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<center>
<h3>IO memory emulation</h3>
</center>
Thanks to Greg Lonnon, UML (as of
2.4.6-4um) has support for I/O memory emulation. This allows a host
file to be specified as an I/O region on the kernel command line.
That file will be mapped into UML's kernel address space where a
driver can locate it and do whatever it wants with the memory,
including providing an interface to it for UML processes to use.
<a name="Driver interface"/><table width="100%" bgcolor="#e0e0e0">
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<b>
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<blockquote head="Driver interface">
Specifying an iomem region on the kernel command line is done as
follows:
<blockquote>
<tt>
<font color="#000000">iomem=<b>name</b>,<b>file</b>
</font>
</tt>
</blockquote>
The name is defined by the driver that will use the I/O area and the
file is the name of the host file that will be mapped in.
<p>
The driver finds its I/O area by calling find_iomem
<blockquote>
<tt>
<font color="#000000">
unsigned long find_iomem(char *driver, unsigned long *len_out)
</font>
</tt>
</blockquote>
which returns the address of the beginning of the area and passes its
length out in len_out. The driver argument is the name of the I/O
area as defined on the command line.
<p>
Once the driver has located its I/O area, it can do whatever it wants
with it, including
<ul>
<li>
providing a file interface to it
</li>
<li>
allowing processes to mmap it into their own address spaces
</li>
<li>
use it as a backing store for some other kind of interface, such as an
X server using it as a framebuffer and providing an X protocol
interface to it
</li>
</ul>
</blockquote>
<a name="The mmapper driver"/><table width="100%" bgcolor="#e0e0e0">
<tr>
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<b>
<font color="black">The mmapper driver</font>
</b>
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<blockquote head="The mmapper driver">
arch/um/drivers/mmapper_kern.c contains a simple example driver which
locates the iomem area named "mmapper" and makes it available for
mapping through /dev/mmapper.
<a href="text/mmapper.c">Here</a> is a little
program which will mmap the /dev/mmapper device and print its contents.
<p>
To run this driver, compile it into the kernel by turning on
CONFIG_MMAPPER, and assign a file to it by putting
<blockquote>
<tt>
<font color="#000000">iomem=mmapper,<b>file</b>
</font>
</tt>
</blockquote>
on the command line.
</blockquote>
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