1. 11 Apr, 2022 1 commit
    • 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>
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