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| modedb default video mode support |
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| 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 |
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| - 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) |
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| 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. |
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| Valid mode specifiers (mode_option argument): |
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| <xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m] |
| <name>[-<bpp>][@<refresh>] |
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| with <xres>, <yres>, <bpp> and <refresh> decimal numbers and <name> a string. |
| Things between square brackets are optional. |
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| 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). |
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| Sample usage: 1024x768M@60m - CVT timing with margins |
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| ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** |
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| What is the VESA(TM) Coordinated Video Timings (CVT)? |
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| From the VESA(TM) Website: |
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| "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." |
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| 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. |
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| 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. |
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| 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. |
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| 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.) |
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| CVT also introduced a new naming convention (should be seen from dmesg output): |
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| <pix>M<a>[-R] |
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| 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 |
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| example: .48M3-R - 800x600 with reduced blanking |
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| Note: VESA(TM) has restrictions on what is a standard CVT timing: |
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| - 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 |
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| If one of the above are not satisfied, the kernel will print a warning but the |
| timings will still be calculated. |
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| ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** |
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| To find a suitable video mode, you just call |
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| 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) |
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| with db/dbsize your non-standard video mode database, or NULL to use the |
| standard video mode database. |
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| 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. |
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| To specify a video mode at bootup, use the following boot options: |
| video=<driver>:<xres>x<yres>[-<bpp>][@refresh] |
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| 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. |
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| Drivers that support modedb boot options |
| Boot Name Cards Supported |
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| 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 |
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| BTW, only a few drivers use this at the moment. Others are to follow |
| (feel free to send patches). |