On 15.12.2014 04:24, Alan Stern wrote: > On Fri, 12 Dec 2014, Florian Echtler wrote: >> 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. Oh, OK, thanks for clearing that up. > Are you maybe using the wrong words? For instance, are you confusing > "DMA" with "consistent" (or "coherent") mappings? That's entirely possible :-) Although I did read the DMA-API-HOWTO, it may very well be that I'm confusing things due to lack of experience. To put my original question differently: I want to make sure no unnecessary memory copies of the image data are done by the driver. From my understanding, this involves using DMA to fill the buffer with data and one of the videobuf2 mechanisms to get suitable buffers which are accessible from user space. >> 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. OK, I will talk to him directly. Best, Florian -- SENT FROM MY DEC VT50 TERMINAL
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