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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

3				modedb default video mode support
6	Currently all frame buffer device drivers have their own video mode databases,
7	which 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)
17	When a frame buffer device receives a video= option it doesn't know, it should
18	consider that to be a video mode option. If no frame buffer device is specified
19	in a video= option, fbmem considers that to be a global video mode option.
21	Valid mode specifiers (mode_option argument):
23	    <xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m][eDd]
24	    <name>[-<bpp>][@<refresh>]
26	with <xres>, <yres>, <bpp> and <refresh> decimal numbers and <name> a string.
27	Things between square brackets are optional.
29	If 'M' is specified in the mode_option argument (after <yres> and before
30	<bpp> and <refresh>, if specified) the timings will be calculated using
31	VESA(TM) Coordinated Video Timings instead of looking up the mode from a table.
32	If 'R' is specified, do a 'reduced blanking' calculation for digital displays.
33	If 'i' is specified, calculate for an interlaced mode.  And if 'm' is
34	specified, add margins to the calculation (1.8% of xres rounded down to 8
35	pixels and 1.8% of yres).
37	       Sample usage: 1024x768M@60m - CVT timing with margins
39	DRM 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
42	if a display is connected. 'D' will force the display to be enabled and use
43	digital output. This is useful for outputs that have both analog and digital
44	signals (e.g. HDMI and DVI-I). For other outputs it behaves like 'e'. If 'd'
45	is specified the output is disabled.
47	You can additionally specify which output the options matches to.
48	To force the VGA output to be enabled and drive a specific mode say:
49	    video=VGA-1:1280x1024@60me
51	Specifying the option multiple times for different ports is possible, e.g.:
52	    video=LVDS-1:d video=HDMI-1:D
54	***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo *****
56	What is the VESA(TM) Coordinated Video Timings (CVT)?
58	From the VESA(TM) Website:
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."
68	This is the third standard approved by VESA(TM) concerning video timings.  The
69	first was the Discrete Video Timings (DVT) which is  a collection of
70	pre-defined modes approved by VESA(TM).  The second is the Generalized Timing
71	Formula (GTF) which is an algorithm to calculate the timings, given the
72	pixelclock, the horizontal sync frequency, or the vertical refresh rate.
74	The GTF is limited by the fact that it is designed mainly for CRT displays.
75	It artificially increases the pixelclock because of its high blanking
76	requirement. This is inappropriate for digital display interface with its high
77	data rate which requires that it conserves the pixelclock as much as possible.
78	Also, GTF does not take into account the aspect ratio of the display.
80	The CVT addresses these limitations.  If used with CRT's, the formula used
81	is a derivation of GTF with a few modifications.  If used with digital
82	displays, the "reduced blanking" calculation can be used.
84	From the framebuffer subsystem perspective, new formats need not be added
85	to the global mode database whenever a new mode is released by display
86	manufacturers. Specifying for CVT will work for most, if not all, relatively
87	new CRT displays and probably with most flatpanels, if 'reduced blanking'
88	calculation is specified.  (The CVT compatibility of the display can be
89	determined from its EDID. The version 1.3 of the EDID has extra 128-byte
90	blocks where additional timing information is placed.  As of this time, there
91	is no support yet in the layer to parse this additional blocks.)
93	CVT also introduced a new naming convention (should be seen from dmesg output):
95	    <pix>M<a>[-R]
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
102		  example:  .48M3-R - 800x600 with reduced blanking
104	Note: VESA(TM) has restrictions on what is a standard CVT timing:
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
110	If one of the above are not satisfied, the kernel will print a warning but the
111	timings will still be calculated.
113	***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo *****
115	To find a suitable video mode, you just call
117	int __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)
123	with db/dbsize your non-standard video mode database, or NULL to use the
124	standard video mode database.
126	fb_find_mode() first tries the specified video mode (or any mode that matches,
127	e.g. there can be multiple 640x480 modes, each of them is tried). If that
128	fails, the default mode is tried. If that fails, it walks over all modes.
130	To specify a video mode at bootup, use the following boot options:
131	    video=<driver>:<xres>x<yres>[-<bpp>][@refresh]
133	where <driver> is a name from the table below.  Valid default modes can be
134	found in linux/drivers/video/modedb.c.  Check your driver's documentation.
135	There may be more modes.
137	    Drivers that support modedb boot options
138	    Boot Name	  Cards Supported
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
150	BTW, 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.
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