On Fri, 13 Nov 2015, Steinar H. Gunderson wrote: > On Fri, Nov 13, 2015 at 10:38:54AM -0500, Alan Stern wrote: > >> So, in general I think it's good news, although I don't fully understand why > >> I still need the kernel-to-userspace copy for isochronous transfer. > > Maybe you can add some debugging to copy_user_enhanced_fast_string(). > > Add a flag, and call dump_stack() in that routine if the flag is set. > > Then set and clear this flag at the appropriate spots in devio.c. > > What exactly am I looking for, beyond the stack trace the kernel already > gives me? Find out where copy_user_enhanced_fast_string is being called from, and using that information, figure out why it was called. That will tell you why this approach isn't yielding true zerocopy performance. 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
