On Wed, 4 May 2016, Felipe Balbi wrote: > > multiple of 512 bytes and the maxpacket size is 1024. Then you either > > that's not common case for testusb. One of the test cases (see below) > exercises exactly small sg entries: > > # try to trigger short OUT processing bugs > echo "test 7a: $COUNT scatterlists, variable size/short entries" > do_test -t 7 -v 579 > BUFLEN=4096 > echo "test 7b: $COUNT scatterlists, variable size/bigger entries" > do_test -t 7 -v 41 > BUFLEN=64 > echo "test 7c: $COUNT scatterlists, variable size/micro entries" > do_test -t 7 -v 63 These tests will fail if you run them on an EHCI host controller. > (above is from tools/usb/hcd-tests.sh) > > > have to split the last entry in two or move it completely into the next > > ring segment. > > right, this is easy enough to do and that we (well, Mathias) already > have working :-) > > > 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. > > that's not a good idea, IMO. HCD drivers should be robust enough in > these situations. Why? Just so that hcd-tests.sh can complete with no errors? I don't know of any actual drivers or user programs that do real work and need such small SG entry sizes. > >> /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. > > see testusb :-) You did mention, on another thread, that you ran > testusb. Your counterexample isn't really testusb; it's hcd-tests.sh. I haven't tried running that. > And, btw, years ago I used to run these tests on daily basis on > MUSB. It also seems that Synopsys' XHCI (part of dwc3 when configured as > dual-role) is immune to this link TRB alignment limitation because I was > running hcd-tests.sh also on a daily basis on TI's AM437x SK and IDK > boards (one as host, one as peripheral). If creating bounce buffers for small SG transfers is so common (i.e., necessary for more than one host controller driver) then it really belongs in a common library somewhere. 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
