On Tue, 9 Aug 2011, Sarah Sharp wrote: > On Tue, Aug 09, 2011 at 08:48:42AM +0200, Laurent Pinchart wrote: > > Hi Sarah, > > > > On Tuesday 09 August 2011 02:30:33 Sarah Sharp wrote: > > > On Tue, Aug 09, 2011 at 12:29:33AM +0200, Laurent Pinchart wrote: > > > > On Tuesday 09 August 2011 00:18:02 Sarah Sharp wrote: > > > So is the header something the UVC video driver allocates, or something > > > userspace allocates? Is the header, jpeg header, and data all confined > > > to one frame (one usb_iso_packet_descriptor in the URB)? > > > > > > Scatter gather list entries can be variable length, so I don't really > > > see where the problem is, as long as the uvcvideo driver knows how long > > > the header for that particular frame is supposed to be. You'd just have > > > one scatterlist per frame, with either two or three entries (I don't > > > know if userspace passes the jpeg header and data as one buffer) that > > > you would re-setup every time an URB completes. > > > > > > Of course, I don't know if the trade off in size of having a scatterlist > > > per frame vs. the performance gain of avoiding the memcopy would be a > > > good choice. But I do think it's feasible. It's not clear to me that this would help at all. If the data can be copied from the buffer directly to userspace, skipping over the header, then using a scatterlist wouldn't reduce the number of copies. Not unless some sort of zerocopy direct-I/O thing were implemented. 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
