Linux – What happened to `vga=ask` in newer kernels

bootframebufferkernellinux

I used to activate framebuffer mode in the "older" kernels with vga=ask, and later setting it to a value, once I've figured out which one works best for me.

Now, this option seems to be gone, the 3.11 kernel tells me it ignores this legacy option.

What should I use instead, when activating framebuffer mode for the text mode consoles and the boot process?

Best Answer

KMS-enabled kernels overrule any vga= setting before init completes, when modesetting is initiated, functionally making whether vga=ask works or not moot.

Instead, use video= with the specific mode desired on the vttys. With video=, you're not limited to VESA modes - any mode supported by the display can be used. It's even possible sometimes with video= to specify modes that a display doesn't natively support, though the result can be ugly, e.g. 2048x1024 on a 1920x1200 native display.

See https://www.kernel.org/doc/Documentation/fb/modedb.txt for video= documentation.

Related Solutions

QEMU – Disable Framebuffer in QEMU Guests

As of 2017, qemu doesn't provide text-mode-only graphics card emulation for x86-64 that would force a guest to stay in text mode.

Current distributions like Fedora 25 come with the bochs_drm kernel module that enables a frame buffer (e.g. 1024x768 graphics mode), by default. In contrast to that, e.g. Debian 8 (stable) doesn't provide this module and thus it stays in old-school text-mode during the complete boot.

Thus, when running qemu from a terminal (e.g. with -display curses) it makes sense to enable a serial console as fail safe:

console=tty0 console=ttyS0

console=tty0 console=ttyS0,115200

(Kernel parameters for the guest, default speed is 9600, both settings works with qemu, make the settings persistent in Fedora via assigning them to GRUB_CMDLINE_LINUX in /etc/sysconfig/grub and executing grub2-mkconfig -o /etc/grub2.cfg or grub2-mkconfig -o /etc/grub2-efi.cfg)

In case nothing else works one can then switch inside qemu via Alt+3 to the serial console, then.

A second measure is to disable the framebuffer via a bochs_drm module parameter - i.e. via setting it on the guest kernel command line:

bochs_drm.fbdev=off

Blacklist Alternative

Alternatively, the bochs_drm module can be blacklisted - i.e. via creating a config under /etc/modprobe.d - say - bochs.conf:

blacklist bochs_drm

Since the initramfs mustn't load the bochs_drm module, as well, one has to make sure that this config is included into the initramfs. On Fedora like distributions this is achieved via:

# dracut -f

UEFI Boot

When booting qemu with an UEFI firmware (e.g. -bios /usr/share/edk2/ovmf/OVMF_CODE.fd) the disabling of the bochs fbdev isn't enough. The Fedora boot then hangs while trying to switch to the bochs framebuffer. Blacklisting the bochs_drm fixes this but it isn't sufficient. One just gets a 640 x 480 graphics mode that isn't reset to text mode by the kernel. Thus, for UEFI guests one has to take the serial console route.

Serial Console

Using the serial console in combination with -display curses yields a suboptimal user experience as the curses interferes with the vt110/vt220 terminal emulation. Thus, it only suffices for emergencies.

A better solution is to completely switch the display off and use the combined serial/monitor Qemu mode:

-display none -serial mon:stdio -echr 2

(where Ctrl+b h displays a help and Ctrl+b c switches between the modes)

With Fedora 27, the Grub2 is configured with serial console support, by default. Thus, it can be controlled via the serial terminal, as well.

Calling resize after login updates the terminal geometry, thus, the resulting terminal behaves as good as a local one.

Multi-User Target

In case the guest image has a graphical login manager installer it makes sense to disable it:

# systemctl set-default multi-user.target

Otherwise, on has to switch to the first virtual console after each boot (e.g. Alt+2 or Alt+3 when using the curses display).