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DRM Lease Manager

The DRM Lease Manager uses the DRM Lease feature, introduced in the Linux kernel version 4.15, to partition display controller output resources between multiple processes.

For more information on the DRM lease functionality, please see the blog posts published by Keith Packard, who developed the feature, at https://keithp.com/blogs/DRM-lease/

This repository contains a user space daemon to create and manage DRM leases, and distribute them to client applications. This implementation is independent of any window system / compositor, such as X11 or wayland, so can be used with clients that directly access a DRM device.

This repository also provides a library that clients can use to communicate with the DRM Lease Manager.

Building

Build this repository requires a recent version of the meson build system.

The basic build procedure is as follows:

meson <build_dir>
ninja -C <build_dir>
sudo ninja -C <build_dir> install

<build_dir> can be any directory name, but build is commonly used.

Configuration

The drm-lease-manager configuration file allows the user to specify the mapping of DRM connectors to DRM leases. The location of the configuration file can be specified with the --config command line option.

The drm-lease-manager has two lease modes. 1st one is a single-plane mode, and the other is a multi-plane mode.

A single-plane mode supports only one plane per output connector. On the other hand, it worked various DRM drivers.

A multi-plane mode supports one or more planes per output connector. On the other hand, this mode did not work some DRM driver such as rockchip without strict connector configuration.

The default is single-plane mode. When you want to use multi-plane mode, use --enable-universal-plane command line option.

The TOML configuration file consists of a list of lease definitions and a set of optional connector configurations.

Lease definitions

This configuration is required both single-plane mode and multi-plane mode.

Each list entry is of the following form:

[[lease]]    
name="My lease"
connectors=["HDMI-A-1", "LVDS-1"]
  • Note: quotes around all string values are mandatory. Additional leases can be configured by adding another entry to the [[lease]] list.

name

The name of the lease can be requested by lease clients.

connectors

A list of connector names to include in the lease. The above example uses HDMI and LVDS, but any supported DRM connector type may be used. If any of these connector names does not exist on the system, the lease will not be created.
A connector can be marked as optional in the connector configuration if necessary.

Connector configuration

This setting is optional not a mandatory. That setting is using in multi-plane mode.

Each connector that appears in the connectors field of a lease configuration can be additionally configured in a table with the connector name.
To continue the example above, the connector configuration could be as follows:

["HDMI-A-1"]
optional = true
planes = [31, 33, 35]

Note that leases are defined as a list named "leases" ([[lease]]), while connectors are defined as tables named after each connector (["connector 1"]).

optional

Creating a lease with a connector that is marked optional will not fail if the connector is unavailable. Lease creation will continue as if the connector name was omitted from the lease definition.

planes

planes defines the list of DRM planes that should be associated with each connector. These planes will be available in any lease that includes the connector. This setting can be used to assign DRM planes that can be shared between multiple connectors.

Default configuration

If no configuration file is specified one DRM lease will be created for each connector on the DRM device (up to the number of available CRTCs). All DRM planes that are used exclusively by the connector will also be added to the lease. To add DRM Planes that are shared with other connectors, a configuration file with a suitable connector configuration must be used.

The names of the DRM leases will have the following pattern:

<device>-<connector name>

So, for example, a DRM lease for the first LVDS device on the device /dev/dri/card0 would be named card0-LVDS-1.

Running

Once installed, running the following command will start the DRM Lease Manager daemon

drm-lease-manager [<path DRM device>]

If no DRM device is specified, the first available device capabale of modesetting will be used. More detailed options can be displayed by specifying the -h flag.

Dynamic lease transfer

When drm-lease-manager is started with the -t option, the ownership of a leases resources can be transferred from one client to another.

This allows the ownership of the leased resources to be transferred without the display being closed and the screen blanking. drm-lease-manager handles the timing of the transfer and manages the references to the DRM device, so that the last framebuffer of the old client stays on screen until the new client presents its first frame.

The transition can be completed without direct communication between the old and new client applications, however, the client that the lease will be transitioned from must be able to handle unexpected lease revocation. Once the lease is revoked, all DRM API calls referring to the DRM resources managed by the lease will fail with -ENOENT. The client should be able to gracefully handle this condition by, for example, pausing or shutting down its rendering operations.

Client API usage

The libdmclient handles all communication with the DRM Lease Manager and provides file descriptors that can be used as if the DRM device was opened directly. Clients only need to replace their calls to drmOpen() and drmClose() with the appropriate libdlmclient API calls.

The client library API is described in dlmclient.h in the libdlmclient directory.

If doxygen is available, building the repository will generate doxygen documentation in the <build_dir>/libdlmclient/docs/html directory.

Examples

Error handling has been omitted for brevity and clarity of examples.

Requesting a lease from the DRM Lease Manager

  struct dlm_lease *lease = dlm_get_lease("card0-HDMI-A-1");
  int drm_device_fd = dlm_lease_fd(lease);

drm_device_fd can now be used to access the DRM device

Releasing a lease

  dlm_release_lease(lease);

Note: drm_device_fd is not usable after calling dlm_release_lease()

Runtime directory

A runtime directory under the /var system directory is used by the drm-lease-manager and clients to communicate with each other.
The default path is /var/run/drm-lease-manager, but can be changed by setting the -Druntime_subdir option during configuration with meson.

The runtime directory can also be specified at runtime by setting the DLM_RUNTIME_PATH environment variable.