Rotate console on startup (Debian)

Figured this out. You may need to add video=efifb to ensure that the framebuffer console is used:

GRUB_CMDLINE_LINUX="video=efifb fbcon=rotate:1"

EDIT: The efifb driver is designed for EFI firmware only, especially Intel-based Apple computers. However, as I've found out, it also works for non-Apple PCs. I am running the proprietary nVidia drivers on my Linux system, and the efifb driver works quite well. I assume it works for me because I am using nVidia drivers, and the "native" fbdev driver conflicts with them.

To be honest, I don't fully understand why the efifb driver makes things work, but if someone else does (or if you can get things working with another framebuffer driver with nVidia drivers installed), please comment below. Thanks!

There is a small but important difference between the documentation and what you have written in your config file:

…the following option that can be passed to the kernel: fbcon=rotate:<n>

While you have written

fbcon=rotate_all:1

rotate_all is not a valid keyword. If you leave out the _all suffix, it should work:

GRUB_CMDLINE_LINUX="fbcon=rotate:1"