1 =================================
2 modedb default video mode support
3 =================================
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
9 - one routine to probe for video modes, which can be used by all frame buffer
11 - one generic video mode database with a fair amount of standard videomodes
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
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.
21 Valid mode specifiers (mode_option argument)::
23 <xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m][eDd]
24 <name>[-<bpp>][@<refresh>]
26 with <xres>, <yres>, <bpp> and <refresh> decimal numbers and <name> a string.
27 Things between square brackets are optional.
31 - NSTC: 480i output, with the CCIR System-M TV mode and NTSC color encoding
32 - PAL: 576i output, with the CCIR System-B TV mode and PAL color encoding
34 If 'M' is specified in the mode_option argument (after <yres> and before
35 <bpp> and <refresh>, if specified) the timings will be calculated using
36 VESA(TM) Coordinated Video Timings instead of looking up the mode from a table.
37 If 'R' is specified, do a 'reduced blanking' calculation for digital displays.
38 If 'i' is specified, calculate for an interlaced mode. And if 'm' is
39 specified, add margins to the calculation (1.8% of xres rounded down to 8
40 pixels and 1.8% of yres).
42 Sample usage: 1024x768M@60m - CVT timing with margins
44 DRM drivers also add options to enable or disable outputs:
46 'e' will force the display to be enabled, i.e. it will override the detection
47 if a display is connected. 'D' will force the display to be enabled and use
48 digital output. This is useful for outputs that have both analog and digital
49 signals (e.g. HDMI and DVI-I). For other outputs it behaves like 'e'. If 'd'
50 is specified the output is disabled.
52 You can additionally specify which output the options matches to.
53 To force the VGA output to be enabled and drive a specific mode say::
55 video=VGA-1:1280x1024@60me
57 Specifying the option multiple times for different ports is possible, e.g.::
59 video=LVDS-1:d video=HDMI-1:D
61 Options can also be passed after the mode, using commas as separator.
63 Sample usage: 720x480,rotate=180 - 720x480 mode, rotated by 180 degrees
67 - margin_top, margin_bottom, margin_left, margin_right (integer):
68 Number of pixels in the margins, typically to deal with overscan on TVs
69 - reflect_x (boolean): Perform an axial symmetry on the X axis
70 - reflect_y (boolean): Perform an axial symmetry on the Y axis
71 - rotate (integer): Rotate the initial framebuffer by x
72 degrees. Valid values are 0, 90, 180 and 270.
73 - panel_orientation, one of "normal", "upside_down", "left_side_up", or
74 "right_side_up". For KMS drivers only, this sets the "panel orientation"
75 property on the kms connector as hint for kms users.
78 -----------------------------------------------------------------------------
80 What is the VESA(TM) Coordinated Video Timings (CVT)?
81 =====================================================
83 From the VESA(TM) Website:
85 "The purpose of CVT is to provide a method for generating a consistent
86 and coordinated set of standard formats, display refresh rates, and
87 timing specifications for computer display products, both those
88 employing CRTs, and those using other display technologies. The
89 intention of CVT is to give both source and display manufacturers a
90 common set of tools to enable new timings to be developed in a
91 consistent manner that ensures greater compatibility."
93 This is the third standard approved by VESA(TM) concerning video timings. The
94 first was the Discrete Video Timings (DVT) which is a collection of
95 pre-defined modes approved by VESA(TM). The second is the Generalized Timing
96 Formula (GTF) which is an algorithm to calculate the timings, given the
97 pixelclock, the horizontal sync frequency, or the vertical refresh rate.
99 The GTF is limited by the fact that it is designed mainly for CRT displays.
100 It artificially increases the pixelclock because of its high blanking
101 requirement. This is inappropriate for digital display interface with its high
102 data rate which requires that it conserves the pixelclock as much as possible.
103 Also, GTF does not take into account the aspect ratio of the display.
105 The CVT addresses these limitations. If used with CRT's, the formula used
106 is a derivation of GTF with a few modifications. If used with digital
107 displays, the "reduced blanking" calculation can be used.
109 From the framebuffer subsystem perspective, new formats need not be added
110 to the global mode database whenever a new mode is released by display
111 manufacturers. Specifying for CVT will work for most, if not all, relatively
112 new CRT displays and probably with most flatpanels, if 'reduced blanking'
113 calculation is specified. (The CVT compatibility of the display can be
114 determined from its EDID. The version 1.3 of the EDID has extra 128-byte
115 blocks where additional timing information is placed. As of this time, there
116 is no support yet in the layer to parse this additional blocks.)
118 CVT also introduced a new naming convention (should be seen from dmesg output)::
122 where: pix = total amount of pixels in MB (xres x yres)
124 a = aspect ratio (3 - 4:3; 4 - 5:4; 9 - 15:9, 16:9; A - 16:10)
125 -R = reduced blanking
127 example: .48M3-R - 800x600 with reduced blanking
129 Note: VESA(TM) has restrictions on what is a standard CVT timing:
131 - aspect ratio can only be one of the above values
132 - acceptable refresh rates are 50, 60, 70 or 85 Hz only
133 - if reduced blanking, the refresh rate must be at 60Hz
135 If one of the above are not satisfied, the kernel will print a warning but the
136 timings will still be calculated.
138 -----------------------------------------------------------------------------
140 To find a suitable video mode, you just call::
142 int __init fb_find_mode(struct fb_var_screeninfo *var,
143 struct fb_info *info, const char *mode_option,
144 const struct fb_videomode *db, unsigned int dbsize,
145 const struct fb_videomode *default_mode,
146 unsigned int default_bpp)
148 with db/dbsize your non-standard video mode database, or NULL to use the
149 standard video mode database.
151 fb_find_mode() first tries the specified video mode (or any mode that matches,
152 e.g. there can be multiple 640x480 modes, each of them is tried). If that
153 fails, the default mode is tried. If that fails, it walks over all modes.
155 To specify a video mode at bootup, use the following boot options::
157 video=<driver>:<xres>x<yres>[-<bpp>][@refresh]
159 where <driver> is a name from the table below. Valid default modes can be
160 found in drivers/video/fbdev/core/modedb.c. Check your driver's documentation.
161 There may be more modes::
163 Drivers that support modedb boot options
164 Boot Name Cards Supported
166 amifb - Amiga chipset frame buffer
167 aty128fb - ATI Rage128 / Pro frame buffer
168 atyfb - ATI Mach64 frame buffer
169 pm2fb - Permedia 2/2V frame buffer
170 pm3fb - Permedia 3 frame buffer
171 sstfb - Voodoo 1/2 (SST1) chipset frame buffer
172 tdfxfb - 3D Fx frame buffer
173 tridentfb - Trident (Cyber)blade chipset frame buffer
174 vt8623fb - VIA 8623 frame buffer
176 BTW, only a few fb drivers use this at the moment. Others are to follow
177 (feel free to send patches). The DRM drivers also support this.