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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+
+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.
+