1 2 3 modedb default video mode support 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] 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 39***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** 40 41What is the VESA(TM) Coordinated Video Timings (CVT)? 42 43From 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 53This is the third standard approved by VESA(TM) concerning video timings. The 54first was the Discrete Video Timings (DVT) which is a collection of 55pre-defined modes approved by VESA(TM). The second is the Generalized Timing 56Formula (GTF) which is an algorithm to calculate the timings, given the 57pixelclock, the horizontal sync frequency, or the vertical refresh rate. 58 59The GTF is limited by the fact that it is designed mainly for CRT displays. 60It artificially increases the pixelclock because of its high blanking 61requirement. This is inappropriate for digital display interface with its high 62data rate which requires that it conserves the pixelclock as much as possible. 63Also, GTF does not take into account the aspect ratio of the display. 64 65The CVT addresses these limitations. If used with CRT's, the formula used 66is a derivation of GTF with a few modifications. If used with digital 67displays, the "reduced blanking" calculation can be used. 68 69From the framebuffer subsystem perspective, new formats need not be added 70to the global mode database whenever a new mode is released by display 71manufacturers. Specifying for CVT will work for most, if not all, relatively 72new CRT displays and probably with most flatpanels, if 'reduced blanking' 73calculation is specified. (The CVT compatibility of the display can be 74determined from its EDID. The version 1.3 of the EDID has extra 128-byte 75blocks where additional timing information is placed. As of this time, there 76is no support yet in the layer to parse this additional blocks.) 77 78CVT 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 89Note: 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 95If one of the above are not satisfied, the kernel will print a warning but the 96timings will still be calculated. 97 98***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** 99 100To find a suitable video mode, you just call 101 102int __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 108with db/dbsize your non-standard video mode database, or NULL to use the 109standard video mode database. 110 111fb_find_mode() first tries the specified video mode (or any mode that matches, 112e.g. there can be multiple 640x480 modes, each of them is tried). If that 113fails, the default mode is tried. If that fails, it walks over all modes. 114 115To specify a video mode at bootup, use the following boot options: 116 video=<driver>:<xres>x<yres>[-<bpp>][@refresh] 117 118where <driver> is a name from the table below. Valid default modes can be 119found in linux/drivers/video/modedb.c. Check your driver's documentation. 120There 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 pm2fb - Permedia 2/2V frame buffer 129 pm3fb - Permedia 3 frame buffer 130 sstfb - Voodoo 1/2 (SST1) chipset frame buffer 131 tdfxfb - 3D Fx frame buffer 132 tridentfb - Trident (Cyber)blade chipset frame buffer 133 vt8623fb - VIA 8623 frame buffer 134 135BTW, only a few drivers use this at the moment. Others are to follow 136(feel free to send patches). 137