On Mon, 24 Jun 2013, Felipe Balbi wrote: > > > you wouldn't notice the difference. The DMA engine is the one which > > > would read the sgtable to figure where the data is scattered, at the end > > > of the day, SW only knows of a single 1044bytes URB and controller is > > > required to generate proper USB packets out of that. > > > > That simply isn't true. The DMA engine in EHCI, for example, is not > > capable of constructing a packet out of discontiguous memory buffers. > > /me goes read EHCI... > > that's a limitation on EHCI though, which can't support data scattered > around memory. It must be contiguous within that page. It really doesn't > have anything to do with SGs, right ? Right. > If EHCI has such a limitation than EHCI alone should have means to > handle drivers which send data scaterred around memory in small chunks. Not only EHCI; also UHCI, OHCI, and I don't know how many others. This is one of the reasons why David Brownell wrote the USB Scatter-Gather library code in usbcore. > > Furthermore, if the HCD doesn't have DMA support then we break the > > transfer up into multiple URBs, each corresponding to a single SG > > element. The data in the SG buffers do not get rearranged, so you > > would inevitably end up with a short packet at the end of one of the > > intermediate URBs. > > that's, again, a limitation of the HCD. MUSB can't handle SGs at all, so > we have to use a contiguous buffer which can hold the entire data to do > the transfer in that case. MUSB shouldn't be copying data to and from bounce buffers just to be able to handle SG tranfers. > DWC3, OTOH, can handle anything. Your data can be anywhere, and it'll > just transfer it, we just need to set the 'chain' bit properly to tell > the internal DMA controller that the USB transfer isn't over yet. Maybe the same is true of xHCI; I don't know. In any case, those HCDs are definitely in the minority. 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
