On Thu, 22 May 2014, Yves Martens wrote: > Hello, > > I read the USB spec, understand when to prefer bulk vs. isochronous pipes. > My usecase requires controlled jitter & guaranteed bandwidth. > But when discussing this with multiple peer engineers, rumor goes isochronous mode has 'issues' (sic), they advise me against using it altogether. > > But when I try to pinpoint, I find nothing: 'isochronous' does not require more patches on this mailing list or in the git history; no panic stories on forums; commercial devices are few but do exist. > > Does the Linux USB implementation supports both equally well? And also the scheduling of a mixture of both pipe types? For both USB 2 and 3? In general, isochronous support isn't quite as good as bulk support. However, it varies with the driver. For uhci-hcd and ohci-hcd, isochronous is supported just as well as bulk. The same is true for high-speed transfers with ehci-hcd, but for full-speed transfers the isochronous support is a little weak. Isochronous support on xhci-hcd is also somewhat weak. It hasn't been tested very well, and there have been repeated reports of problems on Intel's xHCI controllers (other controllers seem to work better). > Do you have an idea why many commercial devices (e.g. AV applications) still would go for bulk? Because the designers don't understand the tradeoffs very well. 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
