Hi Sarah, On Tuesday 09 August 2011 00:18:02 Sarah Sharp wrote: > On Tue, Aug 02, 2011 at 01:44:54AM +0200, Laurent Pinchart wrote: > > On Monday 01 August 2011 18:57:14 Sebastian Andrzej Siewior wrote: > > > * Laurent Pinchart | 2011-08-01 16:04:55 [+0200]: > > > >> IIUC, there's no way to avoid copying the data since it has to > > > >> processed before getting sent to userspace. > > > > > > > > Every UVC packet starts with a header that needs to be removed, so we > > > > unfortunately can't get rid of all memcpy() calls. > > > > > > So we have something like that: > > > +---------------+------------+-----------+ > > > > > > | hdr-to-remove | jpeg header| jpeg-data | > > > > > > +---------------+------------+-----------+ > > > > First of all, UVC devices don't stream MJPEG only. Uncompressed video > > formats are very common. > > Would it help to have UVC use a scatter gather list for every > isochronous frame? Then you would just need to allocate the header to > remove as the first sg entry, and place the jpeg header and data into > the second sg entry, which would avoid the memory copy. There's already > scatter gather list support for bulk transfers, so I think it shouldn't > be too hard to extend to isochronous transfers. Thanks for your proposal. Let's make it even more fun: the header size is not constant, and is specified by the first byte of the header. Good luck coming up with a generic solution for this :-) I hope that the next video-related USB class spec will take this kind of issues into account. -- Regards, Laurent Pinchart -- 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
