Linux – Is it possible to boot an x86 Linux kernel using both 80×25 text mode consoles and VESA framebuffer consoles

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The Linux kernel documents that you can choose to boot Linux with VESA framebuffer consoles instead of the default 80×25 text mode consoles.

For text application testing purposes, one may need both modes simultaneously. For instance, ALT+1 to ALT+3 in 80×25 text mode and ALT+4 to ALT+6 in framebuffer mode.

Is there a way to achieve dual mode text displays in a single boot?

Best Answer

If you boot the kernel in text mode (standard on x86), you can change the VESA mode after boot with vbetool, using the vbemode action.

If you boot the kernel in framebuffer mode (standard on non-x86), you can change the resolution at runtime with fbset.

When you press ALT+L, etc., the kernel probably sends SIGWINCH to the INIT process (pid 1). The default init program probably generates a kbrequest event, you may be able to pick up on that by modifying /etc/inittab and have init run a script that sets the mode for you.

Related Solutions

Linux – GRUB2 and kernel vga= parameter

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

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

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.

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Related Solutions

Linux – GRUB2 and kernel vga= parameter

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

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