Kernel mode setting

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Reason: KMS and rootless X (1.16), see Talk:Kernel mode setting and Xorg#Rootless Xorg (v1.16). (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

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 already enable KMS during the initramfs stage. Add the required module for the video driver to the MODULES array in /etc/mkinitcpio.conf:

  • amdgpu for AMDGPU, or radeon when using the legacy ATI driver.
  • i915 for Intel graphics.
  • nouveau for the open-source Nouveau driver.
  • mgag200 for Matrox graphics.
  • Depending on QEMU graphics in use: virtio-gpu for VirtIO, qxl for QXL, or cirrus for Cirrus.
  • nvidia nvidia_modeset nvidia_uvm nvidia_drm for nvidia driver. See NVIDIA#DRM kernel mode setting for details.

For example to enable early KMS for the Intel graphics driver:

/etc/mkinitcpio.conf
MODULES=(... i915 ...)
Note: Intel users may need to add intel_agp before i915 to suppress the ACPI errors (check the output of lsmod on your running system to see if intel_agp is loaded). This may be required for resuming from hibernation to work with a changed display configuration. If you use PRIME GPU with Intel IGP being your primary GPU and AMD as the discrete one, adding intel_agp may lead to troubles when resuming from hibernation (monitor gets no signal). See [1] for details.

If you are using a custom EDID file (not applicable for the built-in resolutions), you should embed it into initramfs as well:

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

Then regenerate the initramfs.

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-132n which is larger.

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

Problem 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), 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 folder, 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

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/modules/drm/parameters/edid_firmware:

# echo edid/your_edid.bin > /sys/modules/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

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 nomodeset kernel parameter, for Intel graphics card you need to add i915.modeset=0 and for 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.