On Tue, 17 Nov 2015, Steinar H. Gunderson wrote: > On Tue, Nov 17, 2015 at 07:17:31PM +0100, Markus Rechberger wrote: > > 1. the memset on isochronous transfers to empty the buffers in order > > to avoid leaking raw memory to userspace (this costs a lot on intel > > Atoms and is also noticeable on other systems). > > > > 2. the memory fragmentation. Seems like recent systems have a better > > performance here since we did not get that report for several months > > now, or maybe the user behavior changed. > > Some older Linux systems (maybe 2-3 years old) triggered this issue > > way more often. > > I guess if we get transparent zerocopy, both of these are going away > just like with your patch, right? The only difference is really who sets up > the memory area (the kernel or not). > > Alan, could we perhaps let the zerocopy flag make the request fail (instead > of going through a bounce buffer) if direct DMA is not possible? That way, > it would be quite obvious that you need to allocate the memory some other way > instead of silently hitting the issues Markus mention. But what other way of allocating memory is there? With scatter-gather lists, fragmentation isn't an issue. But bounce buffers are unavoidable if the memory isn't accessible to the USB hardware. 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
