On Wed, 21 Aug 2013, Ming Lei wrote: > > By the way, even though it's a little late to ask this... Why did you > > decide to move only the giveback routine into a tasklet, instead of > > moving the entire interrupt handler? > > Looks below reasons in my mind before preparing the tasklet patch: > > 1, from my observation, on ARM, the most time-consuming part is dma > mapping, unmapping and data copy over coherent memory, so moving > giveback out of interrupt handler can decrease USB irq handling time a lot. Of course, this would remain true either way. > 2, moving giveback out of irq handler can be done in usbcore, so changes > can be minimized. This seems to be the most important factor. When you think about it, though, does it really minimize the changes? Consider all the other adjustments we had to make to ehci-hcd: the interrupt QH unlink change and the isochronous stream stuff (which also affects the usb-audio drivers). I'm starting to think that moving the entire handler to a tasklet would have been better. > 3, driver's complete() may do many driver specific things which may increase > irq handling time randomly, so moving complete() to tasklet can help to > decrease HCD irq handling time. This is like the first reason. > Also moving only the giveback routine into a tasklet can avoid dropping > HCD private lock during irq handler, which may simplify HCD code, and > I have figured out ehci cleanup patches for this. I don't think we should make these changes. It's okay to keep the private lock during the giveback call, but let's not remove the other things. My feeling is that at some point we may indeed want to move the entire handler to a tasklet or a threaded IRQ. If that happens, all those simplifications would need to be undone. Better not to do them in the first place, since they add very little overhead. 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
