• Jan Beulich's avatar
    x86/boot: make "vga=current" work with graphics modes · 927aee59
    Jan Beulich authored
    GrUB2 can be told to leave the screen in the graphics mode it has been
    using (or any other one), via "set gfxpayload=keep" (or suitable
    variants thereof). In this case we can avoid doing another mode switch
    ourselves. This in particular avoids possibly setting the screen to a
    less desirable mode: On one of my test systems the set of modes
    reported available by the VESA BIOS depends on whether the interposed
    KVM switch has that machine set as the active one. If it's not active,
    only modes up to 1024x768 get reported, while when active 1280x1024
    modes are also included. For things to always work with an explicitly
    specified mode (via the "vga=" option), that mode therefore needs be a
    1024x768 one.
    For some reason this only works for me with "multiboot2" (and
    "module2"); "multiboot" (and "module") still forces the screen into text
    mode, despite my reading of the sources suggesting otherwise.
    For starters I'm limiting this to graphics modes; I do think this ought
    to also work for text modes, but
    - I can't tell whether GrUB2 can set any text mode other than 80x25
      (I've only found plain "text" to be valid as a "gfxpayload" setting),
    - I'm uncertain whether supporting that is worth it, since I'm uncertain
      how many people would be running their systems/screens in text mode,
    - I'd like to limit the amount of code added to the realmode trampoline.
    For starters I'm also limiting mode information retrieval to raw BIOS
    accesses. This will allow things to work (in principle) also with other
    boot environments where a graphics mode can be left in place. The
    downside is that this then still is dependent upon switching back to
    real mode, so retrieving the needed information from multiboot info is
    likely going to be desirable down the road.
    Signed-off-by: default avatarJan Beulich <jbeulich@suse.com>
    Acked-by: default avatarRoger Pau Monné <roger.pau@citrix.com>
    Acked-by: default avatarHenry Wang <Henry.Wang@arm.com>