This allows to control the backlight level on certain Intel(R) GPUs,
including i915.

This has been taken from intel-gpu-tools which can be found
here: http://cgit.freedesktop.org/xorg/app/intel-gpu-tools/

Originally ported to FreeBSD by "emmex" of the FreeBSD forums.

FreeBSD Build:
  pkg install libpciaccess libdrm
  make
  make install

OpenBSD Build:
  make
  make install

(this installs intel_backlight setuid root, so any user can execute it).

Usage:
  $ intel_backlight
  current backlight value: 30% (281/937)
  $ intel_backlight 50
  current backlight value: 15% (141/937)
  set backlight to 50% (469/937)
  $ intel_backlight incr
  current backlight value: 50% (469/937)
  set backlight to 51% (478/937)
  $ intel_backlight incr
  current backlight value: 51% (478/937)
  set backlight to 60% (562/937)
  $ intel_backlight incr
  current backlight value: 60% (562/937)
  set backlight to 70% (656/937)
  $ intel_backlight decr
  current backlight value: 70% (656/937)
  set backlight to 60% (562/937)
  $ intel_backlight decr
  current backlight value: 60% (562/937)
  set backlight to 51% (478/937)

The included example allows adjusting brightness automatically based on the
input of the isl driver (currently not part of standard FreeBSD).

Usage:
  ./isl_backlight.sh &


OpenBSD Notes:
Requires a kernel capable of concurrent opening of /dev/xf86, if this is to be
run while X is running.  The following patch implements machdep.allowaperture=3
to support this.

Index: arch/amd64/amd64/mem.c
===================================================================
RCS file: /cvs/src/sys/arch/amd64/amd64/mem.c,v
retrieving revision 1.23
diff -u -p -u -p -r1.23 mem.c
--- arch/amd64/amd64/mem.c	14 Mar 2015 03:38:46 -0000	1.23
+++ arch/amd64/amd64/mem.c	26 May 2015 02:52:31 -0000
@@ -93,8 +93,9 @@ mmopen(dev_t dev, int flag, int mode, st
 	        if (suser(p, 0) != 0 || !allowaperture)
 			return (EPERM);
 
-		/* authorize only one simultaneous open() */
-		if (ap_open_count > 0)
+		/* authorize only one simultaneous open() unless
+		 * allowaperture=3 */
+		if (ap_open_count > 0 && allowaperture < 3)
 			return(EPERM);
 		ap_open_count++;
 		break;
@@ -213,6 +214,7 @@ mmmmap(dev_t dev, off_t off, int prot)
 			else
 				return -1;
 		case 2:
+		case 3:
 			/* Allow mapping of the whole 1st megabyte 
 			   for x86emu */
 			if (off <= BIOS_END || !amd64_pa_used(off))