Grub behaving differently when non-UEFI boot options are enabled

[Yann SALMON]

Hello,

The main hard disk of my Dell Precision 3620 is partitioned with a GPT
and Ubuntu is installed and boots (via grub2) in UEFI mode.

What I recently discovered is that the early initialisation of the
graphics card seems to happen differently when the "Enable legacy boot
options" option is activated in the BIOS. Notably, the apparent screen
resolution is not the same.

This is of interest to me because I have difficulties with the Nvidia
driver crashing (after boot) when switching VT (only when legacy boot
options are not enabled).

The question : what sense does it make that this BIOS setting influences
the graphics card so much, considering we are booting an UEFI entry in
each case ? Is this a bug in the Dell BIOS, in grub, in the nvidia
driver ? How can I investigate this ?