1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
|
Contents
~~~~~~~~
1. Summary
2. What is DGA
3. Resolution switching
4. DGA & MPlayer
5. Features of the DGA driver
6. Speed issues
7. Known bugs
8. Future work
A. Some modelines
B. Bug Reports
1. Summary
~~~~~~~~~~
This document tries to explain in some words what DGA is in general and
what the DGA video output driver for mplayer can do (and what it can't).
2. What is DGA?
~~~~~~~~~~~~~~~
DGA is short for Direct Graphics Access and is a means for a program 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 allowed by the kernel only
if you have superuser privileges. You can get these either by logging in
as root or by setting the suid bit on the mplayer excecutable (NOT
recommended!).
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 direct framebuffer access as described above. For
switching the resolution of the video signal you have to rely on the
XVidMode extension.
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.
However DGA has some drawbacks. It seems it is somewhat dependent on the
graphics chip you use and on the implementation of the XServer's video
driver that controls this chip. So it does not work on every system ...
3. Resolution switching
~~~~~~~~~~~~~~~~~~~~~~~
The DGA driver allows for switching the resolution of the output signal.
This avoids the need for doing (slow) software scaling and at the same
time provides a fullscreen image. Ideally it would switch to the exact
resolution (except for honouring aspect ratio) of the video data, but the
XServer only allows switching to resolutions predefined in
/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
out which modes survive with the X11 log file. It can be found at:
/var/log/XFree86.0.log
See appendix A for some sample modeline definitions.
4. DGA & MPlayer
~~~~~~~~~~~~~~~~
DGA is used in two places with MPlayer: The SDL driver can be made to make
use of it (-vo sdl:dga) and within the DGA driver (-vo dga).
The above said is true for both; in the following sections I'll explain
how the DGA driver for MPlayer works.
5. Features of the DGA driver
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.
Doublebuffering means that the next frame of your video is being drawn in
some offscreen memory while the current frame is being displayed. When the
next frame is ready, the graphics chip is just told the location in memory
of the new frame and simply fetches the data to be displayed from there.
In the meantime the other buffer in memory will be filled again with new
video data.
Doublebuffering may be switched on by using the option -double and may be
disabled with -nodouble. Current default option is to disable
doublebuffering. When using the DGA driver, onscreen display (OSD) only
works with doublebuffering enabled. However, enabling doublebuffering may
result in a big speed penalty (on my K6-II+ 525 it used an additional 20% of
CPU time!) depending on the implementation of DGA for your hardware.
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 ...
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.
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.
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 space, 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
EndSection
These entries work fine with my Riva128 chip, using nv.o XServer driver
module.
B. Bug Reports
~~~~~~~~~~~~~~
If you experience troubles with the DGA driver please feel free to file
a bug report to me (e-mail address below). Please start mplayer with the
-v option and include all lines in the bug report that start with vo_dga:
Please do also include the version of X11 you are using, the graphics card
and your CPU type. The X11 driver module (defined in XF86-Config) might
also help. Thanks!
Acki (acki@acki-netz.de, www.acki-netz.de) 18.06.2001
|
