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@ -23,22 +23,21 @@ Contents
2. What is DGA?
~~~~~~~~~~~~~~~
DGA is short for Direct Graphics Access and is a method for a program to
bypass the X-Server and directly modify the framebuffer memory.
DGA is short for Direct Graphics Access and is a means for a programm to
bypass the X-Server and directly modifying the framebuffer memory.
Technically spoken this happens by mapping the framebuffer memory into
the memory range of your process. This is only allowed by the kernel
the memory range of your process. This is just allowed by the kernel
if you have superuser privileges. You can get these either by logging in
as root or by setting the suid bit on the mplayer executable (NOT
recommended!).
as root or by setting the suid bit on the mplayer excecutable.
There are two versions of DGA: DGA1 is used by XFree 3.x.x and DGA2 was
There are two versions of DGA: DGA1 is used by XFree 3.X.X and DGA2 was
introduced with XFree 4.0.1.
DGA1 provides only the direct framebuffer access as described above. For
DGA1 just provides direct framebuffer access as described above. For
switching the resolution of the video signal you have to rely on the
XVidMode extension.
DGA2 incorporated the features of XVidMode extension and also allows
DGA2 incorporates the features of XVidMode extension and also allows
switching the depth of the display. So you may, although basically
running a 32 bit depth XServer, switch to a depth of 15 bits and vice
versa.
@ -58,12 +57,12 @@ Contents
/etc/X11/XF86Config (/etc/X11/XF86Config-4 for XFree 4.0.X respectively).
Those are defined by so-called modelines and depend on the capabilites
of your video hardware. The XServer scans this config file on startup and
disables the modelines not suitable for your hardware. You can find
disables those modelines not suitable for your hardware. You can find
out which modes survive with the X11 log file. It can be found at:
/var/log/XFree86.0.log
/var/log/XFree86.0.log (on Debian systems ... :-) )
See appendix A for some sample modeline definitions.
4. DGA & MPlayer
4. DGA & MPLayer
~~~~~~~~~~~~~~~~
DGA is used in two places with MPlayer: The SDL driver can be made to make
@ -76,47 +75,88 @@ Contents
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The DGA driver is invoked by specifying -vo dga at the command line.
The default behaviour is to switch to a resolution matching the original
resolution of the video as close as possible. It deliberately ignores the
-vm and -fs switches (enabling of video mode switching and fullscreen) -
it always tries to cover as much area of your screen as possible by switching
the video mode, thus refraining to use a single additional cycle of your CPU
to scale the image.
If you don't like the mode it chooses you may force it to choose the mode
matching closest the resolution you specify by -x and -y.
By providing the -v option, the DGA driver will print, among a lot of other
things, a list of all resolutions supported by your current XF86-Config
file.
Having DGA2 you may also force it to use a certain depth by using the -bpp
option. Valid depths are 15, 16, 24 and 32. It depends on your hardware
whether these depths are natively supported or if a (possibly slow)
conversion has to be done.
If you should be lucky enough to have enough offscreen memory left to
put a whole image there, the DGA driver will use doublebuffering, which
results in much smoother movie replaying. It will tell you whether double-
buffering is enabled or not.
... to be continued during the next days ...
# An advantage of this method is that you are in full control of the
# framebuffer and can put your hand on the code that is used for copying
# to the framebuffer. However there are two disadvantages:
# First, you control the screen - no more windowmanager functionally - just
# the one application that uses DGA is visible.
6. Speed issues
~~~~~~~~~~~~~~~
Generally spoken, DGA framebuffer access should be at least as fast as using
the X11 driver with the additional benefit of getting a fullscreen image.
The percentage speed values printed by mplayer have to be interpreted with
some care, as for example, with the X11 driver they do not include the time
used by the X-Server needed for the actual drawing. Hook a terminal to a
serial line of your box and start top to see what is really going on in your
box ...
# Second you
# Do it like this (you got to be root for this):
Generally spoken, the speedup done by using DGA against 'normal' use of X11
highly depends on your graphics card and how well the X-Server module for it
is optimized.
# chown root /usr/local/bin/mplayer
# chmod 750 /usr/local/bin/mplayer
# chmod +s /usr/local/bin/mplayer
If you have a slow system, better use 15 or 16bit depth since they require
only half the memory bandwidth of a 32 bit display.
# Now it works for simple users, too.
# !!!! BUT STAY TUNED !!!!
# This is a BIG security risk! Never do this on a server or on a computer
# can be accessed by more people than only you because they can gain root
# privilegies through suid root mplayer.
# !!!! SO YOU HAVE BEEN WARNED ... !!!!
#
Using a depth of 24bit is even a good idea if your card natively just supports
32 bit depth since it transfers 25% less data compared to the 32/32 mode.
I've seen some avi files already be replayed on a Pentium MMX 266. AMD K6-2
CPUs might work at 400 MHZ and above.
7. Known Bugs
~~~~~~~~~~~~~
Well, according to some developpers of XFree, DGA is quite a beast. They
tell you better not to use it. Its implementation is not always flawless
with every chipset driver for XFree out there.
o with XFree 4.0.3 and nv.o there is a bug resulting in strange colors
o ATI driver requires to switch mode back more than once after finishing
using of DGA
o some drivers simply fail to switch back to normal resolution (use
Ctrl-Alt-Keypad +, - to switch back manually)
o some drivers simply display strange colors
o some drivers lie about the amount of memory they map into the process's
address spacei, thus vo_dga won't use doublebuffering (SIS?)
o some drivers seem to fail to report even a single valid mode. In this
case the DGA driver will crash telling you about a nonsense mode of
100000x100000 or the like ...
o OSD only works with doublebuffering enabled
8. Future work
~~~~~~~~~~~~~~
o use of the new X11 render interface for OSD
o where is my TODO list ???? :-(((
A. Sample modelines
~~~~~~~~~~~~~~~~~~~
Section "Modes"
Identifier "Modes[0]"
# 800x600 @ 60 Hz, 37.8 kHz hsync
Modeline "800x600" 40 800 840 968 1056 600 601 605 628
Modeline "712x600" 35.0 712 740 850 900 400 410 412 425
Modeline "640x480" 25.175 640 664 760 800 480 491 493 525
Modeline "400x300" 20 400 416 480 528 300 301 303 314 Doublescan
Modeline "352x288" 25.10 352 368 416 432 288 296 290 310
Modeline "352x240" 15.750 352 368 416 432 240 244 246 262 Doublescan
Modeline "320x240" 12.588 320 336 384 400 240 245 246 262 Doublescan
# 800x600 @ 60 Hz, 37.8 kHz hsync
Modeline "800x600" 40 800 840 968 1056 600 601 605 628
Modeline "352x288" 25.10 352 368 416 432 288 296 290 310
EndSection