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The Coordinated Video Timings (CVT) is the latest standard approved by VESA
concerning video timings generation. It addresses the limitation of GTF which
is designed mainly for CRT displays. CRT's have a high blanking requirement
(as much as 25% of the horizontal frame length) which artificially increases
the pixelclock. Digital displays, on the other hand, needs to conserve the
pixelclock as much as possible. The GTF also does not take into account the
different aspect ratios in its calculation.
The new function added is fb_find_mode_cvt(). It is called by fb_find_mode()
if it recognizes a mode option string formatted for CVT. The format is:
<xres>x<yres>[M][R][-<bpp>][<at-sign><refresh>][i][m]
The 'M' tells the function to calculate using CVT. On it's own, it will
compute a timing for CRT displays at 60Hz. If the 'R' is specified, 'reduced
blanking' computation will be used, best for flatpanels. The 'i' and the 'm'
is for 'interlaced mode' and 'with margins' respectively.
To determine if CVT was used, check for dmesg for something like this:
CVT Mode - <pix>M<n>[-R], ie: .480M3-R (800x600 reduced blanking)
where: pix - product of xres and yres, in MB
M - is a CVT mode
n - the aspect ratio (3 - 4:3; 4 - 5:4; 9 - 16:9, 15:9; A - 16:10)
-R - reduced blanking
Signed-off-by: Antonino Daplas <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
133 lines
5.8 KiB
Plaintext
133 lines
5.8 KiB
Plaintext
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modedb default video mode support
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Currently all frame buffer device drivers have their own video mode databases,
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which is a mess and a waste of resources. The main idea of modedb is to have
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- one routine to probe for video modes, which can be used by all frame buffer
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devices
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- one generic video mode database with a fair amount of standard videomodes
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(taken from XFree86)
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- the possibility to supply your own mode database for graphics hardware that
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needs non-standard modes, like amifb and Mac frame buffer drivers (which
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use macmodes.c)
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When a frame buffer device receives a video= option it doesn't know, it should
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consider that to be a video mode option. If no frame buffer device is specified
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in a video= option, fbmem considers that to be a global video mode option.
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Valid mode specifiers (mode_option argument):
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<xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m]
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<name>[-<bpp>][@<refresh>]
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with <xres>, <yres>, <bpp> and <refresh> decimal numbers and <name> a string.
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Things between square brackets are optional.
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If 'M' is specified in the mode_option argument (after <yres> and before
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<bpp> and <refresh>, if specified) the timings will be calculated using
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VESA(TM) Coordinated Video Timings instead of looking up the mode from a table.
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If 'R' is specified, do a 'reduced blanking' calculation for digital displays.
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If 'i' is specified, calculate for an interlaced mode. And if 'm' is
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specified, add margins to the calculation (1.8% of xres rounded down to 8
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pixels and 1.8% of yres).
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Sample usage: 1024x768M@60m - CVT timing with margins
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***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo *****
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What is the VESA(TM) Coordinated Video Timings (CVT)?
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From the VESA(TM) Website:
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"The purpose of CVT is to provide a method for generating a consistent
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and coordinated set of standard formats, display refresh rates, and
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timing specifications for computer display products, both those
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employing CRTs, and those using other display technologies. The
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intention of CVT is to give both source and display manufacturers a
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common set of tools to enable new timings to be developed in a
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consistent manner that ensures greater compatibility."
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This is the third standard approved by VESA(TM) concerning video timings. The
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first was the Discrete Video Timings (DVT) which is a collection of
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pre-defined modes approved by VESA(TM). The second is the Generalized Timing
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Formula (GTF) which is an algorithm to calculate the timings, given the
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pixelclock, the horizontal sync frequency, or the vertical refresh rate.
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The GTF is limited by the fact that it is designed mainly for CRT displays.
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It artificially increases the pixelclock because of its high blanking
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requirement. This is inappropriate for digital display interface with its high
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data rate which requires that it conserves the pixelclock as much as possible.
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Also, GTF does not take into account the aspect ratio of the display.
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The CVT addresses these limitations. If used with CRT's, the formula used
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is a derivation of GTF with a few modifications. If used with digital
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displays, the "reduced blanking" calculation can be used.
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From the framebuffer subsystem perspective, new formats need not be added
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to the global mode database whenever a new mode is released by display
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manufacturers. Specifying for CVT will work for most, if not all, relatively
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new CRT displays and probably with most flatpanels, if 'reduced blanking'
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calculation is specified. (The CVT compatibility of the display can be
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determined from its EDID. The version 1.3 of the EDID has extra 128-byte
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blocks where additional timing information is placed. As of this time, there
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is no support yet in the layer to parse this additional blocks.)
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CVT also introduced a new naming convention (should be seen from dmesg output):
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<pix>M<a>[-R]
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where: pix = total amount of pixels in MB (xres x yres)
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M = always present
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a = aspect ratio (3 - 4:3; 4 - 5:4; 9 - 15:9, 16:9; A - 16:10)
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-R = reduced blanking
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example: .48M3-R - 800x600 with reduced blanking
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Note: VESA(TM) has restrictions on what is a standard CVT timing:
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- aspect ratio can only be one of the above values
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- acceptable refresh rates are 50, 60, 70 or 85 Hz only
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- if reduced blanking, the refresh rate must be at 60Hz
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If one of the above are not satisfied, the kernel will print a warning but the
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timings will still be calculated.
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***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo *****
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To find a suitable video mode, you just call
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int __init fb_find_mode(struct fb_var_screeninfo *var,
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struct fb_info *info, const char *mode_option,
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const struct fb_videomode *db, unsigned int dbsize,
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const struct fb_videomode *default_mode,
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unsigned int default_bpp)
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with db/dbsize your non-standard video mode database, or NULL to use the
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standard video mode database.
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fb_find_mode() first tries the specified video mode (or any mode that matches,
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e.g. there can be multiple 640x480 modes, each of them is tried). If that
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fails, the default mode is tried. If that fails, it walks over all modes.
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To specify a video mode at bootup, use the following boot options:
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video=<driver>:<xres>x<yres>[-<bpp>][@refresh]
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where <driver> is a name from the table below. Valid default modes can be
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found in linux/drivers/video/modedb.c. Check your driver's documentation.
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There may be more modes.
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Drivers that support modedb boot options
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Boot Name Cards Supported
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amifb - Amiga chipset frame buffer
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aty128fb - ATI Rage128 / Pro frame buffer
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atyfb - ATI Mach64 frame buffer
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tdfxfb - 3D Fx frame buffer
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tridentfb - Trident (Cyber)blade chipset frame buffer
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BTW, only a few drivers use this at the moment. Others are to follow
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(feel free to send patches).
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