linux/Documentation/fb/modedb.rst
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   1=================================
   2modedb default video mode support
   3=================================
   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
  29Valid names are::
  30
  31  - NSTC: 480i output, with the CCIR System-M TV mode and NTSC color encoding
  32  - NTSC-J: 480i output, with the CCIR System-M TV mode, the NTSC color
  33    encoding, and a black level equal to the blanking level.
  34  - PAL: 576i output, with the CCIR System-B TV mode and PAL color encoding
  35  - PAL-M: 480i output, with the CCIR System-M TV mode and PAL color encoding
  36
  37If 'M' is specified in the mode_option argument (after <yres> and before
  38<bpp> and <refresh>, if specified) the timings will be calculated using
  39VESA(TM) Coordinated Video Timings instead of looking up the mode from a table.
  40If 'R' is specified, do a 'reduced blanking' calculation for digital displays.
  41If 'i' is specified, calculate for an interlaced mode.  And if 'm' is
  42specified, add margins to the calculation (1.8% of xres rounded down to 8
  43pixels and 1.8% of yres).
  44
  45       Sample usage: 1024x768M@60m - CVT timing with margins
  46
  47DRM drivers also add options to enable or disable outputs:
  48
  49'e' will force the display to be enabled, i.e. it will override the detection
  50if a display is connected. 'D' will force the display to be enabled and use
  51digital output. This is useful for outputs that have both analog and digital
  52signals (e.g. HDMI and DVI-I). For other outputs it behaves like 'e'. If 'd'
  53is specified the output is disabled.
  54
  55You can additionally specify which output the options matches to.
  56To force the VGA output to be enabled and drive a specific mode say::
  57
  58    video=VGA-1:1280x1024@60me
  59
  60Specifying the option multiple times for different ports is possible, e.g.::
  61
  62    video=LVDS-1:d video=HDMI-1:D
  63
  64Options can also be passed after the mode, using commas as separator.
  65
  66       Sample usage: 720x480,rotate=180 - 720x480 mode, rotated by 180 degrees
  67
  68Valid options are::
  69
  70  - margin_top, margin_bottom, margin_left, margin_right (integer):
  71    Number of pixels in the margins, typically to deal with overscan on TVs
  72  - reflect_x (boolean): Perform an axial symmetry on the X axis
  73  - reflect_y (boolean): Perform an axial symmetry on the Y axis
  74  - rotate (integer): Rotate the initial framebuffer by x
  75    degrees. Valid values are 0, 90, 180 and 270.
  76  - tv_mode: Analog TV mode. One of "NTSC", "NTSC-443", "NTSC-J", "PAL",
  77    "PAL-M", "PAL-N", or "SECAM".
  78  - panel_orientation, one of "normal", "upside_down", "left_side_up", or
  79    "right_side_up". For KMS drivers only, this sets the "panel orientation"
  80    property on the kms connector as hint for kms users.
  81
  82
  83-----------------------------------------------------------------------------
  84
  85What is the VESA(TM) Coordinated Video Timings (CVT)?
  86=====================================================
  87
  88From the VESA(TM) Website:
  89
  90     "The purpose of CVT is to provide a method for generating a consistent
  91      and coordinated set of standard formats, display refresh rates, and
  92      timing specifications for computer display products, both those
  93      employing CRTs, and those using other display technologies. The
  94      intention of CVT is to give both source and display manufacturers a
  95      common set of tools to enable new timings to be developed in a
  96      consistent manner that ensures greater compatibility."
  97
  98This is the third standard approved by VESA(TM) concerning video timings.  The
  99first was the Discrete Video Timings (DVT) which is  a collection of
 100pre-defined modes approved by VESA(TM).  The second is the Generalized Timing
 101Formula (GTF) which is an algorithm to calculate the timings, given the
 102pixelclock, the horizontal sync frequency, or the vertical refresh rate.
 103
 104The GTF is limited by the fact that it is designed mainly for CRT displays.
 105It artificially increases the pixelclock because of its high blanking
 106requirement. This is inappropriate for digital display interface with its high
 107data rate which requires that it conserves the pixelclock as much as possible.
 108Also, GTF does not take into account the aspect ratio of the display.
 109
 110The CVT addresses these limitations.  If used with CRT's, the formula used
 111is a derivation of GTF with a few modifications.  If used with digital
 112displays, the "reduced blanking" calculation can be used.
 113
 114From the framebuffer subsystem perspective, new formats need not be added
 115to the global mode database whenever a new mode is released by display
 116manufacturers. Specifying for CVT will work for most, if not all, relatively
 117new CRT displays and probably with most flatpanels, if 'reduced blanking'
 118calculation is specified.  (The CVT compatibility of the display can be
 119determined from its EDID. The version 1.3 of the EDID has extra 128-byte
 120blocks where additional timing information is placed.  As of this time, there
 121is no support yet in the layer to parse this additional blocks.)
 122
 123CVT also introduced a new naming convention (should be seen from dmesg output)::
 124
 125    <pix>M<a>[-R]
 126
 127    where: pix = total amount of pixels in MB (xres x yres)
 128           M   = always present
 129           a   = aspect ratio (3 - 4:3; 4 - 5:4; 9 - 15:9, 16:9; A - 16:10)
 130          -R   = reduced blanking
 131
 132          example:  .48M3-R - 800x600 with reduced blanking
 133
 134Note: VESA(TM) has restrictions on what is a standard CVT timing:
 135
 136      - aspect ratio can only be one of the above values
 137      - acceptable refresh rates are 50, 60, 70 or 85 Hz only
 138      - if reduced blanking, the refresh rate must be at 60Hz
 139
 140If one of the above are not satisfied, the kernel will print a warning but the
 141timings will still be calculated.
 142
 143-----------------------------------------------------------------------------
 144
 145To find a suitable video mode, you just call::
 146
 147  int __init fb_find_mode(struct fb_var_screeninfo *var,
 148                          struct fb_info *info, const char *mode_option,
 149                          const struct fb_videomode *db, unsigned int dbsize,
 150                          const struct fb_videomode *default_mode,
 151                          unsigned int default_bpp)
 152
 153with db/dbsize your non-standard video mode database, or NULL to use the
 154standard video mode database.
 155
 156fb_find_mode() first tries the specified video mode (or any mode that matches,
 157e.g. there can be multiple 640x480 modes, each of them is tried). If that
 158fails, the default mode is tried. If that fails, it walks over all modes.
 159
 160To specify a video mode at bootup, use the following boot options::
 161
 162    video=<driver>:<xres>x<yres>[-<bpp>][@refresh]
 163
 164where <driver> is a name from the table below.  Valid default modes can be
 165found in drivers/video/fbdev/core/modedb.c.  Check your driver's documentation.
 166There may be more modes::
 167
 168    Drivers that support modedb boot options
 169    Boot Name     Cards Supported
 170
 171    amifb       - Amiga chipset frame buffer
 172    aty128fb    - ATI Rage128 / Pro frame buffer
 173    atyfb       - ATI Mach64 frame buffer
 174    pm2fb       - Permedia 2/2V frame buffer
 175    pm3fb       - Permedia 3 frame buffer
 176    sstfb       - Voodoo 1/2 (SST1) chipset frame buffer
 177    tdfxfb      - 3D Fx frame buffer
 178    tridentfb   - Trident (Cyber)blade chipset frame buffer
 179    vt8623fb    - VIA 8623 frame buffer
 180
 181BTW, only a few fb drivers use this at the moment. Others are to follow
 182(feel free to send patches). The DRM drivers also support this.
 183