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.

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… */