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modedb.c was moved twice since the modedb documentation was written. Update the path to the current one. Signed-off-by: Jiri Slaby <jslaby@suse.cz> Link: https://lore.kernel.org/r/20200723103454.26167-1-jslaby@suse.cz Signed-off-by: Jonathan Corbet <corbet@lwn.net>
173 lines
7.5 KiB
ReStructuredText
173 lines
7.5 KiB
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=================================
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modedb default video mode support
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=================================
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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][eDd]
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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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DRM drivers also add options to enable or disable outputs:
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'e' will force the display to be enabled, i.e. it will override the detection
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if a display is connected. 'D' will force the display to be enabled and use
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digital output. This is useful for outputs that have both analog and digital
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signals (e.g. HDMI and DVI-I). For other outputs it behaves like 'e'. If 'd'
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is specified the output is disabled.
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You can additionally specify which output the options matches to.
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To force the VGA output to be enabled and drive a specific mode say::
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video=VGA-1:1280x1024@60me
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Specifying the option multiple times for different ports is possible, e.g.::
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video=LVDS-1:d video=HDMI-1:D
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Options can also be passed after the mode, using commas as separator.
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Sample usage: 720x480,rotate=180 - 720x480 mode, rotated by 180 degrees
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Valid options are::
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- margin_top, margin_bottom, margin_left, margin_right (integer):
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Number of pixels in the margins, typically to deal with overscan on TVs
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- reflect_x (boolean): Perform an axial symmetry on the X axis
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- reflect_y (boolean): Perform an axial symmetry on the Y axis
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- rotate (integer): Rotate the initial framebuffer by x
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degrees. Valid values are 0, 90, 180 and 270.
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- panel_orientation, one of "normal", "upside_down", "left_side_up", or
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"right_side_up". For KMS drivers only, this sets the "panel orientation"
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property on the kms connector as hint for kms users.
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-----------------------------------------------------------------------------
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What is the VESA(TM) Coordinated Video Timings (CVT)?
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=====================================================
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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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-----------------------------------------------------------------------------
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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 drivers/video/fbdev/core/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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pm2fb - Permedia 2/2V frame buffer
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pm3fb - Permedia 3 frame buffer
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sstfb - Voodoo 1/2 (SST1) chipset 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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vt8623fb - VIA 8623 frame buffer
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BTW, only a few fb drivers use this at the moment. Others are to follow
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(feel free to send patches). The DRM drivers also support this.
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