1================================= 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 56----------------------------------------------------------------------------- 57 58What is the VESA(TM) Coordinated Video Timings (CVT)? 59===================================================== 60 61From the VESA(TM) Website: 62 63 "The purpose of CVT is to provide a method for generating a consistent 64 and coordinated set of standard formats, display refresh rates, and 65 timing specifications for computer display products, both those 66 employing CRTs, and those using other display technologies. The 67 intention of CVT is to give both source and display manufacturers a 68 common set of tools to enable new timings to be developed in a 69 consistent manner that ensures greater compatibility." 70 71This is the third standard approved by VESA(TM) concerning video timings. The 72first was the Discrete Video Timings (DVT) which is a collection of 73pre-defined modes approved by VESA(TM). The second is the Generalized Timing 74Formula (GTF) which is an algorithm to calculate the timings, given the 75pixelclock, the horizontal sync frequency, or the vertical refresh rate. 76 77The GTF is limited by the fact that it is designed mainly for CRT displays. 78It artificially increases the pixelclock because of its high blanking 79requirement. This is inappropriate for digital display interface with its high 80data rate which requires that it conserves the pixelclock as much as possible. 81Also, GTF does not take into account the aspect ratio of the display. 82 83The CVT addresses these limitations. If used with CRT's, the formula used 84is a derivation of GTF with a few modifications. If used with digital 85displays, the "reduced blanking" calculation can be used. 86 87From the framebuffer subsystem perspective, new formats need not be added 88to the global mode database whenever a new mode is released by display 89manufacturers. Specifying for CVT will work for most, if not all, relatively 90new CRT displays and probably with most flatpanels, if 'reduced blanking' 91calculation is specified. (The CVT compatibility of the display can be 92determined from its EDID. The version 1.3 of the EDID has extra 128-byte 93blocks where additional timing information is placed. As of this time, there 94is no support yet in the layer to parse this additional blocks.) 95 96CVT also introduced a new naming convention (should be seen from dmesg output):: 97 98 <pix>M<a>[-R] 99 100 where: pix = total amount of pixels in MB (xres x yres) 101 M = always present 102 a = aspect ratio (3 - 4:3; 4 - 5:4; 9 - 15:9, 16:9; A - 16:10) 103 -R = reduced blanking 104 105 example: .48M3-R - 800x600 with reduced blanking 106 107Note: VESA(TM) has restrictions on what is a standard CVT timing: 108 109 - aspect ratio can only be one of the above values 110 - acceptable refresh rates are 50, 60, 70 or 85 Hz only 111 - if reduced blanking, the refresh rate must be at 60Hz 112 113If one of the above are not satisfied, the kernel will print a warning but the 114timings will still be calculated. 115 116----------------------------------------------------------------------------- 117 118To find a suitable video mode, you just call:: 119 120 int __init fb_find_mode(struct fb_var_screeninfo *var, 121 struct fb_info *info, const char *mode_option, 122 const struct fb_videomode *db, unsigned int dbsize, 123 const struct fb_videomode *default_mode, 124 unsigned int default_bpp) 125 126with db/dbsize your non-standard video mode database, or NULL to use the 127standard video mode database. 128 129fb_find_mode() first tries the specified video mode (or any mode that matches, 130e.g. there can be multiple 640x480 modes, each of them is tried). If that 131fails, the default mode is tried. If that fails, it walks over all modes. 132 133To specify a video mode at bootup, use the following boot options:: 134 135 video=<driver>:<xres>x<yres>[-<bpp>][@refresh] 136 137where <driver> is a name from the table below. Valid default modes can be 138found in linux/drivers/video/modedb.c. Check your driver's documentation. 139There may be more modes:: 140 141 Drivers that support modedb boot options 142 Boot Name Cards Supported 143 144 amifb - Amiga chipset frame buffer 145 aty128fb - ATI Rage128 / Pro frame buffer 146 atyfb - ATI Mach64 frame buffer 147 pm2fb - Permedia 2/2V frame buffer 148 pm3fb - Permedia 3 frame buffer 149 sstfb - Voodoo 1/2 (SST1) chipset frame buffer 150 tdfxfb - 3D Fx frame buffer 151 tridentfb - Trident (Cyber)blade chipset frame buffer 152 vt8623fb - VIA 8623 frame buffer 153 154BTW, only a few fb drivers use this at the moment. Others are to follow 155(feel free to send patches). The DRM drivers also support this. 156