1
2
3 modedb default video mode support
4
5
6 Currently all frame buffer device drivers have their own video mode databases,
7 which 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
17 When a frame buffer device receives a video= option it doesn't know, it should
18 consider that to be a video mode option. If no frame buffer device is specified
19 in a video= option, fbmem considers that to be a global video mode option.
20
21 Valid mode specifiers (mode_option argument):
22
23 <xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m]
24 <name>[-<bpp>][@<refresh>]
25
26 with <xres>, <yres>, <bpp> and <refresh> decimal numbers and <name> a string.
27 Things between square brackets are optional.
28
29 If 'M' is specified in the mode_option argument (after <yres> and before
30 <bpp> and <refresh>, if specified) the timings will be calculated using
31 VESA(TM) Coordinated Video Timings instead of looking up the mode from a table.
32 If 'R' is specified, do a 'reduced blanking' calculation for digital displays.
33 If 'i' is specified, calculate for an interlaced mode. And if 'm' is
34 specified, add margins to the calculation (1.8% of xres rounded down to 8
35 pixels and 1.8% of yres).
36
37 Sample usage: 1024x768M@60m - CVT timing with margins
38
39 ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo *****
40
41 What is the VESA(TM) Coordinated Video Timings (CVT)?
42
43 From the VESA(TM) Website:
44
45 "The purpose of CVT is to provide a method for generating a consistent
46 and coordinated set of standard formats, display refresh rates, and
47 timing specifications for computer display products, both those
48 employing CRTs, and those using other display technologies. The
49 intention of CVT is to give both source and display manufacturers a
50 common set of tools to enable new timings to be developed in a
51 consistent manner that ensures greater compatibility."
52
53 This is the third standard approved by VESA(TM) concerning video timings. The
54 first was the Discrete Video Timings (DVT) which is a collection of
55 pre-defined modes approved by VESA(TM). The second is the Generalized Timing
56 Formula (GTF) which is an algorithm to calculate the timings, given the
57 pixelclock, the horizontal sync frequency, or the vertical refresh rate.
58
59 The GTF is limited by the fact that it is designed mainly for CRT displays.
60 It artificially increases the pixelclock because of its high blanking
61 requirement. This is inappropriate for digital display interface with its high
62 data rate which requires that it conserves the pixelclock as much as possible.
63 Also, GTF does not take into account the aspect ratio of the display.
64
65 The CVT addresses these limitations. If used with CRT's, the formula used
66 is a derivation of GTF with a few modifications. If used with digital
67 displays, the "reduced blanking" calculation can be used.
68
69 From the framebuffer subsystem perspective, new formats need not be added
70 to the global mode database whenever a new mode is released by display
71 manufacturers. Specifying for CVT will work for most, if not all, relatively
72 new CRT displays and probably with most flatpanels, if 'reduced blanking'
73 calculation is specified. (The CVT compatibility of the display can be
74 determined from its EDID. The version 1.3 of the EDID has extra 128-byte
75 blocks where additional timing information is placed. As of this time, there
76 is no support yet in the layer to parse this additional blocks.)
77
78 CVT also introduced a new naming convention (should be seen from dmesg output):
79
80 <pix>M<a>[-R]
81
82 where: pix = total amount of pixels in MB (xres x yres)
83 M = always present
84 a = aspect ratio (3 - 4:3; 4 - 5:4; 9 - 15:9, 16:9; A - 16:10)
85 -R = reduced blanking
86
87 example: .48M3-R - 800x600 with reduced blanking
88
89 Note: VESA(TM) has restrictions on what is a standard CVT timing:
90
91 - aspect ratio can only be one of the above values
92 - acceptable refresh rates are 50, 60, 70 or 85 Hz only
93 - if reduced blanking, the refresh rate must be at 60Hz
94
95 If one of the above are not satisfied, the kernel will print a warning but the
96 timings will still be calculated.
97
98 ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo *****
99
100 To find a suitable video mode, you just call
101
102 int __init fb_find_mode(struct fb_var_screeninfo *var,
103 struct fb_info *info, const char *mode_option,
104 const struct fb_videomode *db, unsigned int dbsize,
105 const struct fb_videomode *default_mode,
106 unsigned int default_bpp)
107
108 with db/dbsize your non-standard video mode database, or NULL to use the
109 standard video mode database.
110
111 fb_find_mode() first tries the specified video mode (or any mode that matches,
112 e.g. there can be multiple 640x480 modes, each of them is tried). If that
113 fails, the default mode is tried. If that fails, it walks over all modes.
114
115 To specify a video mode at bootup, use the following boot options:
116 video=<driver>:<xres>x<yres>[-<bpp>][@refresh]
117
118 where <driver> is a name from the table below. Valid default modes can be
119 found in linux/drivers/video/modedb.c. Check your driver's documentation.
120 There may be more modes.
121
122 Drivers that support modedb boot options
123 Boot Name Cards Supported
124
125 amifb - Amiga chipset frame buffer
126 aty128fb - ATI Rage128 / Pro frame buffer
127 atyfb - ATI Mach64 frame buffer
128 tdfxfb - 3D Fx frame buffer
129 tridentfb - Trident (Cyber)blade chipset frame buffer
130
131 BTW, only a few drivers use this at the moment. Others are to follow
132 (feel free to send patches).
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