Difference between revisions of "Kernel mode setting"

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The Linux kernel's implementation of 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.
 
The Linux kernel's implementation of 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.
  
{{Note|The proprietary [[nvidia]] and [[catalyst]] drivers also implement kernel mode-setting, but as they do not use the built-in kernel implementation, they lack an fbdev driver for the high-resolution console.}}
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{{Note|The proprietary [[NVIDIA|nvidia]] and [[AMD Catalyst|catalyst]] drivers also implement kernel mode-setting, but as they do not use the built-in kernel implementation, they lack an fbdev driver for the high-resolution console.}}
  
 
== Background ==
 
== Background ==
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[[Intel]], [[Nouveau]] and [[ATI]] drivers already enable KMS automatically for all chipsets. So you need not install it manually.
 
[[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.
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The proprietary [[NVIDIA]] and [[AMD 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 ===
 
=== Early KMS start ===

Revision as of 18:22, 10 November 2012

Summary help replacing me
Information about configuring the Kernel Mode Setting method.
Related
ATI
Intel
Nouveau

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

The Linux kernel's implementation of 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.

Note: The proprietary nvidia and catalyst drivers also implement kernel mode-setting, but as they do not use the built-in kernel implementation, they lack an fbdev driver for the high-resolution console.

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 AMD 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 ATI/AMD cards), i915 (for Intel integrated graphics) 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; e.g. linux>

Troubleshooting

My fonts are too tiny

See changing the default font for how to 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

Polling for connected display devices on older systems can be quite expensive. Poll will happen periodically and can in worst cases take several hundred milliseconds, depending on the hardware. This will cause visible stalls, for example in video playback. These stalls might happen even when your video is on HDP output but you have other non HDP outputs in your hw configuration. If you experience stalls in display output occurring every 10 seconds, disabling polling might help.

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. To disable KMS, add nomodeset as a kernel parameter. See Kernel parameters for more info.