DRM/KMS Platform


The DRM platform plug-in additionally requires the following libraries:

  • WPEBackend-fdo
  • Wayland
  • GLESv2
  • libdrm
  • libgbm
  • libinput
  • libudev

Configuration File Options

If a section named drm is found in the configuration file (see CogShell:config-file), the following options will be honored:

Option Type Default
device-scale-factor float 1.0
disable-atomic-modesetting boolean detect
renderer string "modeset"

The device-scale-factor option indicates a scaling factor to be applied to the rendered content. This is particularly useful for displays with a high DPI to avoid rendered content to appear abnormally small. A typical setting for displays around the 192DPI mark would be 2.0. Note that currently no attempt is done to try guessing a suitable value, and values other than the default need to be explicitly set.

The disable-atomic-modesetting option can be used to explicitly disable usage of atomic mode setting. This is a feature supported by many modern GPU drivers and it will be used by default when available. In some rare cases—mostly buggy or incomplete drivers—it might need to be manually disable its usage by setting this option to true.

The renderer option controls how renderer content will be displayed. The default value is "modeset", which attaches rendered frames directly to the output. Using the value "gles" will “paint” frames onto a quad using OpenGL ES. The main reason to use the latter is that it supports output rotation.


The following parameters can be passed to the platform plug-in during initialization (e.g. using cog --platform-params=…):

Parameter Type Default
renderer string modeset
rotation number 0

The renderer parameter is the same as the configuration file option of the same name.

The rotation parameter indicates the initial output rotation applied.

Environment Variables

The following environment variables can be set to further tweak how the DRM plug-in operates:

Variable Type Default

By default the preferred mode for the first found connected output is used (if available), otherwise the mode with most resolution. Setting COG_PLATFORM_DRM_VIDEO_MODE instructs the plug-in to pick a particular video mode, while COG_PLATFORM_DRM_MODE_MAX can be used to limit which modes are considered. Mode strings are formatted as WxH@R (Width and Height in pixels, Refresh rate in Hertz), for example 1920x1080@60 for a typical Full-HD mode. The R part can be omitted in COG_PLATFORM_DRM_MODE_MAX.

Setting COG_PLATFORM_DRM_CURSOR to a non-empty string enables showing the mouse cursor pointer.

Output Rotation

When using the OpenGL ES renderer using gles as value for the renderer parameter, it is possible to rotate the output by multiples of 90 degrees. This can be set in two ways:

  • During initialization via the rotation parameter.
  • At run time by modifying the CogDrmPlatform.rotation object property.

In both cases the value is an integer in the [0, 3] range, which is the amount 90 degree counter-clockwise turns to apply. For example a value of 3 would result in 3 × 90 = 270 degrees.

The following example shows how to change the rotation at run time:

int main()
    g_autoptr(CogPlatform) plat = cog_platform_new("drm", NULL);

    g_autoptr(CogShell) shell = cog_shell_new("example", FALSE);
    cog_platform_setup(plat, shell, "renderer=gles", NULL);

    /* Set the rotation. */
    g_object_set(plat, "rotation", 3);

    /* Now proceed to run a GMainLoop normally… */