On Thu, 5 Jul 2007, Matthew Dharm wrote: > On Mon, Jul 02, 2007 at 09:15:03AM +0200, Oliver Neukum wrote: > > Am Montag, 2. Juli 2007 schrieb Alan Stern: > > > If you look at usbmon logs of real usb-storage data transfers you'll > > > see that multi-page sg elements occur quite frequently. (Of course, > > > that doesn't prevent us from transferring only one page per URB.) > > > > OK, but the storage driver allocates the URBs on the fly. If we change > > that to preallocation we need to have enough URBs for the worst case. > > Of course, in the storage case we control the worst-case. By adjusting the > parameter which controls how long a sg list the SCSI core will send us, we > can effectively limit the worst-case number of URBs needed.
But we don't control max_sectors, so we can't effectively limit the total transfer size. While I agree that it would be nice to preallocate enough URBs and memory that only one IRQ would be needed, I don't see anything wrong with preallocating a reasonably large amount and re-using it as needed. So we might get 3 or 4 times as many IRQs for particularly large transfers; that doesn't seem too bad. It's a justifiable time-space tradeoff. A reasonably large amount might be enough for a 32 KB transfer at full speed or a 120 KB transfer (the default maximum) at high speed. Or we might want to take into account the actual storage requirements of the HCDs; UHCI is much the worst in that regard. Alan Stern ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ linux-usb-devel@lists.sourceforge.net To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
