On Friday 18 May 2007, David Brownell wrote: > On Thursday 17 May 2007, Hans Petter Selasky wrote: > > Isn't that the job of "usb_buffer_alloc()", to allocate memory in blocks > > of PAGE_SIZE bytes? > > Not at all. The "size" parameter is in bytes, and the utility > code it calls goes to some lengths to *avoid* that kind of wastage. > As they say: UTSL! > > The original reason to have such routines was to allocate small > buffers for HID devices on systems where DMA map/unmap operations > were absurdly expensive on such small buffers. (Think IOMMU ops > across a slowish bus.)
The usb_buffer_alloc name is misleading. Many USB driver developers believe it is a generic purpose buffer allocator. I'd like to change usb_buffer_alloc to make it a general purpose allocator. A parameter would tell the function to just allocate memory (default behaviour) or to allocate a DMA coherent mapping as well. We could even pass the URB pointer to the function to set the URB_TRANSFER_NO_DMA_MAP automatically (although usb_buffer_alloc can be used to allocate the setup buffer, so the API might have to be a little different). > > Is it right understood that the Linux USB stack does currently not use > > scatter and gathering? To me it seems like that, hence in the URB Linux > > only has a single DMA address value. > > Let me guess -- you didn't read my response on that topic ... ? Laurent Pinchart ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ linux-usb-devel@lists.sourceforge.net To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
