modedb.rst 7.5 KB

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  1. =================================
  2. modedb default video mode support
  3. =================================
  4. Currently all frame buffer device drivers have their own video mode databases,
  5. which is a mess and a waste of resources. The main idea of modedb is to have
  6. - one routine to probe for video modes, which can be used by all frame buffer
  7. devices
  8. - one generic video mode database with a fair amount of standard videomodes
  9. (taken from XFree86)
  10. - the possibility to supply your own mode database for graphics hardware that
  11. needs non-standard modes, like amifb and Mac frame buffer drivers (which
  12. use macmodes.c)
  13. When a frame buffer device receives a video= option it doesn't know, it should
  14. consider that to be a video mode option. If no frame buffer device is specified
  15. in a video= option, fbmem considers that to be a global video mode option.
  16. Valid mode specifiers (mode_option argument)::
  17. <xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m][eDd]
  18. <name>[-<bpp>][@<refresh>]
  19. with <xres>, <yres>, <bpp> and <refresh> decimal numbers and <name> a string.
  20. Things between square brackets are optional.
  21. If 'M' is specified in the mode_option argument (after <yres> and before
  22. <bpp> and <refresh>, if specified) the timings will be calculated using
  23. VESA(TM) Coordinated Video Timings instead of looking up the mode from a table.
  24. If 'R' is specified, do a 'reduced blanking' calculation for digital displays.
  25. If 'i' is specified, calculate for an interlaced mode. And if 'm' is
  26. specified, add margins to the calculation (1.8% of xres rounded down to 8
  27. pixels and 1.8% of yres).
  28. Sample usage: 1024x768M@60m - CVT timing with margins
  29. DRM drivers also add options to enable or disable outputs:
  30. 'e' will force the display to be enabled, i.e. it will override the detection
  31. if a display is connected. 'D' will force the display to be enabled and use
  32. digital output. This is useful for outputs that have both analog and digital
  33. signals (e.g. HDMI and DVI-I). For other outputs it behaves like 'e'. If 'd'
  34. is specified the output is disabled.
  35. You can additionally specify which output the options matches to.
  36. To force the VGA output to be enabled and drive a specific mode say::
  37. video=VGA-1:1280x1024@60me
  38. Specifying the option multiple times for different ports is possible, e.g.::
  39. video=LVDS-1:d video=HDMI-1:D
  40. Options can also be passed after the mode, using commas as separator.
  41. Sample usage: 720x480,rotate=180 - 720x480 mode, rotated by 180 degrees
  42. Valid options are::
  43. - margin_top, margin_bottom, margin_left, margin_right (integer):
  44. Number of pixels in the margins, typically to deal with overscan on TVs
  45. - reflect_x (boolean): Perform an axial symmetry on the X axis
  46. - reflect_y (boolean): Perform an axial symmetry on the Y axis
  47. - rotate (integer): Rotate the initial framebuffer by x
  48. degrees. Valid values are 0, 90, 180 and 270.
  49. - panel_orientation, one of "normal", "upside_down", "left_side_up", or
  50. "right_side_up". For KMS drivers only, this sets the "panel orientation"
  51. property on the kms connector as hint for kms users.
  52. -----------------------------------------------------------------------------
  53. What is the VESA(TM) Coordinated Video Timings (CVT)?
  54. =====================================================
  55. From the VESA(TM) Website:
  56. "The purpose of CVT is to provide a method for generating a consistent
  57. and coordinated set of standard formats, display refresh rates, and
  58. timing specifications for computer display products, both those
  59. employing CRTs, and those using other display technologies. The
  60. intention of CVT is to give both source and display manufacturers a
  61. common set of tools to enable new timings to be developed in a
  62. consistent manner that ensures greater compatibility."
  63. This is the third standard approved by VESA(TM) concerning video timings. The
  64. first was the Discrete Video Timings (DVT) which is a collection of
  65. pre-defined modes approved by VESA(TM). The second is the Generalized Timing
  66. Formula (GTF) which is an algorithm to calculate the timings, given the
  67. pixelclock, the horizontal sync frequency, or the vertical refresh rate.
  68. The GTF is limited by the fact that it is designed mainly for CRT displays.
  69. It artificially increases the pixelclock because of its high blanking
  70. requirement. This is inappropriate for digital display interface with its high
  71. data rate which requires that it conserves the pixelclock as much as possible.
  72. Also, GTF does not take into account the aspect ratio of the display.
  73. The CVT addresses these limitations. If used with CRT's, the formula used
  74. is a derivation of GTF with a few modifications. If used with digital
  75. displays, the "reduced blanking" calculation can be used.
  76. From the framebuffer subsystem perspective, new formats need not be added
  77. to the global mode database whenever a new mode is released by display
  78. manufacturers. Specifying for CVT will work for most, if not all, relatively
  79. new CRT displays and probably with most flatpanels, if 'reduced blanking'
  80. calculation is specified. (The CVT compatibility of the display can be
  81. determined from its EDID. The version 1.3 of the EDID has extra 128-byte
  82. blocks where additional timing information is placed. As of this time, there
  83. is no support yet in the layer to parse this additional blocks.)
  84. CVT also introduced a new naming convention (should be seen from dmesg output)::
  85. <pix>M<a>[-R]
  86. where: pix = total amount of pixels in MB (xres x yres)
  87. M = always present
  88. a = aspect ratio (3 - 4:3; 4 - 5:4; 9 - 15:9, 16:9; A - 16:10)
  89. -R = reduced blanking
  90. example: .48M3-R - 800x600 with reduced blanking
  91. Note: VESA(TM) has restrictions on what is a standard CVT timing:
  92. - aspect ratio can only be one of the above values
  93. - acceptable refresh rates are 50, 60, 70 or 85 Hz only
  94. - if reduced blanking, the refresh rate must be at 60Hz
  95. If one of the above are not satisfied, the kernel will print a warning but the
  96. timings will still be calculated.
  97. -----------------------------------------------------------------------------
  98. To find a suitable video mode, you just call::
  99. int __init fb_find_mode(struct fb_var_screeninfo *var,
  100. struct fb_info *info, const char *mode_option,
  101. const struct fb_videomode *db, unsigned int dbsize,
  102. const struct fb_videomode *default_mode,
  103. unsigned int default_bpp)
  104. with db/dbsize your non-standard video mode database, or NULL to use the
  105. standard video mode database.
  106. fb_find_mode() first tries the specified video mode (or any mode that matches,
  107. e.g. there can be multiple 640x480 modes, each of them is tried). If that
  108. fails, the default mode is tried. If that fails, it walks over all modes.
  109. To specify a video mode at bootup, use the following boot options::
  110. video=<driver>:<xres>x<yres>[-<bpp>][@refresh]
  111. where <driver> is a name from the table below. Valid default modes can be
  112. found in drivers/video/fbdev/core/modedb.c. Check your driver's documentation.
  113. There may be more modes::
  114. Drivers that support modedb boot options
  115. Boot Name Cards Supported
  116. amifb - Amiga chipset frame buffer
  117. aty128fb - ATI Rage128 / Pro frame buffer
  118. atyfb - ATI Mach64 frame buffer
  119. pm2fb - Permedia 2/2V frame buffer
  120. pm3fb - Permedia 3 frame buffer
  121. sstfb - Voodoo 1/2 (SST1) chipset frame buffer
  122. tdfxfb - 3D Fx frame buffer
  123. tridentfb - Trident (Cyber)blade chipset frame buffer
  124. vt8623fb - VIA 8623 frame buffer
  125. BTW, only a few fb drivers use this at the moment. Others are to follow
  126. (feel free to send patches). The DRM drivers also support this.