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diff --git a/Documentation/fb/modedb.txt b/Documentation/fb/modedb.txt new file mode 100644 index 0000000..ec4dee7 --- /dev/null +++ b/Documentation/fb/modedb.txt @@ -0,0 +1,136 @@ + + + modedb default video mode support + + +Currently all frame buffer device drivers have their own video mode databases, +which is a mess and a waste of resources. The main idea of modedb is to have + + - one routine to probe for video modes, which can be used by all frame buffer + devices + - one generic video mode database with a fair amount of standard videomodes + (taken from XFree86) + - the possibility to supply your own mode database for graphics hardware that + needs non-standard modes, like amifb and Mac frame buffer drivers (which + use macmodes.c) + +When a frame buffer device receives a video= option it doesn't know, it should +consider that to be a video mode option. If no frame buffer device is specified +in a video= option, fbmem considers that to be a global video mode option. + +Valid mode specifiers (mode_option argument): + + <xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m] + <name>[-<bpp>][@<refresh>] + +with <xres>, <yres>, <bpp> and <refresh> decimal numbers and <name> a string. +Things between square brackets are optional. + +If 'M' is specified in the mode_option argument (after <yres> and before +<bpp> and <refresh>, if specified) the timings will be calculated using +VESA(TM) Coordinated Video Timings instead of looking up the mode from a table. +If 'R' is specified, do a 'reduced blanking' calculation for digital displays. +If 'i' is specified, calculate for an interlaced mode. And if 'm' is +specified, add margins to the calculation (1.8% of xres rounded down to 8 +pixels and 1.8% of yres). + + Sample usage: 1024x768M@60m - CVT timing with margins + +***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** + +What is the VESA(TM) Coordinated Video Timings (CVT)? + +From the VESA(TM) Website: + + "The purpose of CVT is to provide a method for generating a consistent + and coordinated set of standard formats, display refresh rates, and + timing specifications for computer display products, both those + employing CRTs, and those using other display technologies. The + intention of CVT is to give both source and display manufacturers a + common set of tools to enable new timings to be developed in a + consistent manner that ensures greater compatibility." + +This is the third standard approved by VESA(TM) concerning video timings. The +first was the Discrete Video Timings (DVT) which is a collection of +pre-defined modes approved by VESA(TM). The second is the Generalized Timing +Formula (GTF) which is an algorithm to calculate the timings, given the +pixelclock, the horizontal sync frequency, or the vertical refresh rate. + +The GTF is limited by the fact that it is designed mainly for CRT displays. +It artificially increases the pixelclock because of its high blanking +requirement. This is inappropriate for digital display interface with its high +data rate which requires that it conserves the pixelclock as much as possible. +Also, GTF does not take into account the aspect ratio of the display. + +The CVT addresses these limitations. If used with CRT's, the formula used +is a derivation of GTF with a few modifications. If used with digital +displays, the "reduced blanking" calculation can be used. + +From the framebuffer subsystem perspective, new formats need not be added +to the global mode database whenever a new mode is released by display +manufacturers. Specifying for CVT will work for most, if not all, relatively +new CRT displays and probably with most flatpanels, if 'reduced blanking' +calculation is specified. (The CVT compatibility of the display can be +determined from its EDID. The version 1.3 of the EDID has extra 128-byte +blocks where additional timing information is placed. As of this time, there +is no support yet in the layer to parse this additional blocks.) + +CVT also introduced a new naming convention (should be seen from dmesg output): + + <pix>M<a>[-R] + + where: pix = total amount of pixels in MB (xres x yres) + M = always present + a = aspect ratio (3 - 4:3; 4 - 5:4; 9 - 15:9, 16:9; A - 16:10) + -R = reduced blanking + + example: .48M3-R - 800x600 with reduced blanking + +Note: VESA(TM) has restrictions on what is a standard CVT timing: + + - aspect ratio can only be one of the above values + - acceptable refresh rates are 50, 60, 70 or 85 Hz only + - if reduced blanking, the refresh rate must be at 60Hz + +If one of the above are not satisfied, the kernel will print a warning but the +timings will still be calculated. + +***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** + +To find a suitable video mode, you just call + +int __init fb_find_mode(struct fb_var_screeninfo *var, + struct fb_info *info, const char *mode_option, + const struct fb_videomode *db, unsigned int dbsize, + const struct fb_videomode *default_mode, + unsigned int default_bpp) + +with db/dbsize your non-standard video mode database, or NULL to use the +standard video mode database. + +fb_find_mode() first tries the specified video mode (or any mode that matches, +e.g. there can be multiple 640x480 modes, each of them is tried). If that +fails, the default mode is tried. If that fails, it walks over all modes. + +To specify a video mode at bootup, use the following boot options: + video=<driver>:<xres>x<yres>[-<bpp>][@refresh] + +where <driver> is a name from the table below. Valid default modes can be +found in linux/drivers/video/modedb.c. Check your driver's documentation. +There may be more modes. + + Drivers that support modedb boot options + Boot Name Cards Supported + + amifb - Amiga chipset frame buffer + aty128fb - ATI Rage128 / Pro frame buffer + atyfb - ATI Mach64 frame buffer + pm2fb - Permedia 2/2V frame buffer + pm3fb - Permedia 3 frame buffer + sstfb - Voodoo 1/2 (SST1) chipset frame buffer + tdfxfb - 3D Fx frame buffer + tridentfb - Trident (Cyber)blade chipset frame buffer + vt8623fb - VIA 8623 frame buffer + +BTW, only a few drivers use this at the moment. Others are to follow +(feel free to send patches). |
