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.
Largely based on a patch by Markus Rechberger with some updates. The original
patch can be found at:
http://sundtek.de/support/devio_mmap_v0.4.diff
Signed-off-by: Steinar H. Gunderson <sesse@google.com>
Signed-off-by: Markus Rechberger <mrechberger@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
f7d34b445a