readme

linux driver edition

The allocator shown here  exploits high memory. This document explains
how  a user can  deal   with drivers uses   this  allocator and how  a
programmer can link in the module.


	User's manual
	=============


One of the most compelling problems with any DMA-capable device is the
allocation  of a suitable  memory buffer. The "allocator" module tries
to deal with  the problem in  a clean way.  The module is  able to use
high   memory  (above the  one   used in  normal   operation)  for DMA
allocation.

To prevent  the  kernel for using   high memory,  so  that it  remains
available for  DMA, you should  pass a  command  line argument to  the
kernel.  Command line arguments  can be passed to  Lilo, to Loadlin or
to whichever loader  you are using  (unless it's very poor in design).
For Lilo, either use  "append=" in  /etc/lilo.conf or add  commandline
arguments to the  interactive prompt. For  example, I have a 32MB  box
and reserve two megs for DMA:

In lilo.conf:
	image = /zImage
	label = linux
	append = "mem=30M"

Or, interactively:
	LILO: linux mem=30M

Once  the kernel is booted  with the  right command-line argument, any
driver  linked   with  the  allocator   module  will  be able   to get
DMA-capable memory without  much  trouble (unless the  various drivers
need more memory than available).

The module implements an alloc/free  mechanism,  so that it can  serve
multiple drivers  at the  same time. Note  however that  the allocator
uses all of  high memory and assumes to  be the only piece of software
using such memory.


	Programmer's manual
	===================

The allocator  can be either  linked to a  device driver or used  as a
separate module. If  linked to the driver, you  must not define MODULE
when compiling allocator.c, and  the driver must call allocator_init()
before using  the allocator  and must call  allocator_cleanup() before
unloading.   This  is  usually  done  from  within  init_module()  and
cleanup_module(). If the allocator is  linked to a driver, it won't be
possible for several drivers to allocate high DMA memory, as explained
above.

It  is  possible, on  the  other  hand, to  compile  the  module as  a
standalone module, so  that several modules can rely  on the allocator
for they DMA buffers. To  compile the allocator as a standalone module
you need  to define  MODULE when compiling  allocator.c . This  is the
default  in this  distribution, by  virtue of  the  provided Makefile.
Drivers   using   a   standalone   allocator  won't   need   to   call
allocator_init() nor allocator_cleanup().

The allocator exports the following functions (declared in allocator.h):

   unsigned long allocator_allocate_dma (unsigned long bytes,
					 int priority);

	This function returns a physical address, over high_memory,
	which corresponds to an area of at least "bytes" bytes.
	The area will be owned by the module calling the function.
	The returned address can be passed to device boards, to instruct
	their DMA controllers, via phys_to_bus(). The address can be used
	by C code after vremap()/ioremap(). The "priority" argument should
	be GFP_KERNEL or GFP_ATOMIC, according to the context of the
	caller; it is used to call kmalloc(), as the allocator must keep
	track of any region it gives away. In case of error the function
	returns 0, and the caller is expected to issue a -ENOMEM error.


   int allocator_free_dma (unsigned long address);

	This function is the reverse of the previous one. If a driver
	doesn't free the DMA memory it allocated, the allocator will
	consider such memory as busy. Note, however, that
	allocator_cleanup() calls kfree() on every region it reclaimed,
	so that a driver with the allocator linked in can avoid calling
	allocator_free_dma() at unload time. The return value is 0
	or -EINVAL if the address is not handled by allocator.