On Fri, 12 Dec 2014, Florian Echtler wrote: > Hello Alan, > > On 11.12.2014 20:36, Alan Stern wrote: > > On Sun, 7 Dec 2014, Florian Echtler wrote: > >> - Can I always use DMA on the USB side (for bulk transfers), or does > >> this in any way require support from the USB device's hardware? (I'm > >> guessing no, but a definite answer would be great.) > > DMA requires hardware support in the USB host controller, not in the > > USB device. Most current USB host controllers support DMA; only a few > > of the oldest ones don't. > thanks - good to know for sure. AFAICT if I want to transfer bulk > messages using DMA, I can't just use kalloc and usb_bulk_msg, but need > usb_alloc_coherent, usb_fill_bulk_urb and usb_submit_urb, correct? That's not right. kmalloc and usb_bulk_msg will make use of DMA if the host controller driver supports it. In fact, they don't get any choice in the matter; the host controller driver is what makes the decision about whether to use DMA. Are you maybe using the wrong words? For instance, are you confusing "DMA" with "consistent" (or "coherent") mappings? > Also, can you suggest any documentation regarding the relation between > videobuf2 and DMA? Not me. Other people like Laurent Pinchart have worked on video over USB. 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
