On Sun, 5 Feb 2006, David Brownell wrote: > > > A: 100 bytes at addr1, 512 bytes at addr2 > > > B: 512 bytes at addr2, 100 bytes at addr2 > > > > > > Request A always requires copying for the packet straddling offset 100 > > > (relative to message start) but request B never requires copying (so it > > > could support a full zerocopy I/O model via get_user_pages). That's a > > > key semantic issue which Badari's patch does not address. > > > > That's a very good point. iovec aio needs to address alignment issues. > > The simple way to meet Badari's current goal is: calculate the length > of the whole iovec, use that to allocate the right size i/o buffer, then > have the AIO write path gather data into that, and the AIO read "retry" > path scatter data out of it after reads complete.
Okay. That sounds like the kind of thing that should already be written somewhere. If not, it should be added in some central place where other drivers can use it too. > Then there'd be the asymmetry that IN endpoints can be halted using read() > but not with readv() -- similarly for OUT and write() -- but that's a very > livable issue. You mean with a "wrong-direction" I/O call? Why can't that be done with readv/writev? Especially if the total transfer length is zero? > > The easiest answer for now is to require all buffers to be aligned on > > 512-byte boundaries (or 64 for full speed), in terms of both the buffer's > > start address and length (except for the last buffer in an iovec, which > > can have arbitrary length). > > Those are appropriate requirements with O_DIRECT file descriptors, where > userspace already expects extra restrictions on syscall parameters. > > I think USB's restriction would be that packets must never span pages; > neither usb_request nor urb restrict buffer start addresses, other than > for dma-safety. (The "512 byte alignment" restriction makes sense for > disk-centric file I/O.) You're right; what I said was needlessly severe. Even O_DIRECT doesn't need the buffer-start restriction, although it does need the buffer length restriction. Is there any generic code in the kernel for making these sorts of checks? Alan Stern ------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Do you grep through log files for problems? Stop! Download the new AJAX search engine that makes searching your log files as easy as surfing the web. DOWNLOAD SPLUNK! http://sel.as-us.falkag.net/sel?cmd=lnk&kid=103432&bid=230486&dat=121642 _______________________________________________ linux-usb-devel@lists.sourceforge.net To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
