To sum up, DMA I/O buffers should be allocated separately from regular
data structures, using kmalloc(). Multiple buffers with non-overlapping
lifetimes can share a region like a C union. Multiple output buffers with
overlapping lifetimes can share a region like a C struct. On some
architectures this can be done without penalty, one others there's no
penalty provided that the total size of the buffers is <= 16 bytes. Although output buffers may share a region with other data, they probably shouldn't.
I like simpler rules. Mine would be: "Use usb_buffer_alloc() in preference to kmalloc(), for i/o memory you're re-using."
Trying to write code that's cacheline-aware is error prone. Better to use memory that's known to be dma-coherent, and not have to worry about all those corner cases.
- Dave
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