Bufflink - fast lightweight kernel/userland communication

Bufflink provides a quick and easy way for kernel code to create a
ring buffer and write to it from  any context (process, bottom half
or interrupt). A userland process can access it via a character device
which behaves mostly like a FIFO. For more information, see
/usr/src/linux/Documentation/bufflink.txt after you have applied the
patch below.

This distribution contains the following files

README			This file
bufflink-0.3.patch	Patch against Linux 2.2.12 kernel
bufflink_test.c		Kernel module for testing bufflink
devtest.c		Userland program for testing with bufflink_test


To make bufflink available in your kernel:
    Apply the patch
    # cd /usr/src/linux
    # patch -p1 < /foo/bar/bufflink-0.3.patch
    Configure your kernel
    # make config (or make menuconfig or make xconfig or ...)
    Say Y to CONFIG_EXPERIMENTAL
    Say Y to CONFIG_BUFFLINK_KERNEL
    Say Y or M to CONFIG_BUFFLINK
    (If you say M to CONFIG_BUFFLINK then you may want to add the line
    alias char-dev-120 bufflink
    to /etc/conf.modules or else you will have to remember to
    "insmod bufflink" when necessary)
    Build your kernel and "make modules" and "make modules_install"
    Reboot on the new kernel

To test bufflink:
    Build the kernel module bufflink_test (in any directory) as follows:
    cc -Wall -O2 -D__KERNEL__ -DMODULE -c bufflink_test.c
    then insmod it:
    # insmod ./bufflink_test.o
    and look at the kernel log (wherever you have printk appearing)
    to see what minor it has picked (almost certainly 0).
    Ensure there is a device node with the right major/minor:
    # mknod /dev/bufflink0 c 120 0
    (replacing the 0 with whatever minor bufflink_test logged and
    replacing 120 with whatever major appears against "bufflink" when
    you do "cat /proc/devices").
    Compile the userland side:
    cc -o devtest devtest.c

    You can then write to the ring buffer:
    ./devtest w 8
    ./devtest w 8
    (will write 16 bytes in two lots of 8).
    ./devtest r 2
    will read and print the first 2 immediately.
    ./devtest r 40
    will read and print the remaining 14 *immediately* (i.e. it will
    not block waiting for the full 40).
    ./devtest r 40
    will block waiting for more data in the ring buffer. Leave it in
    the background or another window, then do
    ./devtest w 10
    to write 10 bytes to the ring buffer. That wakes up the reader
    immediately so that it sees the 10 bytes.
    Play with devtest to convince yourself of the behaviour when an
    overflow occurs.