On Wed, Jun 22, 2011 at 10:35:44AM -0700, matt mooney wrote: > On 10:30 Wed 22 Jun , matt mooney wrote: > > On 20:02 Wed 22 Jun , Kirill Smelkov wrote: > > > There are cases, when 80% max isochronous bandwidth is too limiting. > > > > > > For example I have two USB video capture cards which stream uncompressed > > > video, and to stream full NTSC + PAL videos we'd need > > > > > > NTSC 640x480 YUV422 @30fps ~17.6 MB/s > > > PAL 720x576 YUV422 @25fps ~19.7 MB/s > > > > > > isoc bandwidth. > > > > > > Now, due to limited alt settings in capture devices NTSC one ends up > > > streaming with max_pkt_size=2688 and PAL with max_pkt_size=2892, both > > > with interval=1. In terms of microframe time allocation this gives > > > > > > NTSC ~53us > > > PAL ~57us > > > > > > and together > > > > > > ~110us > 100us == 80% of 125us uframe time. > > > > > > So those two devices can't work together simultaneously because the'd > > > over allocate isochronous bandwidth. > > > > > > 80% seemed a bit arbitrary to me, and I've tried to raise it to 90% and > > > both devices started to work together, so I though sometimes it would be > > > a good idea for users to override hardcoded default of max 80% isoc > > > bandwidth. > > > > There is nothing arbitrary about 80%. The USB 2.0 Specification constrains > > periodic transfers for high-speed endpoints to 80% of the microframe. See > > section 5.6.4. > > > > Looking at the patch, I see that you probably already knew that. > > So I digress and defer to the USB experts ;) Yes, it was meant as 80% being arbitrary chosen by USB 2.0 specification. Notes taken - I'll clarify patch description. Thanks for commenting, Kirill -- 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
