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
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
50    video=VGA-1:1280x1024@60me
51
52Specifying the option multiple times for different ports is possible, e.g.::
53
54    video=LVDS-1:d video=HDMI-1:D
55
56Options can also be passed after the mode, using commas as separator.
57
58       Sample usage: 720x480,rotate=180 - 720x480 mode, rotated by 180 degrees
59
60Valid options are::
61
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.
71
72
73-----------------------------------------------------------------------------
74
75What is the VESA(TM) Coordinated Video Timings (CVT)?
76=====================================================
77
78From the VESA(TM) Website:
79
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."
87
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.
93
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.
99
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.
103
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.)
112
113CVT also introduced a new naming convention (should be seen from dmesg output)::
114
115    <pix>M<a>[-R]
116
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
121
122	  example:  .48M3-R - 800x600 with reduced blanking
123
124Note: VESA(TM) has restrictions on what is a standard CVT timing:
125
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
129
130If one of the above are not satisfied, the kernel will print a warning but the
131timings will still be calculated.
132
133-----------------------------------------------------------------------------
134
135To find a suitable video mode, you just call::
136
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)
142
143with db/dbsize your non-standard video mode database, or NULL to use the
144standard video mode database.
145
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.
149
150To specify a video mode at bootup, use the following boot options::
151
152    video=<driver>:<xres>x<yres>[-<bpp>][@refresh]
153
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::
157
158    Drivers that support modedb boot options
159    Boot Name	  Cards Supported
160
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
170
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.
173