Although most scatter-gather buffers are page aligned, occasionally one of them isn't. This can happen when the sg and st drivers use direct I/O and the user's buffer doesn't start on a page boundary; the first s-g buffer in the list will coincide with the start of the user buffer even though all the later ones will start at page boundaries.
The usb-storage driver has trouble with unaligned buffers. More accurately, it has trouble with s-g entries having bad lengths. A buffer that occupies an entire page or is the _last_ entry in the s-g list is okay, but if the _first_ entry isn't aligned properly it is likely to have a bad length. [For linux-scsi readers: This has to do with the way USB works. Data transfers are divided up into packets, and packets have a maximum size (typically 64 or 512 bytes). A packet can't span two s-g entries; hence if an s-g entry's length isn't divisible by the maxpacket size then one of the packets for that entry must be short, i.e., smaller than the maximum size. But USB specifies that short packets always indicate the end of a transfer. Thus if any s-g entry but the last has a bad length, the data transfer will terminate prematurely.] Alignment to a 512-byte boundary would suffice and page alignment would be more than enough. The cleanest way to fix the problem would be to have st and sg refuse to do direct I/O when the user's buffer isn't page aligned. That's probably too draconian (but it would allow us to give the user process an appropriate error code right away). So instead I'm going to change the usb-storage driver to make it reject any s-g transfer that doesn't meet the entry length requirements. My question is this: What's the best way to inform the SCSI midlayer about what went wrong? Return a DID_ABORT code? Or would something else be better? Alan Stern ------------------------------------------------------- This SF. Net email is sponsored by: GoToMyPC GoToMyPC is the fast, easy and secure way to access your computer from any Web browser or wireless device. Click here to Try it Free! https://www.gotomypc.com/tr/OSDN/AW/Q4_2003/t/g22lp?Target=mm/g22lp.tmpl _______________________________________________ [EMAIL PROTECTED] To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
