📄 user space device drivers.htm
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<h3>User-space device drivers</h3>
<p>It is not always necessary to write a device driver for a
device, especially in applications where no two applications
will compete for the device. The most useful example of this
is a memory-mapped device, but you can also do this with
devices in I/O space (devices accessed with <tt>inb()</tt> and
<tt>outb()</tt>, etc.). If your process is running as
superuser (root), you can use the <tt>mmap()</tt> call to map
some of your process memory to actual memory locations, by
<tt>mmap()</tt>'ing a section of /dev/mem. When you have done
this mapping, it is pretty easy to write and read from real
memory addresses just as you would read and write any
variables.
</p><p>If your driver needs to respond to interrupts, then you
really need to be working in kernel space, and need to write a
real device driver, as there is no good way at this time to
deliver interrupts to user processes. Although the DOSEMU
project has created something called the SIG (Silly Interrupt
Generator) which allows interrupts to be posted to user
processes (I believe through the use of signals), the SIG is
not particularly fast, and should be thought of as a last
resort for things like DOSEMU.
</p><p>An interrupt is an asyncronous notification posted by the
hardware to alert the device driver of some condition. You have
likely dealt with `IRQ's when setting up your hardware; an IRQ
is an ``Interrupt ReQuest line,'' which is triggered when the
device wants to talk to the driver. This may be because it has
data to give to the drive, or because it is now ready to
receive data, or because of some other ``exceptional
condition'' that the driver needs to know about. It is similar
to user-level processes receiving a signal, so similar that the
same <tt>sigaction</tt> structure is used in the kernel to deal
with interrupts as is used in user-level programs to deal with
signals. Where the user-level has its signals delivered to it
by the kernel, the kernel has interrupt delivered to it by
hardware.
</p><p>If your driver must be accessible to multiple processes at
once, and/or manage contention for a resource, then you also
need to write a real device driver at the kernel level, and a
user-space device driver will not be sufficient or even
possible.
</p><h4>Example: <tt>vgalib</tt></h4>
<p>A good example of a user-space driver is the <tt>vgalib</tt>
library. The standard <tt>read()</tt> and <tt>write()</tt>
calls are really inadequate for writing a really fast graphics
driver, and so instead there is a library which acts
conceptually like a device driver, but runs in user space. Any
processes which use it <b>must</b> run setuid root, because it
uses the <tt>ioperm()</tt> system call. It is possible for a
process that is not setuid root to write to /dev/mem if you
have a group <tt>mem</tt> or <tt>kmem</tt> which is allowed
write permission to /dev/mem and the process is properly
setgid, but only a process running as root can execute the
<tt>ioperm()</tt> call.
</p><p>There are several I/O ports associated with VGA graphics.
<b>vgalib</b> creates symbolic names for this with
<tt>#define</tt> statements, and then issues the
<tt>ioperm()</tt> call like this to make it possible for the
process to read and write directly from and to those ports:
</p><pre> if (ioperm(CRT_IC, 1, 1)) {
printf("VGAlib: can't get I/O permissions \n");
exit (-1);
}
ioperm(CRT_IM, 1, 1);
ioperm(ATT_IW, 1, 1);
<i>[...]</i>
</pre>
It only needs to do error checking once, because the only
reason for the <tt>ioperm()</tt> call to fail is that it is not
being called by the superuser, and this status is not going to
change.
<p><img src="user%20space%20device%20drivers_files/warning.gif" alt="[Careful!]">After making this
call, the process is allowed to use <tt>inb</tt> and
<tt>outb</tt> machine instructions, but only on the specified
ports. These instructions can be accessed without writing
directly in assembly by including <tt><linux></linux></tt>, but
will only work if you compile <b>with optimization on,</b> by
giving the <tt>-O<i>?</i></tt> to gcc. Read
<tt><linux/asm.h></tt> for details.
</p><p>After arranging for port I/O, <tt>vgalib</tt> arranges for
writing directly to kernel memory with the following code:
</p><pre> /* open /dev/mem */
if ((mem_fd = open("/dev/mem", O_RDWR) ) < 0) {
printf("VGAlib: can't open /dev/mem \n");
exit (-1);
}
/* mmap graphics memory */
if ((graph_mem = malloc(GRAPH_SIZE + (PAGE_SIZE-1))) == NULL) {
printf("VGAlib: allocation error \n");
exit (-1);
}
if ((unsigned long)graph_mem % PAGE_SIZE)
graph_mem += PAGE_SIZE - ((unsigned long)graph_mem % PAGE_SIZE);
graph_mem = (unsigned char *)mmap(
(caddr_t)graph_mem,
GRAPH_SIZE,
PROT_READ|PROT_WRITE,
MAP_SHARED|MAP_FIXED,
mem_fd,
GRAPH_BASE
);
if ((long)graph_mem < 0) {
printf("VGAlib: mmap error \n");
exit (-1);
}
</pre>
It first opens /dev/mem, then allocates memory enough so that
the mapping can be done on a page (4 KB) boundary, and then
attempts the map. <tt>GRAPH_SIZE</tt> is the size of VGA
memory, and <tt>GRAPH_BASE</tt> is the first address of VGA
memory in /dev/mem. Then by writing to the address that is
returned by <tt>mmap()</tt>, the process is actually writing to
screen memory.
<h4>Example: mouse conversion</h4>
<p>If you want a driver that acts a bit more like a
kernel-level driver, but does not live in kernel space, you can
also make a fifo, or named pipe. This usually lives in the
/dev/ directory (although it doesn't need to) and acts
substantially like a device once set up. However, fifo's are
one-directional only--they have one reader and one writer.
</p><p>For instance, it used to be that if you had a PS/2-style
mouse, and wanted to run XFree86, you had to create a fifo
called /dev/mouse, and run a program called mconv which read
PS/2 mouse ``droppings'' from /dev/psaux, and wrote the
equivalent microsoft-style ``droppings'' to /dev/mouse. Then
XFree86 would read the ``droppings'' from /dev/mouse, and it
would be as if there were a microsoft mouse connected to
/dev/mouse. Even though XFree86 is now able to read PS/2 style
``droppings'', the concepts in this example still stand. (If
you have a better example, I'd be glad to see it.)
</p><h4>The evil instruction</h4><p>Don't use the <tt>cli()</tt> instruction. It's possible to
use it as root to disable interrupts, and one particular program
used to used to use it--the <b>clock</b> program. However, this
kills SMP machines. If you need to use <tt>cli()</tt>, you need
a kernel-space driver, and a user-space driver will only cause
grief as more and more Linux users use SMP machines.
</p><p>Copyright (C) 1992, 1993, 1994, 1995, 1996 Michael K. Johnson,
johnsonm@redhat.com.<br>
</p><p>
</p><p></p><hr size="3">
<p><b><a name="Messages">Messages</a></b>
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<dl compact="compact">
<dt> 1. <img src="user%20space%20device%20drivers_files/question.gif" alt="Question:" align="middle" height="15" width="15">
<a href="http://tldp.org/LDP/khg/HyperNews/get/devices/fake/1.html">
What is SMP?</a><i></i> </dt>
<dt> -> <img src="user%20space%20device%20drivers_files/more.gif" alt="More:" align="middle" height="15" width="15">
<a href="http://tldp.org/LDP/khg/HyperNews/get/devices/fake/1/1.html">
SMP: Two Definitions?</a> <i> by Reinhold J. Gerharz</i> </dt>
<dt> -> <img src="user%20space%20device%20drivers_files/more.gif" alt="More:" align="middle" height="15" width="15">
<a href="http://tldp.org/LDP/khg/HyperNews/get/devices/fake/1/1/1.html">
Only one definition for Linux...</a> <i> by <a href="http://tldp.org/LDP/khg/%7Ejohnsonm">Michael K. Johnson</a></i> </dt>
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