Based on kernel version 4.2. Page generated on 2015-09-09 12:12 EST.
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][eDd] 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 DRM 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 42 if a display is connected. 'D' will force the display to be enabled and use 43 digital output. This is useful for outputs that have both analog and digital 44 signals (e.g. HDMI and DVI-I). For other outputs it behaves like 'e'. If 'd' 45 is specified the output is disabled. 46 47 You can additionally specify which output the options matches to. 48 To force the VGA output to be enabled and drive a specific mode say: 49 video=VGA-1:1280x1024@60me 50 51 Specifying the option multiple times for different ports is possible, e.g.: 52 video=LVDS-1:d video=HDMI-1:D 53 54 ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** 55 56 What is the VESA(TM) Coordinated Video Timings (CVT)? 57 58 From the VESA(TM) Website: 59 60 "The purpose of CVT is to provide a method for generating a consistent 61 and coordinated set of standard formats, display refresh rates, and 62 timing specifications for computer display products, both those 63 employing CRTs, and those using other display technologies. The 64 intention of CVT is to give both source and display manufacturers a 65 common set of tools to enable new timings to be developed in a 66 consistent manner that ensures greater compatibility." 67 68 This is the third standard approved by VESA(TM) concerning video timings. The 69 first was the Discrete Video Timings (DVT) which is a collection of 70 pre-defined modes approved by VESA(TM). The second is the Generalized Timing 71 Formula (GTF) which is an algorithm to calculate the timings, given the 72 pixelclock, the horizontal sync frequency, or the vertical refresh rate. 73 74 The GTF is limited by the fact that it is designed mainly for CRT displays. 75 It artificially increases the pixelclock because of its high blanking 76 requirement. This is inappropriate for digital display interface with its high 77 data rate which requires that it conserves the pixelclock as much as possible. 78 Also, GTF does not take into account the aspect ratio of the display. 79 80 The CVT addresses these limitations. If used with CRT's, the formula used 81 is a derivation of GTF with a few modifications. If used with digital 82 displays, the "reduced blanking" calculation can be used. 83 84 From the framebuffer subsystem perspective, new formats need not be added 85 to the global mode database whenever a new mode is released by display 86 manufacturers. Specifying for CVT will work for most, if not all, relatively 87 new CRT displays and probably with most flatpanels, if 'reduced blanking' 88 calculation is specified. (The CVT compatibility of the display can be 89 determined from its EDID. The version 1.3 of the EDID has extra 128-byte 90 blocks where additional timing information is placed. As of this time, there 91 is no support yet in the layer to parse this additional blocks.) 92 93 CVT also introduced a new naming convention (should be seen from dmesg output): 94 95 <pix>M<a>[-R] 96 97 where: pix = total amount of pixels in MB (xres x yres) 98 M = always present 99 a = aspect ratio (3 - 4:3; 4 - 5:4; 9 - 15:9, 16:9; A - 16:10) 100 -R = reduced blanking 101 102 example: .48M3-R - 800x600 with reduced blanking 103 104 Note: VESA(TM) has restrictions on what is a standard CVT timing: 105 106 - aspect ratio can only be one of the above values 107 - acceptable refresh rates are 50, 60, 70 or 85 Hz only 108 - if reduced blanking, the refresh rate must be at 60Hz 109 110 If one of the above are not satisfied, the kernel will print a warning but the 111 timings will still be calculated. 112 113 ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** 114 115 To find a suitable video mode, you just call 116 117 int __init fb_find_mode(struct fb_var_screeninfo *var, 118 struct fb_info *info, const char *mode_option, 119 const struct fb_videomode *db, unsigned int dbsize, 120 const struct fb_videomode *default_mode, 121 unsigned int default_bpp) 122 123 with db/dbsize your non-standard video mode database, or NULL to use the 124 standard video mode database. 125 126 fb_find_mode() first tries the specified video mode (or any mode that matches, 127 e.g. there can be multiple 640x480 modes, each of them is tried). If that 128 fails, the default mode is tried. If that fails, it walks over all modes. 129 130 To specify a video mode at bootup, use the following boot options: 131 video=<driver>:<xres>x<yres>[-<bpp>][@refresh] 132 133 where <driver> is a name from the table below. Valid default modes can be 134 found in linux/drivers/video/modedb.c. Check your driver's documentation. 135 There may be more modes. 136 137 Drivers that support modedb boot options 138 Boot Name Cards Supported 139 140 amifb - Amiga chipset frame buffer 141 aty128fb - ATI Rage128 / Pro frame buffer 142 atyfb - ATI Mach64 frame buffer 143 pm2fb - Permedia 2/2V frame buffer 144 pm3fb - Permedia 3 frame buffer 145 sstfb - Voodoo 1/2 (SST1) chipset frame buffer 146 tdfxfb - 3D Fx frame buffer 147 tridentfb - Trident (Cyber)blade chipset frame buffer 148 vt8623fb - VIA 8623 frame buffer 149 150 BTW, only a few fb drivers use this at the moment. Others are to follow 151 (feel free to send patches). The DRM drivers also support this.