On Mon, 22 Mar 2010, Pete Zaitcev wrote: > On Fri, 19 Mar 2010 11:17:08 -0400 (EDT) > Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > > Pete, I removed the test for IOMMU coalescing in mon_text.c. I figured > > it didn't matter since we were only going to look at the first 32 bytes > > of the first scatterlist buffer anyway. Is this valid? > > In my opinion, no. Even 2 bytes may end crossing a page boundary. > Why not use (urb->transfer_flags & URB_DMA_SG_COMBINED) like you > did for the binary reader? On second thought, I'm not so sure this is necessary. Host controller hardware can't handle scatter-gather transfers unless the boundary between scatterlist elements occurs either at a packet boundary or at a page boundary -- and in fact, the host controller drivers don't handle the case where a page boundary isn't also a packet boundary. Because of this restriction, and because high-speed bulk endpoints must have a maxpacket length of 512, usb-storage (currently the only scatter-gather USB driver) requires 512-byte alignment of the data buffers. Any future driver using the scatter-gather library can be expected to include the same requirement. Hence it is not possible for 2 bytes to cross a page boundary. 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
