Re: [PATCH v4 1/8] x86/boot: make "vga=current" work with graphics modes

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Mar 31, 2022 at 11:44:10AM +0200, Jan Beulich wrote:
> 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: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, Roger.