2modedb default video mode support
   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
   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)
  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.
  21Valid mode specifiers (mode_option argument)::
  23    <xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m][eDd]
  24    <name>[-<bpp>][@<refresh>]
  26with <xres>, <yres>, <bpp> and <refresh> decimal numbers and <name> a string.
  27Things between square brackets are optional.
  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).
  37       Sample usage: 1024x768M@60m - CVT timing with margins
  39DRM drivers also add options to enable or disable outputs:
  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.
  47You can additionally specify which output the options matches to.
  48To force the VGA output to be enabled and drive a specific mode say::
  50    video=VGA-1:1280x1024@60me
  52Specifying the option multiple times for different ports is possible, e.g.::
  54    video=LVDS-1:d video=HDMI-1:D
  56Options can also be passed after the mode, using commas as separator.
  58       Sample usage: 720x480,rotate=180 - 720x480 mode, rotated by 180 degrees
  60Valid options are::
  62  - margin_top, margin_bottom, margin_left, margin_right (integer):
  63    Number of pixels in the margins, typically to deal with overscan on TVs
  64  - reflect_x (boolean): Perform an axial symmetry on the X axis
  65  - reflect_y (boolean): Perform an axial symmetry on the Y axis
  66  - rotate (integer): Rotate the initial framebuffer by x
  67    degrees. Valid values are 0, 90, 180 and 270.
  68  - panel_orientation, one of "normal", "upside_down", "left_side_up", or
  69    "right_side_up". For KMS drivers only, this sets the "panel orientation"
  70    property on the kms connector as hint for kms users.
  75What is the VESA(TM) Coordinated Video Timings (CVT)?
  78From the VESA(TM) Website:
  80     "The purpose of CVT is to provide a method for generating a consistent
  81      and coordinated set of standard formats, display refresh rates, and
  82      timing specifications for computer display products, both those
  83      employing CRTs, and those using other display technologies. The
  84      intention of CVT is to give both source and display manufacturers a
  85      common set of tools to enable new timings to be developed in a
  86      consistent manner that ensures greater compatibility."
  88This is the third standard approved by VESA(TM) concerning video timings.  The
  89first was the Discrete Video Timings (DVT) which is  a collection of
  90pre-defined modes approved by VESA(TM).  The second is the Generalized Timing
  91Formula (GTF) which is an algorithm to calculate the timings, given the
  92pixelclock, the horizontal sync frequency, or the vertical refresh rate.
  94The GTF is limited by the fact that it is designed mainly for CRT displays.
  95It artificially increases the pixelclock because of its high blanking
  96requirement. This is inappropriate for digital display interface with its high
  97data rate which requires that it conserves the pixelclock as much as possible.
  98Also, GTF does not take into account the aspect ratio of the display.
 100The CVT addresses these limitations.  If used with CRT's, the formula used
 101is a derivation of GTF with a few modifications.  If used with digital
 102displays, the "reduced blanking" calculation can be used.
 104From the framebuffer subsystem perspective, new formats need not be added
 105to the global mode database whenever a new mode is released by display
 106manufacturers. Specifying for CVT will work for most, if not all, relatively
 107new CRT displays and probably with most flatpanels, if 'reduced blanking'
 108calculation is specified.  (The CVT compatibility of the display can be
 109determined from its EDID. The version 1.3 of the EDID has extra 128-byte
 110blocks where additional timing information is placed.  As of this time, there
 111is no support yet in the layer to parse this additional blocks.)
 113CVT also introduced a new naming convention (should be seen from dmesg output)::
 115    <pix>M<a>[-R]
 117    where: pix = total amount of pixels in MB (xres x yres)
 118           M   = always present
 119           a   = aspect ratio (3 - 4:3; 4 - 5:4; 9 - 15:9, 16:9; A - 16:10)
 120          -R   = reduced blanking
 122          example:  .48M3-R - 800x600 with reduced blanking
 124Note: VESA(TM) has restrictions on what is a standard CVT timing:
 126      - aspect ratio can only be one of the above values
 127      - acceptable refresh rates are 50, 60, 70 or 85 Hz only
 128      - if reduced blanking, the refresh rate must be at 60Hz
 130If one of the above are not satisfied, the kernel will print a warning but the
 131timings will still be calculated.
 135To find a suitable video mode, you just call::
 137  int __init fb_find_mode(struct fb_var_screeninfo *var,
 138                          struct fb_info *info, const char *mode_option,
 139                          const struct fb_videomode *db, unsigned int dbsize,
 140                          const struct fb_videomode *default_mode,
 141                          unsigned int default_bpp)
 143with db/dbsize your non-standard video mode database, or NULL to use the
 144standard video mode database.
 146fb_find_mode() first tries the specified video mode (or any mode that matches,
 147e.g. there can be multiple 640x480 modes, each of them is tried). If that
 148fails, the default mode is tried. If that fails, it walks over all modes.
 150To specify a video mode at bootup, use the following boot options::
 152    video=<driver>:<xres>x<yres>[-<bpp>][@refresh]
 154where <driver> is a name from the table below.  Valid default modes can be
 155found in drivers/video/fbdev/core/modedb.c.  Check your driver's documentation.
 156There may be more modes::
 158    Drivers that support modedb boot options
 159    Boot Name     Cards Supported
 161    amifb       - Amiga chipset frame buffer
 162    aty128fb    - ATI Rage128 / Pro frame buffer
 163    atyfb       - ATI Mach64 frame buffer
 164    pm2fb       - Permedia 2/2V frame buffer
 165    pm3fb       - Permedia 3 frame buffer
 166    sstfb       - Voodoo 1/2 (SST1) chipset frame buffer
 167    tdfxfb      - 3D Fx frame buffer
 168    tridentfb   - Trident (Cyber)blade chipset frame buffer
 169    vt8623fb    - VIA 8623 frame buffer
 171BTW, only a few fb drivers use this at the moment. Others are to follow
 172(feel free to send patches). The DRM drivers also support this.