linux/Documentation/fb/modedb.txt
<<
>>
Prefs
   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][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    video=VGA-1:1280x1024@60me
  50
  51Specifying 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
  56What is the VESA(TM) Coordinated Video Timings (CVT)?
  57
  58From 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
  68This is the third standard approved by VESA(TM) concerning video timings.  The
  69first was the Discrete Video Timings (DVT) which is  a collection of
  70pre-defined modes approved by VESA(TM).  The second is the Generalized Timing
  71Formula (GTF) which is an algorithm to calculate the timings, given the
  72pixelclock, the horizontal sync frequency, or the vertical refresh rate.
  73
  74The GTF is limited by the fact that it is designed mainly for CRT displays.
  75It artificially increases the pixelclock because of its high blanking
  76requirement. This is inappropriate for digital display interface with its high
  77data rate which requires that it conserves the pixelclock as much as possible.
  78Also, GTF does not take into account the aspect ratio of the display.
  79
  80The CVT addresses these limitations.  If used with CRT's, the formula used
  81is a derivation of GTF with a few modifications.  If used with digital
  82displays, the "reduced blanking" calculation can be used.
  83
  84From the framebuffer subsystem perspective, new formats need not be added
  85to the global mode database whenever a new mode is released by display
  86manufacturers. Specifying for CVT will work for most, if not all, relatively
  87new CRT displays and probably with most flatpanels, if 'reduced blanking'
  88calculation is specified.  (The CVT compatibility of the display can be
  89determined from its EDID. The version 1.3 of the EDID has extra 128-byte
  90blocks where additional timing information is placed.  As of this time, there
  91is no support yet in the layer to parse this additional blocks.)
  92
  93CVT 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
 104Note: 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
 110If one of the above are not satisfied, the kernel will print a warning but the
 111timings will still be calculated.
 112
 113***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo *****
 114
 115To find a suitable video mode, you just call
 116
 117int __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
 123with db/dbsize your non-standard video mode database, or NULL to use the
 124standard video mode database.
 125
 126fb_find_mode() first tries the specified video mode (or any mode that matches,
 127e.g. there can be multiple 640x480 modes, each of them is tried). If that
 128fails, the default mode is tried. If that fails, it walks over all modes.
 129
 130To specify a video mode at bootup, use the following boot options:
 131    video=<driver>:<xres>x<yres>[-<bpp>][@refresh]
 132
 133where <driver> is a name from the table below.  Valid default modes can be
 134found in linux/drivers/video/modedb.c.  Check your driver's documentation.
 135There 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
 150BTW, 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.
 152
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.