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INTRODUCTION
The mbuff.o module and /dev/mbuff is intended to be used as a
shared memory device making memory allocated in the kernel using
vmalloc possible to map in the user space. Such memory does not need
to be reserved at the system startup and its size is not limited by memory
fragmentation. The allocated memory is logically (but not physically)
countinuous. It can not be swapped out, so is well suited for real time
applications, especially communication between real time tasks and user space
or other high bandwidth kernel-user data exchange.
INSTALLATION
Make sure you have in /usr/src/linux the configured sources of currently
running kernel.
The support for the "misc device driver" should be ON in the kernel config.
The module is tested with 2.0.36-38, 2.2.12-15 and 2.3.99pre3 SMP and single
processor kernels.
To test:
make test
lsmod
dmesg
lsmod (repeat until you see usage count returning to 0).
rmmod mbuff
Remove -DSHM_DEMO from CFLAGS in Makefile.
cp -a ./mbuff /dev/mbuff
Set the uid,gid and permissions of /dev/mbuff according to your security policy.
make clean; make
cp mbuff.o /lib/modules/`uname -r`/misc
depmod -a
modprobe mbuff
BASIC USAGE
The simplest example is the file demo.c distributed with the package.
The
void * mbuff_alloc(const char *name, int size)
function allocates new area and maps it, or just maps already existing area.
The function returns the pointer to the mapped area or NULL in the case
of failure (no /dev/mbuff, bad permisions, mbuff.o not loaded,
not enough memory, or size greater than the size of already allocated area).
The first call does real allocation (swapping out some programs if neccesary),
the next calls should use the "size" argument equal or less than the one
used at the first allocation.
mbuff_alloc should be called by each process accessing the memory, as well
as in kernel module.
Every process calling mbuff_alloc is responsible for freeing it before exit.
It can be done with
void mbuff_free(const char *name, void * mbuf)
function. It will unmap the memory and decrease usage counter, so when the last
process unmaps the memory, it will be freed. "mbuf" should be the pointer
returned initially by mbuff_alloc.
For people who often forget to deallocate the memory, there is
void * mbuff_attach(const char *name, int size)
function - it works like mbuff_alloc, exept it does not increase usage counter
- memory can be deallocated automatically on munmap (e.g. process gets killed).
To unmap it earlier use
void mbuff_detach(const char *name, void * mbuf)
function. All it does is just munmap call.
It makes sense to use mbuff_attach and mbuff_detach only in user space.
I would advice to declare "volatile" all pointers operating on the shared
area - this prevents compiler from guessing the contents.
WARNING: mbuff_allocate calls vmalloc, may need to swap out some memory
- do not call it from real time nor interrupt nor timer context.
This code may call schedule() !
It should be safe to call it from RT-FIFO handler. And of course it may
be called in init_module().
HOW IT WORKS
Well, for details I have to say "read the sources". If you are really
interested, compile with debugging and run "make test".
Important thing to note are return codes from ioctl functions. They return
the size of the area (if it exists at the time of return), or a negative
error code. IOCTL_MBUFF_DEALLOCATE returns size of the area if it still
exists, 0 if it has been just definitively deallocated or negative -EINVAL
if there is no such named area. You do not need to worry if on the last
DEALLOCATE call you still get positive value - it means just something
else is using it.
More documentation will follow in next versions. The home site for the
module is http://crds.chemie.unibas.ch/PCI-MIO-E/ - get mbuff-*.tar.gz
--
Tomasz Motylewski
<motyl@stan.chemie.unibas.ch>, <motyl@ip.pl>
P.S. I should also mention that there exists another driver
("portable_shm") written by Paolo Mantegazza which is based on this code
- it is distributed with RTAI version of real time Linux -
http://www.aero.polimi.it/projects/rtai/ . It is written for 2.2 SMP
kernel. However I would recommend you to use my version of the driver :-)
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