What can compositor do with DRM devices once it has found them?

These devices respond to a number of ioctls. All DRM devices respond to a set of common ioctls that cover display controller functionality, and there are driver-specific ioctls used chiefly by Mesa for rendering operations. In addition, kernel writes data to open DRM file descriptors whenever an event happened and userspace has requested to know about it.

The list of ioctls is pretty large, so this post covers only a small subset.

Versions

DRM_IOCTL_VERSION returns human-readable version, name, description and (release?) date of DRM device.

Device capabilities

DRM_IOCTL_GET_CAP returns capabilities of a DRM device.

Many of these capabilities refer to specific DRM ioctls, so they may not make much sense yet.

Framebuffer capabilities

  • DRM_CAP_ADDFB2_MODIFIERS. boolean. Framebuffers support pixel format modifiers.
  • DRM_CAP_DUMB_BUFFER. boolean. The device can create dumb framebuffers (DRM_IOCTL_MODE_CREATE_DUMB et al)
  • DRM_CAP_DUMB_PREFERRED_DEPTH. integer. The preferred pixel depth of dumb framebuffers.
  • DRM_CAP_DUMB_PREFER_SHADOW. boolean. Non-sequential writes or reading from dumb framebuffer is slow.

Syncobj capabilities

  • DRM_CAP_SYNCOBJ. boolean. The device supports DRM sync objects.
  • DRM_CAP_SYNCOBJ_TIMELINE. boolean. The device supports DRM timeline sync objects (they can signal at specific points in their lifetimes).

Page flipping capabilities

  • DRM_CAP_ASYNC_PAGE_FLIP. boolean. The device can pageflip without delay (tearing pageflip).
  • DRM_CAP_PAGE_FLIP_TARGET. boolean. The device can pageflip with a specified target (DRM_MODE_PAGE_FLIP_TARGET_{ABSOLUTE,RELATIVE}).

Other capabilities

  • DRM_CAP_CURSOR_HEIGHT, DRM_CAP_CURSOR_WIDTH integer. return valid (not necessary the maximum) hardware cursor size.
  • DRM_CAP_PRIME. bitfield. The device can export buffers to other GPUs and/or import buffers from them.

Obsolete capabilities

  • DRM_CAP_CRTC_IN_VBLANK_EVENT. boolean. Always true since Linux 4.12.
  • DRM_CAP_TIMESTAP_MONOTONIC. boolean. Always true since Linux 4.15.
  • DRM_CAP_VBLANK_HIGH_CRTC. boolean. Always true since Linux 2.6.39

Client capabilities

DRM client may indicate it has certain capabilites. This is necessary to activate backward-incompatible functionality that would otherwise break old clients.

Client capabilities are set using DRM_IOCTL_SET_CLIENT_CAP.

  • DRM_CLIENT_CAP_STEREO_3D. boolean. Client can handle stereo 3D, so show it the corresponding modes in the list of modes.
  • DRM_CLIENT_CAP_UNIVERSAL_PLANES. integer1. If > 0 then client can handle multiple planes, show it planes beyond primary and cursor ones in the list of planes.
  • DRM_CLIENT_CAP_ATOMIC. boolean. Client can use atomic modesetting ioctls. Also enables universal planes and aspect ratio.
  • DRM_CLIENT_CAP_ASPECT_RATIO. boolean. Client can handle aspect ratio in modes, so provide it.
  • DRM_CLIENT_CAP_WRITEBACK_CONNECTORS. boolean. Client can handle writeback connectors (connectors that capture output and save it into a framebuffer).

References

  • DRM uAPI
  • Linux drivers/gpu/drm/drm_ioctl.c.

Footnotes

  1. Xserver modesetting DDX is completely broken: it asks for universal planes, but is not ready to handle them. To avoid blowing up users' desktops, any process with name starting with X is required to set this capability to 2 to really enable universal planes. Yes, really. ↩︎