[PATCH] PCI: document sysfs rom file interface

idr gently pointed out today that not only is the sysfs rom file
interface somewhat unintuitive (despite my efforts and initial
implementation), but it's also undocumented!  This patch to
Documentation/filesystems/sysfs-pci.txt corrects the latter problem; the
former is a userland ABI now though, so we're stuck with it for awhile
at least.
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Documentation/filesystems/sysfs-pci.txt

 Accessing PCI device resources through sysfs
+--------------------------------------------
 
 sysfs, usually mounted at /sys, provides access to PCI resources on platforms
 that support it.  For example, a given bus might look like this:
@@ -47,14 +48,21 @@ files, each with their own function.
   binary - file contains binary data
   cpumask - file contains a cpumask type
 
-The read only files are informational, writes to them will be ignored.
-Writable files can be used to perform actions on the device (e.g. changing
-config space, detaching a device).  mmapable files are available via an
-mmap of the file at offset 0 and can be used to do actual device programming
-from userspace.  Note that some platforms don't support mmapping of certain
-resources, so be sure to check the return value from any attempted mmap.
+The read only files are informational, writes to them will be ignored, with
+the exception of the 'rom' file.  Writable files can be used to perform
+actions on the device (e.g. changing config space, detaching a device).
+mmapable files are available via an mmap of the file at offset 0 and can be
+used to do actual device programming from userspace.  Note that some platforms
+don't support mmapping of certain resources, so be sure to check the return
+value from any attempted mmap.
+
+The 'rom' file is special in that it provides read-only access to the device's
+ROM file, if available.  It's disabled by default, however, so applications
+should write the string "1" to the file to enable it before attempting a read
+call, and disable it following the access by writing "0" to the file.
 
 Accessing legacy resources through sysfs
+----------------------------------------
 
 Legacy I/O port and ISA memory resources are also provided in sysfs if the
 underlying platform supports them.  They're located in the PCI class heirarchy,
@@ -75,6 +83,7 @@ simply dereference the returned pointer (after checking for errors of course)
 to access legacy memory space.
 
 Supporting PCI access on new platforms
+--------------------------------------
 
 In order to support PCI resource mapping as described above, Linux platform
 code must define HAVE_PCI_MMAP and provide a pci_mmap_page_range function.