Kernel mode setting

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Revision as of 17:06, 26 July 2012 by DSpider (Talk | contribs) (Disabling modesetting: added both a temporary and a permanent solution)

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Kernel Mode Setting (KMS) is a method for setting display resolution and depth in the kernel space rather than user space.

KMS enables native resolution in the framebuffer and allows for instant console (tty) switching. KMS also enables newer technologies (such as DRI2) which will help reduce artifacts and increase 3D performance, even kernel space power-saving.

Background

Previously, setting up the video card was the job of the X server. Because of this, it was not easily possible to have fancy graphics in virtual consoles. Also, each time a switch from X to a virtual console was made (Template:Keypress), the server had to give control over the video card to the kernel, which was slow and caused flickering. The same "painful" process happened when the control was given back to the X server (Template:Keypress).

With Kernel Mode Setting (KMS), the kernel is now able to set the mode of the video card. This makes fancy graphics during bootup, virtual console and X fast switching possible, among other things.

Installation

At first, note that for any method you use, you should always disable:

  • Any "vga=" options in your bootloader as these will conflict with the native resolution enabled by KMS.
  • Any "video=" lines that enable a framebuffer that conflicts with the driver.
  • Any other framebuffer drivers (such as uvesafb).

Late KMS start

Intel, Nouveau and ATI drivers already enable KMS automatically for all chipsets. So you need not install it manually.

The proprietary NVIDIA and ATI Catalyst drivers do not use the open driver stack. In order to use KMS you should replace them with open source drivers.

Early KMS start

To load KMS as early as possible in boot process, add the module radeon (for radeon cards), i915 (for intel cards) or nouveau (for nvidia cards) to the MODULES line in /etc/mkinitcpio.conf:

/etc/mkinitcpio.conf
MODULES="i915"
or
MODULES="radeon"
or
MODULES="nouveau"

Rebuild your kernel image (refer to the mkinitcpio article for more info):

# mkinitcpio -p [name of your kernel preset]

Troubleshooting

My fonts are too tiny

See changing the default font for how change your console font to a large font. Terminus font in [community] is available in many sizes, including larger sizes.

Issue upon bootloading and dmesg

If you see an error code of 0x00000010 (2) while booting up, (You will get about 10 lines of text, the last part denoting that error code), then add the following line into /etc/modprobe.d/modprobe.conf:

options drm_kms_helper poll=0

Forcing modes

From the nouveau wiki:

A mode can be forced on the kernel command line. Unfortunately, the command line option video is poorly documented in the DRM case. Bit and pieces on how to use it can be found in

The format is:

video=<conn>:<xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m][eDd]
  • <conn>: Connector, e.g. DVI-I-1, see your kernel log.
  • <xres> x <yres>: resolution
  • M: compute a CVT mode?
  • R: reduced blanking?
  • -<bpp>: color depth
  • @<refresh>: refresh rate
  • i: interlaced (non-CVT mode)
  • m: margins?
  • e: output forced to on
  • d: output forced to off
  • D: digital output forced to on (e.g. DVI-I connector)

You can override the modes of several outputs using "video" several times, for instance, to force DVI to 1024x768 at 85 Hz and TV-out off:

video=DVI-I-1:1024x768@85 video=TV-1:d

Disabling modesetting

You may want to disable KMS for various reasons, such as getting a blank screen or a "no signal" error from the display, when using the Catalyst driver, etc. When the bootloader menu shows up, press the Template:Keypress key (Syslinux) or Template:Keypress (for GRUB) and add nomodeset to the kernel line (GRUB) or at the end of the string (for Syslinux).

GRUB:

kernel /vmlinuz-linux root=/dev/sda3 ro quiet nomodeset

Syslinux:

> .linux ../vmlinuz-linux root=/dev/sda3 ro quiet initrd=../initramfs-linux.img nomodeset

To make the change permanent:

  • Edit /boot/syslinux/syslinux.cfg and add it to the APPEND line.
  • Edit /etc/default/grub with GRUB_CMDLINE_LINUX_DEFAULT="quiet nomodeset" and re-generate the .cfg with grub-mkconfig -o /boot/grub/grub.cfg.