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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](https://mesonbuild.com/Getting-meson.html) 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 TOML configuration file consists of a list of lease definitions
and a set of optional connector configurations.
### Lease definitions
Each list entry is of the following form:
```toml
[[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
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:
```toml
["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
```c
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
```c
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.
|
