Linux – Where in GRUB2 source code is the handover/jump to kernel code

Maybe it is deprecated by grub2, however it should still work and must continue to work. It works for me using grub2 and debian wheezy as well as rhel7.3 (confirmed 01-2017). Also according to the official documentation it is not deprecated:

https://www.kernel.org/doc/Documentation/x86/boot.txt

If the command line provided by the boot loader is entered by the user, the user may expect the following command line options to work. They should normally not be deleted from the kernel command line even though not all of them are actually meaningful to the kernel.

vga= here is either an integer (in C notation, either decimal, octal, or hexadecimal) or one of the strings "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask" (meaning 0xFFFD). This value should be entered into the vid_mode field, as it is used by the kernel before the command line is parsed.

And https://www.kernel.org/doc/Documentation/svga.txt will tell you all you need to know. Below a few examples of resolutions I commonly use:

vga=0x31b --> 1280x1024x32
vga=0x34d --> 1600x900x32
vga=0x31e --> 1600x1200x32

To get a list of supported modes use:

vga=ask

For a 1024×768x24 screen which I found works on pretty much all systems and monitors I use (this time in decimal and in hex):

vga=792
vga=0x318

Boot loaders, or Grub for that matter, rebuilds the memory map like e820, I guess this is the reason why you are seeing different values between GRUB and the EFI stub loader.

There is a comment in the Linux source code which says that EFI allows "more than the max 128 entries that can fit in the e820 legacy (zeropage) memory map.". This seems not to be the case according to the numbers you posted, hence I doubt that adding add_efi_memmap is helpful... However, it certainly doesn't hurt to parse this table too...