On Tue, 3 May 2016, Felipe Balbi wrote: > Hi, > > Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> writes: > > On Mon, 2 May 2016, Mathias Nyman wrote: > > > >> The current implemenentation restart the sent pattern for each entry in > >> the sg list. The receiving end expects a continuous pattern, and test > >> will fail unless scatterilst entries happen to be aligned with the > >> pattern > > > > Ah. We didn't spot this earlier because for non-xHCI controllers, the > > scatterlist entries _must_ be aligned with the maxpacket size and > > therefore with the pattern. > > interesting. We actually found a similar issue with XHCI. scatterlist > has to be aligned to wMaxPacketSize but only before a link TRB. Mathias > has been working on a solution which involves memcpy()ing enough bytes > to align to wMaxPacketSize before the link TRB (it's very infrequent as > we have 256 TRBs per segment on XHCI), but if you know of a nicer way, > we're all ears :-) You should be able to handle this without memcpy'ing anything. If the individual scatterlist entries are large enough, you can simply end the ring segment early. Either put a link TRB before the physical segment end, at the last point where the cumulative transfer size is divisible by maxpacket, or else fill out from there to the end of the segment with no-op TRBs. Common case: Each scatterlist entry is a multiple of 512 bytes and the maxpacket size is 1024. Then you either have to split the last entry in two or move it completely into the next ring segment. This approach doesn't work quite as well if the scatterlist entries are very small. For instance, if they are 8 bytes or smaller then you might have to fill out the segment with 128 or more no-op TRBs, which is not so good if the segment can hold only 256 TRBs. I suppose we could simply rule this out by requiring SG transfers to have a minimum entry size of 128 bytes, or something like that. > /me goes dig EHCI Not sure that will help. The same issue could arise there, if the scatterlist entries were smaller than 512 bytes. The driver doesn't handle this case properly, but it works out okay because the case never comes up. 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
