Mode setting

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Regulating access to the hardware is a fundamental kernel task. The Direct Rendering Manager and KMS are part of the Linux kernel. The KMS does only the mode setting.

Mode setting is a software operation that activates a display mode (screen resolution, color depth, and refresh rate) for a computer's display controller.

In kernel mode-setting (KMS), the display mode is set by the kernel. In user-space mode-setting (UMS), the display mode is set by a userland process.

Kernel mode-setting is more flexible and allows displaying of an error in the case of a fatal error in the kernel, even when using a user-space display server.

User-space mode setting would require superuser privileges for direct hardware access, so kernel-based mode setting removes the need for superuser privileges for the user-space graphics server.

Atomic mode setting[edit]

Atomic mode setting means setting the output modes of a single graphics device. Collecting the video mode, encoder and connector setup over the whole graphics card into a single operation avoids flicker. Either the whole set succeeds, or it fails.[1]


Microsoft Windows[edit]

Microsoft Windows versions that are NT-based use kernel mode setting. The kernel error display made possible by kernel mode setting is known as the Blue Screen of Death.


The role of KMS (Kernel mode-setting), Linux example
The Linux Graphics Stack 2013
Wayland compositors require KMS (and also OpenGL ES and EGL)
The place of certain Linux kernel modules
evdev is the Linux kernel module that receives data from various Input devices such as Keyboard, Mouse, Touch-Pad, etc. The data is passed to the Display server (e.g. the X.Org Server or some Wayland compositor only to be passed further to the wayland client respectively X client. Some applications require a minimal latency

The Linux kernel got the prerequisite for kernel-based mode setting by accepting Intel's GEM in version 2.6.28, released in December 2008.[2] This will be[dated info] replaced by Tungstens Graphics TTM (Translation Table Maps) memory manager which supports the GEM API.[3] TTM was developed for the free and open-source drivers for Radeon and S3 Graphics graphic chipsets (see Free and open-source graphics device driver).[4] Support for Intel GMA graphic chipsets was accepted in version 2.6.29, which was released on March 23, 2009.[5] Support for pre-R600 ATI Radeon graphics cards was accepted in version 2.6.31, which was released on September 9, 2009.[6] Support for R600 and R700 was in development within DRM and was merged in version 2.6.32.[7] Support for Evergreen (R800) was merged in version 2.6.34. As Nvidia did not release all the needed documentation for its graphics chip, development proceeded under the nouveau project, which uses reverse engineering to build a working open-source driver for Nvidia cards. Nouveau was accepted in version 2.6.33 of the kernel, which was released on December 10, 2009. Kernel-based mode setting is not only supported by the nouveau driver, it is required.[8] Wayland compositors (e.g. Weston) and kmscon depend on kernel mode setting via ioctl.


FreeBSD has support for both kernel-based mode setting and GEM for later generations of Intel GPUs (IronLake, SandyBridge, and IvyBridge) starting with version 9.1.[9]


OpenBSD has kernel-based mode setting support for Intel and Radeon GPUs. Starting with version 5.4 of OpenBSD, support for Intel GPUs is available. With the release of version 5.5, the implementation has been extended to add support for Radeon chipsets as well.


  • At the Linux Plumbers Conference 2013, it was suggested to split GEM and KMS.[10]
  • There is the Atomic Display Framework;[11][12] it was presented by Google's Android-Team at the Linux Plumbers Conference 2013.
  • There is the Common Display Framework.[13]

See also[edit]


  1. ^ Paalanen, Pekka. "From pre-history to beyond the global thermonuclear war". Blog. Retrieved 6 June 2014. 
  2. ^ "Linux 2 6 28". Linux Kernel Newbies. Retrieved 2013-02-14. 
  3. ^ Larabel, Michael (2008-08-26). "A GEM-ified TTM Manager For Radeon". Phoronix. Retrieved 2013-02-14. 
  4. ^ Larabel, Michael (2009-06-10). "TTM Memory Manager Gets Ready For Release". Phoronix. Retrieved 2013-02-14. 
  5. ^ "Linux 2 6 29". Linux Kernel Newbies. Retrieved 2013-02-14. 
  6. ^ "Linux 2 6 31". Linux Kernel Newbies. 2009-09-09. Retrieved 2013-02-14. 
  7. ^ Larabel, Michael (2009-09-30). "AMD R600/700 2D Performance: Open vs. Closed Drivers". Phoronix. Retrieved 2013-02-14. 
  8. ^ "nouveau/ KernelModeSetting". 24 August 2013. Retrieved 2014-08-11. 
  9. ^ "FreeBSD 9.1-RELEASE Release Notes". FreeBSD Foundation. 30 December 2012. 
  10. ^
  11. ^
  12. ^
  13. ^

External links[edit]