On Fri, 18 Jun 2010, Oliver Neukum wrote: > > Storage carefully keeps each URB's buffer > > distinct, and doesn't try to map/combine > > them into a single ginormous transfer, > > holding the equivalent of multiple Ethernet > > packets. That statement is a little ambiguous. To put it more explicitly: usb-storage uses the scatter-gather library, which either constructs a separate URB for each buffer in an SG list, or else passes the entire list down to the HCD in a single URB (if the HCD supports scatterlist operation). > Doesn't the storage driver do exactly that? It builds > a single transfer out of multiple buffers. It seems to me > that we can build a single transfer out of multiple > buffers provided all but the last buffers meet some > size constraint. The requirement is that each scatterlist buffer except the last must be a multiple of the maxpacket length. With full-speed and high-speed USB this works out because the maxpacket length is either 64 or 512, and disk I/O is done by pages or (rarely) by sectors. It doesn't work out so well with Wireless USB, where the maxpacket length can be something weird like 1536 or 3072. Then the only way to handle it is to use bounce buffers. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
