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This article or section needs expansion.

Reason: KMS and rootless X (1.16), see Talk:Kernel mode setting and Xorg#Rootless Xorg. (Discuss in Talk:Kernel mode setting)

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 driver (since 364.12) also implements kernel mode-setting, but it does not use the built-in kernel implementation and it lacks 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 (`Ctrl+Alt+F2`), 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 (`Alt+F7` when X runs in VT7).

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

The factual accuracy of this article or section is disputed.

Reason: Since mkinitcpio v32, the `kms` hook is included by default, therefore most setups will have early loading enabled by default. (Discuss in Talk:Kernel mode setting#kms hook is now included by default)

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, ATI and AMDGPU drivers already enable KMS automatically for all chipsets, so you need not install it manually.

The proprietary NVIDIA driver supports KMS (since 364.12), which has to be manually enabled.

### Early KMS start

Tip: If you encounter problems with the resolution, you can check whether enforcing the mode helps.

KMS is typically initialized after the initramfs stage. However, it is possible to enable KMS already during the initramfs stage. Add the required module for the video driver to the initramfs configuration file:

- `amdgpu` for AMDGPU, or `radeon` when using the legacy ATI driver.
- `i915` for Intel graphics.
- `nouveau` for the open-source Nouveau driver.
- `nvidia nvidia_modeset nvidia_uvm nvidia_drm` for the out-of-tree nvidia and nvidia-open drivers. See NVIDIA#DRM kernel mode setting for details.

- `mgag200` for Matrox graphics.
- Depending on QEMU graphics in use (qemu option `-vga type` or libvirt `<video><model type='type'>`[1]):
  - `bochs` for `std` (qemu) and `vga`/`bochs` (libvirt),
  - `virtio-gpu` for `virtio`,
  - `qxl` for `qxl`,
  - `vmwgfx` for `vmware` (qemu) and `vmvga` (libvirt),
  - `cirrus` for `cirrus`.
- Depending on VirtualBox graphics controller:
  - `vmwgfx` for VMSVGA,
  - `vboxvideo` for VBoxVGA or VBoxSVGA.

Initramfs configuration instructions are slightly different depending on the initramfs generator you use.

#### mkinitcpio

For in-tree modules, add `kms` to the HOOKS array in `/etc/mkinitcpio.conf`.

For out-of-tree modules, place the module names in the MODULES array. For example, to enable early KMS for the NVIDIA graphics driver:

    /etc/mkinitcpio.conf

    MODULES=(... nvidia nvidia_modeset nvidia_uvm nvidia_drm ...)

Note: If you use PRIME Graphics Processing Unit (GPU) with Intel Integrated Graphics Processors (IGP) being your primary GPU and AMD as the discrete one, `intel_agp` may lead to troubles when resuming from hibernation (monitor gets no signal). See [2] for details.

If you are using the #Forcing modes and EDID method, you should embed the custom file into initramfs as well:

    /etc/mkinitcpio.conf

    FILES=(/usr/lib/firmware/edid/your_edid.bin)

Then regenerate the initramfs.

#### Booster

If you use Booster, you can load required modules with this config change:

    /etc/booster.yaml

    modules_force_load: i915

If you are using the #Forcing modes and EDID method, you should embed the custom file into your booster images as well:

    /etc/booster.yaml

    extra_files: /usr/lib/firmware/edid/your_edid.bin

Then regenerate the booster images.

## Troubleshooting

### My fonts are too tiny

See Linux console#Fonts for how to change your console font to a large font. The Terminus font (terminus-font) is available in many sizes, such as `ter-132b` which is larger.

Alternatively, disabling modesetting might switch to lower resolution and make fonts appear larger.

### Problem upon bootloading and dmesg

This article or section is out of date.

Reason: This section was added in 2011 and 2012. Is it still relevant a decade later? (Discuss in Talk:Kernel mode setting)

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 output to a HDP (Hardware Debug Port), but you have other non HDP outputs in your hardware 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), use:

    /etc/modprobe.d/modprobe.conf

    options drm_kms_helper poll=0

## Forcing modes and EDID

If your native resolution is not automatically configured or no display at all is detected, then your monitor might send none or just a skewed EDID file. The kernel will try to catch this case and will set one of the most typical resolutions.

In case you have the EDID file for your monitor, you merely need to explicitly enforce it (see below). However, most often one does not have direct access to a sane file and it is necessary to either extract an existing one and fix it or to generate a new one.

Generating new EDID binaries for various resolutions and configurations is possible during kernel compilation by following the upstream documentation (also see here for a short guide). Other solutions are outlined in details in this article. Extracting an existing one is in most cases easier, e.g. if your monitor works fine under Windows, you might have luck extracting the EDID from the corresponding driver, or if a similar monitor works which has the same settings, you may use `get-edid` from the read-edid package. You can also try looking in `/sys/class/drm/*/edid`.

After having prepared your EDID, place it in a directory, e.g. called `edid` under `/usr/lib/firmware` and copy your binary into it.

To load it at boot, specify the following in the kernel command line:

    drm.edid_firmware=edid/your_edid.bin

For kernels older than 4.13, use this line instead:

    drm_kms_helper.edid_firmware=edid/your_edid.bin

In order to apply it only to a specific connector, use:

    drm.edid_firmware=VGA-1:edid/your_edid.bin

If you want to set multiple edid files, use:

    drm.edid_firmware=VGA-1:edid/your_edid.bin,VGA-2:edid/your_other_edid.bin

For the built-in resolutions, refer to the table below. The Name column specifies the name which one is supposed to use in order to enforce its usage.

Resolution | Name
--- | ---
800x600 | edid/800x600.bin
1024x768 | edid/1024x768.bin
1280x1024 | edid/1280x1024.bin
1600x1200 (kernel 3.10 or higher) | edid/1600x1200.bin
1680x1050 | edid/1680x1050.bin
1920x1080 | edid/1920x1080.bin

If you are doing early KMS, you must include the custom EDID file in the initramfs, otherwise you will run into problems.

The value of the `drm.edid_firmware` parameter may also be altered after boot by writing to `/sys/module/drm/parameters/edid_firmware`:

    # echo edid/your_edid.bin > /sys/module/drm/parameters/edid_firmware

This will only take affect for newly plugged in displays, already plugged-in screens will continue to use their existing EDID settings. For external displays, replugging them is sufficient to see the effect however.

Since kernel 3.15, to load an EDID after boot, you can use debugfs instead of a kernel command line parameter if the kernel is not in lockdown mode. This is very useful if you swap the monitors on a connector or just for testing. Once you have an EDID file as above, run:

    # cat correct-edid.bin > /sys/kernel/debug/dri/0/HDMI-A-2/edid_override

And to disable:

    # echo -n reset > /sys/kernel/debug/dri/0/HDMI-A-2/edid_override

### Forcing modes

Warning: The method described below is somehow incomplete because e.g. Xorg does not take into account the resolution specified, so users are encouraged to use the method described above. However, specifying resolution with `video=` command line may be useful in some scenarios.

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. Bits and pieces on how to use it can be found in
  - https://docs.kernel.org/fb/modedb.html
  - https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/drm_fb_helper.c

The format is:

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

- `<conn>`: Connector, e.g. DVI-I-1, see `/sys/class/drm/` for available connectors
- `<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

To get the name and current status of connectors, you can use the following shell oneliner:

    $ for p in /sys/class/drm/*/status; do con=${p%/status}; echo -n "${con#*/card?-}: "; cat $p; done

    DVI-I-1: connected
    HDMI-A-1: disconnected
    VGA-1: disconnected

## Disabling modesetting

You may want to disable KMS for various reasons. To disable KMS, add `nomodeset` as a kernel parameter. See Kernel parameters for more info.

Along with the `nomodeset` kernel parameter, for an Intel graphics card, you need to add `i915.modeset=0`, and for an Nvidia graphics card, you need to add `nouveau.modeset=0`. For Nvidia Optimus dual-graphics system, you need to add all the three kernel parameters (i.e. `"nomodeset i915.modeset=0 nouveau.modeset=0"`).

Note: Some Xorg drivers will not work with KMS disabled. See the wiki page on your specific driver for details.
Categories:

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- Kernel

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