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. If we really want to do zerocopy I/O then we should not use a bounce buffer. But the only way to avoid bounce buffers may be to give user programs a way of allocating memory pages below 4 GB, because lots of USB hardware can only do 32-bit DMA. Is there an API for allocating user memory below 4 GB? Would a new MMAP flag be acceptable? 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
