalloc()
Use GFP_DMA flag on the kmalloc() call, too, whenever possible.
I'll disagree. As a rule, avoid that unless you know you need it. In most cases, USB drivers don't.
This will help avoid bounce buffering on some systems.
USB has a better solution, in 2.5 kernels: usb_buffer_alloc(). See Documentation/usb/dma.txt for more info ... including the pointer to DMA-mapping.txt, giving "complete" rules.
On "some systems" (like SA-1111 OHCI!) that call arranges
to return memory that fits into those awkward DMA windows.
("Under 1 MByte" in that case; "Under 16 MBytes" for ISA.)- Dave
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