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DT Fragment Loader

A Linux Kernel Module for Advanced Device Tree Operations

Applying Fragments

Fragments are applied based on the "active fragments" that are passed into the module during init.

Active Fragments

The "active fragments" string should be a comma separated list of "fragment ids" to activate. These can either match location and compat properties of a fragment with the format l%d_c%d; or just a param string. For example: "l1_c3,l2_c4,custom_param".

The kernel commandline can provide a string as a module parameter, active_fragments="" or dt_fragment_loader.active_fragments="".

The device-tree can provide an active fragments string as a property at /dt-fragments/active-fragments.

We also detect duplicate fragment ids while parsing active fragments. A "duplicate fragment id" is defined as a fragment id that has either the same location or param value as another fragment id.

In event of "duplicate" fragment ids. The applied fragment id is chosen via the following precendence:

  1. The first fragment id provided by the kernel command line
  2. The first fragment id provided by the device-tree

Thus, this allows the device tree to provide a "default" for a location which may be overridden by the kernel command line.

Fragment Application Timing

Once this module is loaded, fragments are immediately applied.

Fragments are applied in the order that they are numbered in the dt-fragments node. e.g. fragment@0 is applied before fragment@1.

Operations are also applied in the same order via their numbering within their fragment. e.g. override@0 is applied before override@1.

Operations within fragments are continuously applied as they are parsed. Which means that the init of drivers for moved nodes may occur before fragment-loader is complete.

Writing Fragments

Fragment nodes are to be added to /dt-fragments.

Fragment Nodes

Fragments define a set of device tree modifications, and the "fragment id" that activates them.

Fragment node Property List

  • [node reg] - The reg value is used to define the order of which fragments should be applied.
  • [child nodes] - All children of a fragment should be operation nodes.
  • location - An arbitrary enumeration of a physical hardware location in a system.
  • compat - An arbitrary enumeration of compatible hardware that could be present at a physical location.
  • param - An arbitrary string for "fragment id" params that does not rely on location or compat enumerations.

Operation Nodes

Operation nodes are the child nodes under a fragment that define the device-tree modifications to perform.

All operations are described in detail under sub-headings below.

Generic Operation Node Property List

  • [node name] - The name of the node defines the type of operation to apply.
  • [node reg] - The reg value is used to define the order of which operations should be applied while applying a fragment.

The override Operation Node

The override operation provides the ability to copy and overwrite device-tree properties, and to move new child nodes.

Child nodes are moved to allow them to keep the same phandle without creating duplicate phandles. This means they are removed from the out-of-tree fragment when applied.

Further, child nodes can not overwrite an existing node. Attempting to do so will result in an error from fragment-loader. Modifying existing nodes should be done via modifying properties with seperate override operations.

Override Node Property List

  • [node name] - Must be override@N
  • target - A phandle reference of the in tree device-tree node to modify.
  • _overlay_ (child node) - The out-of-tree properties/nodes that are to be copied/moved into the "target" respectively.

Example /dt-fragment Node

/{
  dt-fragments {
    status = "okay";
    active-fragments = "l0_c4";

    fragment-component-name@0 {
      location = <0>;
      compat = <4>;
      param = "a_second_custom_enable_str";

      override@0 {
        target = <&target_node>;

        _overlay_ {
          status = "okay";
          arbitrary-prop = <23>;
          arbitrary-child-node@0 {
            status = "disabled";
          };
        };
      };
    };
  };
};