On Tue, 4 Mar 2014, David Laight wrote: > > > Notice the request length: 1536. That's three 512-byte sectors. A > > > little unusual, since most I/O is done in units of pages, which are > > > 4096 bytes. > > Ok, we can't have SuperSpeed mass storage devices broken, so it looks > > like we'll have to revert the last patch to add scatter-gather to the > > ASIX driver to avoid that breakage. That means Mathias is going to need > > to revert those two commits then, since he's taking over pushing xHCI > > driver bug fixes this kernel. > > > > Greg, Dave, Freddy, how do you want to handle reverting commit > > 3804fad45411? Should that come through Dave's networking tree or Greg's > > USB tree? > > I'm not sure what those two commits have to do with this problem. > > In order to support a request with a non-terminal buffer that isn't > a multiple of 1k you need something to stop a LINK TRB being in the > middle of the transfer. > Otherwise the 1536 byte transfer (expected as a 1k block followed by > 512byte one) can probably get sent as a 512byte block (terminating the > bulk data request) followed by a spurious 1k block. These should > generate horrid errors from the target disk. David is right; this problem can't be fixed simply by reverting patches. The real problem is that the block layer has handed the USB stack an SG list that xhci-hcd cannot handle at all, in its current form. There are only two reasonable ways to fix this: Add appropriate TRB fragment handling into xhci-hcd, or use bounce buffers for non-aligned requests. In theory the block layer could be taught about the need for these bounce buffers, but that would be only a partial solution. It would help for mass-storage transfers, but not for networking (which doesn't use the block layer). An alternative is to work around this particular problem by identifying the code that submits the 3-sector SG element, and changing it to use an even number of sectors. But obviously that doesn't solve the underlying issue. I think in the end there is no real choice but to bite the bullet and implement the TRB fragmentation rules. 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
