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The Linux BootPrompt-HowTo: Boot Arguments for Video Frame Buffer Drivers Next Previous Contents

5. Boot Arguments for Video Frame Buffer Drivers

The `video=' argument (not avail. in v2.0 kernels) is used when the frame buffer device abstraction layer is built into the kernel. If that sounds complicated, well it isn't really too bad. It basically means that instead of having a different video program (the X11R6 server) for each brand of video card (e.g. XF86_S3, XF86_SVGA, ...), the kernel would have a built in driver available for each video card and export a single interface for the video program so that only one X11R6 server (XF86_FBDev) would be required. This is similar to how networking is now - the kernel has drivers available for each brand of network card and exports a single network interface so that just one version of a network program (like Netscape) will work for all systems, regardless of the underlying brand of network card.

The typical format of this argument is video=name:option1,option2,... where name is the name of a generic option or of a frame buffer driver. The video= option is passed from linux/init/main.c into linux/drivers/video/fbmem.c for further processing. Here it is checked for some generic options before trying to match to a known driver name. Once a driver name match is made, the comma separated option list is then passed into that particular driver for final processing. The list of valid driver names can be found by reading down the fb_drivers array in the file fbmem.c mentioned above.

Information on the options that each driver supports will eventually be found in linux/Documentation/fb/ but currently (v2.2) only a few are described there. Unfortunately the number of video drivers and the number of options for each one is content for another document itself and hence too much to list here.

If there is no Documentation file for your card, you will have to get the option information directly from the driver. Go to linux/drivers/video/ and look in the appropriate ???fb.c file (the ??? will be based on the card name). In there, search for a function with _setup in its name and you should see what options the driver tries to match, such as font or mode or...

5.1 The `video=map:...' Argument

This option is used to set/override the console to frame buffer device mapping. A comma separated list of numbers sets the mapping, with the value of option N taken to be the frame buffer device number for console N.

5.2 The `video=scrollback:...' Argument

A number after the colon will set the size of memory allocated for the scrollback buffer. (Use Shift and Page Up or Page Down keys to scroll.) A suffix of `k' or `K' after the number will indicate that the number is to be interpreted as kilobytes instead of bytes.

5.3 The `video=vc:...' Argument

A number, or a range of numbers (e.g. video=vc:2-5) will specify the first, or the first and last frame buffer virtual console(s). The use of this option also has the effect of setting the frame buffer console to not be the default console.

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