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2modedb default video mode support
3=================================
4
5
6Currently all frame buffer device drivers have their own video mode databases,
7which is a mess and a waste of resources. The main idea of modedb is to have
8
9 - one routine to probe for video modes, which can be used by all frame buffer
10 devices
11 - one generic video mode database with a fair amount of standard videomodes
12 (taken from XFree86)
13 - the possibility to supply your own mode database for graphics hardware that
14 needs non-standard modes, like amifb and Mac frame buffer drivers (which
15 use macmodes.c)
16
17When a frame buffer device receives a video= option it doesn't know, it should
18consider that to be a video mode option. If no frame buffer device is specified
19in a video= option, fbmem considers that to be a global video mode option.
20
21Valid mode specifiers (mode_option argument)::
22
23 <xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m][eDd]
24 <name>[-<bpp>][@<refresh>]
25
26with <xres>, <yres>, <bpp> and <refresh> decimal numbers and <name> a string.
27Things between square brackets are optional.
28
29If 'M' is specified in the mode_option argument (after <yres> and before
30<bpp> and <refresh>, if specified) the timings will be calculated using
31VESA(TM) Coordinated Video Timings instead of looking up the mode from a table.
32If 'R' is specified, do a 'reduced blanking' calculation for digital displays.
33If 'i' is specified, calculate for an interlaced mode. And if 'm' is
34specified, add margins to the calculation (1.8% of xres rounded down to 8
35pixels and 1.8% of yres).
36
37 Sample usage: 1024x768M@60m - CVT timing with margins
38
39DRM drivers also add options to enable or disable outputs:
40
41'e' will force the display to be enabled, i.e. it will override the detection
42if a display is connected. 'D' will force the display to be enabled and use
43digital output. This is useful for outputs that have both analog and digital
44signals (e.g. HDMI and DVI-I). For other outputs it behaves like 'e'. If 'd'
45is specified the output is disabled.
46
47You can additionally specify which output the options matches to.
48To force the VGA output to be enabled and drive a specific mode say::
49
50 video=VGA-1:1280x1024@60me
51
52Specifying the option multiple times for different ports is possible, e.g.::
53
54 video=LVDS-1:d video=HDMI-1:D
55
56Options can also be passed after the mode, using commas as separator.
57
58 Sample usage: 720x480,rotate=180 - 720x480 mode, rotated by 180 degrees
59
60Valid options are::
61
62 - margin_top, margin_bottom, margin_left, margin_right (integer):
63 Number of pixels in the margins, typically to deal with overscan on TVs
64 - reflect_x (boolean): Perform an axial symmetry on the X axis
65 - reflect_y (boolean): Perform an axial symmetry on the Y axis
66 - rotate (integer): Rotate the initial framebuffer by x
67 degrees. Valid values are 0, 90, 180 and 270.
68
69
70-----------------------------------------------------------------------------
71
72What is the VESA(TM) Coordinated Video Timings (CVT)?
73=====================================================
74
75From the VESA(TM) Website:
76
77 "The purpose of CVT is to provide a method for generating a consistent
78 and coordinated set of standard formats, display refresh rates, and
79 timing specifications for computer display products, both those
80 employing CRTs, and those using other display technologies. The
81 intention of CVT is to give both source and display manufacturers a
82 common set of tools to enable new timings to be developed in a
83 consistent manner that ensures greater compatibility."
84
85This is the third standard approved by VESA(TM) concerning video timings. The
86first was the Discrete Video Timings (DVT) which is a collection of
87pre-defined modes approved by VESA(TM). The second is the Generalized Timing
88Formula (GTF) which is an algorithm to calculate the timings, given the
89pixelclock, the horizontal sync frequency, or the vertical refresh rate.
90
91The GTF is limited by the fact that it is designed mainly for CRT displays.
92It artificially increases the pixelclock because of its high blanking
93requirement. This is inappropriate for digital display interface with its high
94data rate which requires that it conserves the pixelclock as much as possible.
95Also, GTF does not take into account the aspect ratio of the display.
96
97The CVT addresses these limitations. If used with CRT's, the formula used
98is a derivation of GTF with a few modifications. If used with digital
99displays, the "reduced blanking" calculation can be used.
100
101From the framebuffer subsystem perspective, new formats need not be added
102to the global mode database whenever a new mode is released by display
103manufacturers. Specifying for CVT will work for most, if not all, relatively
104new CRT displays and probably with most flatpanels, if 'reduced blanking'
105calculation is specified. (The CVT compatibility of the display can be
106determined from its EDID. The version 1.3 of the EDID has extra 128-byte
107blocks where additional timing information is placed. As of this time, there
108is no support yet in the layer to parse this additional blocks.)
109
110CVT also introduced a new naming convention (should be seen from dmesg output)::
111
112 <pix>M<a>[-R]
113
114 where: pix = total amount of pixels in MB (xres x yres)
115 M = always present
116 a = aspect ratio (3 - 4:3; 4 - 5:4; 9 - 15:9, 16:9; A - 16:10)
117 -R = reduced blanking
118
119 example: .48M3-R - 800x600 with reduced blanking
120
121Note: VESA(TM) has restrictions on what is a standard CVT timing:
122
123 - aspect ratio can only be one of the above values
124 - acceptable refresh rates are 50, 60, 70 or 85 Hz only
125 - if reduced blanking, the refresh rate must be at 60Hz
126
127If one of the above are not satisfied, the kernel will print a warning but the
128timings will still be calculated.
129
130-----------------------------------------------------------------------------
131
132To find a suitable video mode, you just call::
133
134 int __init fb_find_mode(struct fb_var_screeninfo *var,
135 struct fb_info *info, const char *mode_option,
136 const struct fb_videomode *db, unsigned int dbsize,
137 const struct fb_videomode *default_mode,
138 unsigned int default_bpp)
139
140with db/dbsize your non-standard video mode database, or NULL to use the
141standard video mode database.
142
143fb_find_mode() first tries the specified video mode (or any mode that matches,
144e.g. there can be multiple 640x480 modes, each of them is tried). If that
145fails, the default mode is tried. If that fails, it walks over all modes.
146
147To specify a video mode at bootup, use the following boot options::
148
149 video=<driver>:<xres>x<yres>[-<bpp>][@refresh]
150
151where <driver> is a name from the table below. Valid default modes can be
152found in linux/drivers/video/modedb.c. Check your driver's documentation.
153There may be more modes::
154
155 Drivers that support modedb boot options
156 Boot Name Cards Supported
157
158 amifb - Amiga chipset frame buffer
159 aty128fb - ATI Rage128 / Pro frame buffer
160 atyfb - ATI Mach64 frame buffer
161 pm2fb - Permedia 2/2V frame buffer
162 pm3fb - Permedia 3 frame buffer
163 sstfb - Voodoo 1/2 (SST1) chipset frame buffer
164 tdfxfb - 3D Fx frame buffer
165 tridentfb - Trident (Cyber)blade chipset frame buffer
166 vt8623fb - VIA 8623 frame buffer
167
168BTW, only a few fb drivers use this at the moment. Others are to follow
169(feel free to send patches). The DRM drivers also support this.