modedb default video mode support


Currently all frame buffer device drivers have their own video mode databases,
which is a mess and a waste of resources. The main idea of modedb is to have

  - one routine to probe for video modes, which can be used by all frame buffer
    devices
  - one generic video mode database with a fair amount of standard videomodes
    (taken from XFree86)
  - the possibility to supply your own mode database for graphics hardware that
    needs non-standard modes, like amifb and Mac frame buffer drivers (which
    use macmodes.c)

When a frame buffer device receives a video= option it doesn't know, it should
consider that to be a video mode option. If no frame buffer device is specified
in a video= option, fbmem considers that to be a global video mode option.

Valid mode specifiers (mode_option argument):

    <xres>x<yres>[-<bpp>][@<refresh>]
    <name>[-<bpp>][@<refresh>]

with <xres>, <yres>, <bpp> and <refresh> decimal numbers and <name> a string.
Things between square brackets are optional.

To find a suitable video mode, you just call

int __init fb_find_mode(struct fb_var_screeninfo *var,
                        struct fb_info *info, const char *mode_option,
                        const struct fb_videomode *db, unsigned int dbsize,
                        const struct fb_videomode *default_mode,
                        unsigned int default_bpp)

with db/dbsize your non-standard video mode database, or NULL to use the
standard video mode database.

fb_find_mode() first tries the specified video mode (or any mode that matches,
e.g. there can be multiple 640x480 modes, each of them is tried). If that
fails, the default mode is tried. If that fails, it walks over all modes.

BTW, only a few drivers use this at the moment. Others are to follow
(feel free to send patches).