On Mon, 16 Nov 2015, Christoph Hellwig wrote: > On Mon, Nov 16, 2015 at 08:03:12PM +0100, Steinar H. Gunderson wrote: > > The original use case: DVB capture on embedded devices. > > > > My use case: USB3 uncompressed HD video capture (from multiple cards). > > > > Both of these hit (beyond the speed boost from zerocopy) the problem that > > as time goes by, memory gets more fragmented and usbfs fails allocation. > > Allocating memory up-front solves that. > > As said I think you should just use get_user_pages() on user memory, > and bounce buffer if it doesn't fit the DMA mask. > > Thіs also allows the user to use hugetlbs if they need large contiguous > allocations for performance reasons. In other words, you're suggesting we do this: Check that userspace requested zerocopy (otherwise the user program might try to access other data stored in the same cache lines as the buffer while the I/O is in progres); Call get_user_pages (or get_user_pages_fast? -- it's not clear which should be used) for this buffer; Use the array of pages returned by that routine to populate a scatter-gather list (sg_alloc_table_from_pages); Pass that list to dma_map_sg. Is that right? Does dma_map_sg check the page addresses against the DMA mask and automatically create a bounce buffer, or do we have to do that manually? Documentation/DMA-API-HOWTO.txt doesn't discuss this. 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
