uvesafb: documentation

uvesafb - A Generic Driver for VBE2+ compliant video cards

==========================================================

1. Requirements

---------------

uvesafb should work with any video card that has a Video BIOS compliant

with the VBE 2.0 standard.

Unlike other drivers, uvesafb makes use of a userspace helper called

v86d.  v86d is used to run the x86 Video BIOS code in a simulated and

controlled environment.  This allows uvesafb to function on arches other

than x86.  Check the v86d documentation for a list of currently supported

arches.

v86d source code can be downloaded from the following website:

  http://dev.gentoo.org/~spock/projects/uvesafb

Please refer to the v86d documentation for detailed configuration and

installation instructions.

Note that the v86d userspace helper has to be available at all times in

order for uvesafb to work properly.  If you want to use uvesafb during

early boot, you will have to include v86d into an initramfs image, and

either compile it into the kernel or use it as an initrd.

2. Caveats and limitations

--------------------------

uvesafb is a _generic_ driver which supports a wide variety of video

cards, but which is ultimately limited by the Video BIOS interface.

The most important limitations are:

- Lack of any type of acceleration.

- A strict and limited set of supported video modes.  Often the native

  or most optimal resolution/refresh rate for your setup will not work

  with uvesafb, simply because the Video BIOS doesn't support the

  video mode you want to use.  This can be especially painful with

  widescreen panels, where native video modes don't have the 4:3 aspect

  ratio, which is what most BIOS-es are limited to.

- Adjusting the refresh rate is only possible with a VBE 3.0 compliant

  Video BIOS.  Note that many nVidia Video BIOS-es claim to be VBE 3.0

  compliant, while they simply ignore any refresh rate settings.

3. Configuration

----------------

uvesafb can be compiled either as a module, or directly into the kernel.

In both cases it supports the same set of configuration options, which

are either given on the kernel command line or as module parameters, e.g.:

 video=uvesafb:1024x768-32,mtrr:3,ywrap (compiled into the kernel)

 # modprobe uvesafb mode=1024x768-32 mtrr=3 scroll=ywrap  (module)

Accepted options:

ypan    Enable display panning using the VESA protected mode

        interface.  The visible screen is just a window of the

        video memory, console scrolling is done by changing the

        start of the window.  Available on x86 only.

ywrap   Same as ypan, but assumes your gfx board can wrap-around

        the video memory (i.e. starts reading from top if it

        reaches the end of video memory).  Faster than ypan.

        Available on x86 only.

redraw  Scroll by redrawing the affected part of the screen, this

        is the safe (and slow) default.

(If you're using uvesafb as a module, the above three options are

 used a parameter of the scroll option, e.g. scroll=ypan.)

vgapal  Use the standard VGA registers for palette changes.

pmipal  Use the protected mode interface for palette changes.

        This is the default if the protected mode interface is

        available.  Available on x86 only.

mtrr:n  Setup memory type range registers for the framebuffer

        where n:

              0 - disabled (equivalent to nomtrr) (default)

              1 - uncachable

              2 - write-back

              3 - write-combining

              4 - write-through

        If you see the following in dmesg, choose the type that matches

        the old one.  In this example, use "mtrr:2".

...

mtrr: type mismatch for e0000000,8000000 old: write-back new: write-combining

...

nomtrr  Do not use memory type range registers.

vremap:n

        Remap 'n' MiB of video RAM.  If 0 or not specified, remap memory

        according to video mode.

vtotal:n

        If the video BIOS of your card incorrectly determines the total

        amount of video RAM, use this option to override the BIOS (in MiB).

<mode>  The mode you want to set, in the standard modedb format.  Refer to

        modedb.txt for a detailed description.  When uvesafb is compiled as

        a module, the mode string should be provided as a value of the

        'mode' option.

vbemode:x

        Force the use of VBE mode x.  The mode will only be set if it's

        found in the VBE-provided list of supported modes.

        NOTE: The mode number 'x' should be specified in VESA mode number

        notation, not the Linux kernel one (eg. 257 instead of 769).

        HINT: If you use this option because normal <mode> parameter does

        not work for you and you use a X server, you'll probably want to

        set the 'nocrtc' option to ensure that the video mode is properly

        restored after console <-> X switches.

nocrtc  Do not use CRTC timings while setting the video mode.  This option

        has any effect only if the Video BIOS is VBE 3.0 compliant.  Use it

        if you have problems with modes set the standard way.  Note that

        using this option implies that any refresh rate adjustments will

        be ignored and the refresh rate will stay at your BIOS default (60 Hz).

noedid  Do not try to fetch and use EDID-provided modes.

noblank Disable hardware blanking.

v86d:path

        Set path to the v86d executable. This option is only available as

        a module parameter, and not as a part of the video= string.  If you

        need to use it and have uvesafb built into the kernel, use

        uvesafb.v86d="path".

Additionally, the following parameters may be provided.  They all override the

EDID-provided values and BIOS defaults.  Refer to your monitor's specs to get

the correct values for maxhf, maxvf and maxclk for your hardware.

maxhf:n     Maximum horizontal frequency (in kHz).

maxvf:n     Maximum vertical frequency (in Hz).

maxclk:n    Maximum pixel clock (in MHz).

4. The sysfs interface

----------------------

uvesafb provides several sysfs nodes for configurable parameters and

additional information.

Driver attributes:

/sys/bus/platform/drivers/uvesafb

  - v86d (default: /sbin/v86d)

    Path to the v86d executable. v86d is started by uvesafb

    if an instance of the daemon isn't already running.

Device attributes:

/sys/bus/platform/drivers/uvesafb/uvesafb.0

  - nocrtc

    Use the default refresh rate (60 Hz) if set to 1.

  - oem_product_name

  - oem_product_rev

  - oem_string

  - oem_vendor

    Information about the card and its maker.

  - vbe_modes

    A list of video modes supported by the Video BIOS along with their

    VBE mode numbers in hex.

  - vbe_version

    A BCD value indicating the implemented VBE standard.

5. Miscellaneous

----------------

Uvesafb will set a video mode with the default refresh rate and timings

from the Video BIOS if you set pixclock to 0 in fb_var_screeninfo.

--

 Michal Januszewski <spock@gentoo.org>

 Last updated: 2007-06-16

 Documentation of the uvesafb options is loosely based on vesafb.txt.