Configuring Video Modes on UEFI and Legacy BIOS Systems

The Linux kernel handles video mode configuration through bootloader parameters and kernel graphics drivers. Modern UEFI systems use Kernel Mode Setting (KMS) automatically, but you can override modes when needed. Legacy BIOS systems still use older mechanisms if you’re maintaining older hardware.

UEFI Systems: Modern Video Mode Configuration

On UEFI systems with GRUB2, add video parameters to the kernel command line in /etc/default/grub:

GRUB_CMDLINE_LINUX="video=1920x1080-32@60"

The syntax format is:

video=<connector>:<xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m][eDd]

Where:

• <connector>: Optional display connector name (HDMI-1, DP-2, eDP-1, LVDS-1, etc.)
• <xres>x<yres>: Resolution in pixels
• M: Reduced blanking
• R: CVT reduced blanking v2 (more aggressive reduction)
• -<bpp>: Bits per pixel (8, 16, 24, 32)
• @<refresh>: Refresh rate in Hz
• i: Interlaced mode
• m: Margins around the display
• e, D, d: Enable/disable output or deep color

For multi-monitor setups, specify modes per connector:

GRUB_CMDLINE_LINUX="video=HDMI-1:1920x1080-32@60 video=DP-2:3840x2160-32@60"

To force a specific connector when detection fails:

GRUB_CMDLINE_LINUX="video=eDP-1:1366x768-32@60"

Configuring GRUB Graphics Mode

Improve boot-time visuals by setting GRUB’s graphics terminal:

GRUB_GFXMODE=1920x1080x32
GRUB_GFXPAYLOAD_LINUX=keep
GRUB_TERMINAL=gfxterm

The GRUB_GFXPAYLOAD_LINUX=keep option preserves the GRUB resolution throughout the kernel boot process, eliminating display flicker as the kernel takes over.

After editing /etc/default/grub, regenerate the configuration:

# Debian/Ubuntu
sudo update-grub

# RHEL/Fedora/CentOS
sudo grub2-mkconfig -o /boot/grub2/grub.cfg

# Systems with separate EFI partition
sudo grub2-mkconfig -o /boot/efi/EFI/ubuntu/grub.cfg

Legacy BIOS Systems: vga Parameter

Older BIOS systems use the 16-bit boot protocol with the vga= kernel parameter, which accepts decimal values derived from VESA mode numbers.

The conversion formula is:

Linux_kernel_mode_number = VESA_mode_number + 0x200

Common VESA mode conversions:

Resolution	256 Color	15-bit	16-bit	24-bit	32-bit
640×480	0x101	0x110	0x111	0x112	—
800×600	0x103	0x113	0x114	0x115	0x10E
1024×768	0x105	0x116	0x117	0x118	0x10A
1280×1024	0x107	0x119	0x11A	0x11B	0x10D

For example, to set 1024×768 at 16-bit color (VESA 0x117):

# Add 0x200: 0x117 + 0x200 = 0x317
# Convert to decimal: 0x317 = 791
GRUB_CMDLINE_LINUX="vga=791"

Convert hexadecimal to decimal with Python:

python3 -c "print(0x317)"  # Output: 791

Querying Available Video Modes

GRUB Command Line

At the GRUB boot menu, press c to enter the command line and type:

vbeinfo

This displays all VESA modes your firmware supports:

VBE info: version: 2.0
total memory: 4096 KiB
List of compatible video modes:
0x101: 640 x 480 x 8 Packed
0x110: 640 x 480 x 15 Direct
0x111: 640 x 480 x 16 Direct
0x112: 640 x 480 x 24 Direct
0x103: 800 x 600 x 8 Packed
0x113: 800 x 600 x 15 Direct
...
Configured VBE mode = 0x101

Using modetest

For modern systems, query connected displays and their capabilities:

# AMD GPUs
modetest -M amdgpu

# Intel GPUs
modetest -M i915

# NVIDIA (when using nouveau driver)
modetest -M nouveau

This shows all detected displays and available resolutions with connector names.

Kernel Mode Setting (KMS)

Modern systems with KMS enabled (amdgpu, i915, nouveau, radeon, etc.) handle mode-setting automatically. The kernel graphics driver typically handles everything without manual intervention unless you need to:

• Force a specific resolution before the display manager starts
• Work around driver detection or EDID bugs
• Debug display problems
• Configure framebuffer output on headless systems

Check if KMS is active:

cat /proc/cmdline | grep -o 'video=[^ ]*'

Examine kernel logs for mode-setting activity:

dmesg | grep -E 'drm|kms|framebuffer'

Look for messages like [drm] Initialized amdgpu or i915 ... initialized.

To disable KMS (rarely needed):

GRUB_CMDLINE_LINUX="nomodeset"

This forces the kernel to use simple framebuffer drivers instead of hardware-specific drivers. Expect degraded performance and limited resolution.

Troubleshooting Video Mode Issues

Verify firmware support: Use vbeinfo in GRUB to confirm the mode exists on your system.

Check connector names: Use modetest to list connected displays with their exact connector names:

modetest | head -50

Use the exact connector name from output in your video= parameter.

Force fallback modes: Remove custom video= parameters and let the driver auto-detect:

GRUB_CMDLINE_LINUX=""

Review kernel logs: Detailed information appears in dmesg:

dmesg | grep -i 'video\|mode\|display'

Look for messages indicating failed mode-setting or unsupported resolutions.

Update BIOS/firmware: Video mode support varies by firmware revision. Firmware updates often improve compatibility and fix display bugs.

Check EDID data: The kernel reads display capabilities via EDID. On some displays, EDID is corrupted or missing. Force a known-good mode explicitly:

GRUB_CMDLINE_LINUX="video=HDMI-1:1920x1080-32@60"

Disable Secure Boot temporarily: On EFI systems, some firmware has quirks with certain video modes under Secure Boot. Disabling Secure Boot can help isolate firmware-specific issues.

Reduce resolution at boot: If high-resolution modes fail, the kernel may lack sufficient firmware-allocated memory. Try a lower resolution:

GRUB_CMDLINE_LINUX="video=1024x768-32@60"

Enable verbose graphics logging: Get detailed driver debugging output:

GRUB_CMDLINE_LINUX="drm.debug=0xff"

This produces voluminous kernel log output but identifies low-level driver issues. Check dmesg after boot.