On Thu, Nov 26, 2015 at 01:19:13AM +0100, Steinar H. Gunderson wrote: > Add a new interface for userspace to preallocate memory that can be > used with usbfs. This gives two primary benefits: > > - Zerocopy; data no longer needs to be copied between the userspace > and the kernel, but can instead be read directly by the driver from > userspace's buffers. This works for all kinds of transfers (even if > nonsensical for control and interrupt transfers); isochronous also > no longer need to memset() the buffer to zero to avoid leaking kernel data. > > - Once the buffers are allocated, USB transfers can no longer fail due to > memory fragmentation; previously, long-running programs could run into > problems finding a large enough contiguous memory chunk, especially on > embedded systems or at high rates. > > Memory is allocated by using mmap() against the usbfs file descriptor, > and similarly deallocated by munmap(). Once memory has been allocated, > using it as pointers to a bulk or isochronous operation means you will > automatically get zerocopy behavior. Note that this also means you cannot > modify outgoing data until the transfer is complete. The same holds for > data on the same cache lines as incoming data; DMA modifying them at the > same time could lead to your changes being overwritten. > > There's a new capability USBDEVFS_CAP_MMAP that userspace can query to see > if the running kernel supports this functionality, if just trying mmap() is > not acceptable. Shouldn't there also be some sort of documentation update with this patch as well, so that people know of the new functionality we are adding? thanks, greg k-h -- 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
