forked from Minki/linux
uvesafb: documentation
Documentation for the uvesafb driver. Signed-off-by: Michal Januszewski <spock@gentoo.org> Signed-off-by: Antonino Daplas <adaplas@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Michal Januszewski
Linus Torvalds
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Documentation/fb/uvesafb.txt
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Documentation/fb/uvesafb.txt
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uvesafb - A Generic Driver for VBE2+ compliant video cards
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==========================================================
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1. Requirements
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---------------
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uvesafb should work with any video card that has a Video BIOS compliant
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with the VBE 2.0 standard.
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Unlike other drivers, uvesafb makes use of a userspace helper called
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v86d. v86d is used to run the x86 Video BIOS code in a simulated and
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controlled environment. This allows uvesafb to function on arches other
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than x86. Check the v86d documentation for a list of currently supported
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arches.
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v86d source code can be downloaded from the following website:
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http://dev.gentoo.org/~spock/projects/uvesafb
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Please refer to the v86d documentation for detailed configuration and
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installation instructions.
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Note that the v86d userspace helper has to be available at all times in
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order for uvesafb to work properly. If you want to use uvesafb during
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early boot, you will have to include v86d into an initramfs image, and
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either compile it into the kernel or use it as an initrd.
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2. Caveats and limitations
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--------------------------
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uvesafb is a _generic_ driver which supports a wide variety of video
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cards, but which is ultimately limited by the Video BIOS interface.
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The most important limitations are:
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- Lack of any type of acceleration.
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- A strict and limited set of supported video modes. Often the native
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or most optimal resolution/refresh rate for your setup will not work
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with uvesafb, simply because the Video BIOS doesn't support the
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video mode you want to use. This can be especially painful with
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widescreen panels, where native video modes don't have the 4:3 aspect
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ratio, which is what most BIOS-es are limited to.
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- Adjusting the refresh rate is only possible with a VBE 3.0 compliant
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Video BIOS. Note that many nVidia Video BIOS-es claim to be VBE 3.0
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compliant, while they simply ignore any refresh rate settings.
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3. Configuration
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----------------
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uvesafb can be compiled either as a module, or directly into the kernel.
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In both cases it supports the same set of configuration options, which
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are either given on the kernel command line or as module parameters, e.g.:
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video=uvesafb:1024x768-32,mtrr:3,ywrap (compiled into the kernel)
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# modprobe uvesafb mode=1024x768-32 mtrr=3 scroll=ywrap (module)
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Accepted options:
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ypan Enable display panning using the VESA protected mode
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interface. The visible screen is just a window of the
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video memory, console scrolling is done by changing the
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start of the window. Available on x86 only.
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ywrap Same as ypan, but assumes your gfx board can wrap-around
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the video memory (i.e. starts reading from top if it
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reaches the end of video memory). Faster than ypan.
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Available on x86 only.
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redraw Scroll by redrawing the affected part of the screen, this
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is the safe (and slow) default.
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(If you're using uvesafb as a module, the above three options are
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used a parameter of the scroll option, e.g. scroll=ypan.)
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vgapal Use the standard VGA registers for palette changes.
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pmipal Use the protected mode interface for palette changes.
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This is the default if the protected mode interface is
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available. Available on x86 only.
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mtrr:n Setup memory type range registers for the framebuffer
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where n:
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0 - disabled (equivalent to nomtrr) (default)
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1 - uncachable
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2 - write-back
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3 - write-combining
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4 - write-through
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If you see the following in dmesg, choose the type that matches
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the old one. In this example, use "mtrr:2".
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...
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mtrr: type mismatch for e0000000,8000000 old: write-back new: write-combining
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...
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nomtrr Do not use memory type range registers.
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vremap:n
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Remap 'n' MiB of video RAM. If 0 or not specified, remap memory
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according to video mode.
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vtotal:n
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If the video BIOS of your card incorrectly determines the total
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amount of video RAM, use this option to override the BIOS (in MiB).
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<mode> The mode you want to set, in the standard modedb format. Refer to
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modedb.txt for a detailed description. When uvesafb is compiled as
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a module, the mode string should be provided as a value of the
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'mode' option.
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vbemode:x
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Force the use of VBE mode x. The mode will only be set if it's
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found in the VBE-provided list of supported modes.
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NOTE: The mode number 'x' should be specified in VESA mode number
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notation, not the Linux kernel one (eg. 257 instead of 769).
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HINT: If you use this option because normal <mode> parameter does
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not work for you and you use a X server, you'll probably want to
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set the 'nocrtc' option to ensure that the video mode is properly
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restored after console <-> X switches.
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nocrtc Do not use CRTC timings while setting the video mode. This option
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has any effect only if the Video BIOS is VBE 3.0 compliant. Use it
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if you have problems with modes set the standard way. Note that
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using this option implies that any refresh rate adjustments will
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be ignored and the refresh rate will stay at your BIOS default (60 Hz).
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noedid Do not try to fetch and use EDID-provided modes.
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noblank Disable hardware blanking.
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v86d:path
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Set path to the v86d executable. This option is only available as
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a module parameter, and not as a part of the video= string. If you
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need to use it and have uvesafb built into the kernel, use
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uvesafb.v86d="path".
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Additionally, the following parameters may be provided. They all override the
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EDID-provided values and BIOS defaults. Refer to your monitor's specs to get
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the correct values for maxhf, maxvf and maxclk for your hardware.
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maxhf:n Maximum horizontal frequency (in kHz).
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maxvf:n Maximum vertical frequency (in Hz).
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maxclk:n Maximum pixel clock (in MHz).
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4. The sysfs interface
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----------------------
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uvesafb provides several sysfs nodes for configurable parameters and
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additional information.
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Driver attributes:
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/sys/bus/platform/drivers/uvesafb
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- v86d (default: /sbin/v86d)
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Path to the v86d executable. v86d is started by uvesafb
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if an instance of the daemon isn't already running.
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Device attributes:
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/sys/bus/platform/drivers/uvesafb/uvesafb.0
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- nocrtc
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Use the default refresh rate (60 Hz) if set to 1.
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- oem_product_name
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- oem_product_rev
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- oem_string
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- oem_vendor
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Information about the card and its maker.
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- vbe_modes
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A list of video modes supported by the Video BIOS along with their
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VBE mode numbers in hex.
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- vbe_version
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A BCD value indicating the implemented VBE standard.
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5. Miscellaneous
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----------------
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Uvesafb will set a video mode with the default refresh rate and timings
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from the Video BIOS if you set pixclock to 0 in fb_var_screeninfo.
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--
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Michal Januszewski <spock@gentoo.org>
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Last updated: 2007-06-16
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Documentation of the uvesafb options is loosely based on vesafb.txt.
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