On Wed, 12 Jun 2019, Christoph Hellwig wrote: > On Wed, Jun 12, 2019 at 01:46:06PM +0200, Oliver Neukum wrote: > > > Thay is someething the virt_boundary prevents. But could still give > > > you something like: > > > > > > 1536 4096 4096 1024 > > > > > > or > > > 1536 16384 8192 4096 16384 512 > > > > That would kill the driver, if maxpacket were 1024. > > > > USB has really two kinds of requirements > > > > 1. What comes from the protocol > > 2. What comes from the HCD > > > > The protocol wants just multiples of maxpacket. XHCI can satisfy > > that in arbitrary scatter/gather. Other HCs cannot. > > We have no real way to enforce that for the other HCs unfortunately. > I can't really think of any better way to handle their limitations > except for setting max_segments to 1 or bounce buffering. Would it be okay to rely on the assumption that USB block devices never have block size < 512? (We could even add code to the driver to enforce this, although refusing to handle such devices at all might be worse than getting an occasional error.) As I mentioned before, the only HCD that sometimes ends up with maxpacket = 1024 but is unable to do full SG is vhci-hcd, and that one shouldn't be too hard to fix. Alan Stern
