Modules (External projects)

Zephyr relies on the source code of several externally maintained projects in order to avoid reinventing the wheel and to reuse as much well-established, mature code as possible when it makes sense. In the context of Zephyr’s build system those are called modules. These modules must be integrated with the Zephyr build system, as described in more detail in other sections on this page.

To be classified as a candidate for being included in the default list of modules, an external project is required to have its own life-cycle outside the Zephyr Project, that is, reside in its own repository, and have its own contribution and maintenance workflow and release process. Zephyr modules should not contain code that is written exclusively for Zephyr. Instead, such code should be contributed to the main zephyr tree.

Modules to be included in the default manifest of the Zephyr project need to provide functionality or features endorsed and approved by the project Technical Steering Committee and should comply with the module licensing requirements and contribution guidelines. They should also have a Zephyr developer that is committed to maintain the module codebase.

Zephyr depends on several categories of modules, including but not limited to:

Debugger integration
Silicon vendor Hardware Abstraction Layers (HALs)
Cryptography libraries
File Systems
Inter-Process Communication (IPC) libraries

Additionally, in some cases modules (particularly vendor HALs) can contain references to optional binary blobs.

This page summarizes a list of policies and best practices which aim at better organizing the workflow in Zephyr modules.

Module Repositories

All modules included in the default manifest shall be hosted in repositories under the zephyrproject-rtos GitHub organization.
The module repository codebase shall include a module.yml file in a zephyr/ folder at the root of the repository.
Module repository names should follow the convention of using lowercase letters and dashes instead of underscores. This rule will apply to all new module repositories, except for repositories that are directly tracking external projects (hosted in Git repositories); such modules may be named as their external project counterparts.

Note

Existing module repositories that do not conform to the above convention do not need to be renamed to comply with the above convention.
Module repositories names should be explicitly set in the zephyr/module.yml file.
Modules should use “zephyr” as the default name for the repository main branch. Branches for specific purposes, for example, a module branch for an LTS Zephyr version, shall have names starting with the ‘zephyr_’ prefix.
If the module has an external (upstream) project repository, the module repository should preserve the upstream repository folder structure.

Note

It is not required in module repositories to maintain a ‘master’ branch mirroring the master branch of the external repository. It is not recommended as this may generate confusion around the module’s main branch, which should be ‘zephyr’.
Modules should expose all provided header files with an include pathname beginning with the module-name. (E.g., mcuboot should expose its bootutil/bootutil.h as “mcuboot/bootutil/bootutil.h”.)

Synchronizing with upstream

It is preferred to synchronize a module repository with the latest stable release of the corresponding external project. It is permitted, however, to update a Zephyr module repository with the latest development branch tip, if this is required to get important updates in the module codebase. When synchronizing a module with upstream it is mandatory to document the rationale for performing the particular update.

Requirements for allowed practices

Changes to the main branch of a module repository, including synchronization with upstream code base, may only be applied via pull requests. These pull requests shall be verifiable by Zephyr CI and mergeable (e.g. with the Rebase and merge, or Create a merge commit option using Github UI). This ensures that the incoming changes are always reviewable, and the downstream module repository history is incremental (that is, existing commits, tags, etc. are always preserved). This policy also allows to run Zephyr CI, git lint, identity, and license checks directly on the set of changes that are to be brought into the module repository.

Note

Force-pushing to a module’s main branch is not allowed.

Allowed practices

The following practices conform to the above requirements and should be followed in all modules repositories. It is up to the module code owner to select the preferred synchronization practice, however, it is required that the selected practice is consistently followed in the respective module repository.

Updating modules with a diff from upstream: Upstream changes brought as a single snapshot commit (manual diff) in a pull request against the module’s main branch, which may be merged using the Rebase & merge operation. This approach is simple and should be applicable to all modules with the downside of suppressing the upstream history in the module repository.

Note

The above practice is the only allowed practice in modules where the external project is not hosted in an upstream Git repository.

The commit message is expected to identify the upstream project URL, the version to which the module is updated (upstream version, tag, commit SHA, if applicable, etc.), and the reason for the doing the update.

Updating modules by merging the upstream branch: Upstream changes brought in by performing a Git merge of the intended upstream branch (e.g. main branch, latest release branch, etc.) submitting the result in pull request against the module main branch, and merging the pull request using the Create a merge commit operation. This approach is applicable to modules with an upstream project Git repository. The main advantages of this approach is that the upstream repository history (that is, the original commit SHAs) is preserved in the module repository. The downside of this approach is that two additional merge commits are generated in the downstream main branch.

Contributing to Zephyr modules

Individual Roles & Responsibilities

To facilitate management of Zephyr module repositories, the following individual roles are defined.

Administrator: Each Zephyr module shall have an administrator who is responsible for managing access to the module repository, for example, for adding individuals as Collaborators in the repository at the request of the module owner. Module administrators are members of the Administrators team, that is a group of project members with admin rights to module GitHub repositories.

Module owner: Each module shall have a module code owner. Module owners will have the overall responsibility of the contents of a Zephyr module repository. In particular, a module owner will:

coordinate code reviewing in the module repository
be the default assignee in pull-requests against the repository’s main branch
request additional collaborators to be added to the repository, as they see fit
regularly synchronize the module repository with its upstream counterpart following the policies described in Synchronizing with upstream
be aware of security vulnerability issues in the external project and update the module repository to include security fixes, as soon as the fixes are available in the upstream code base
list any known security vulnerability issues, present in the module codebase, in Zephyr release notes.

Note

Module owners are not required to be Zephyr Maintainers.

Merger: The Zephyr Release Engineering team has the right and the responsibility to merge approved pull requests in the main branch of a module repository.

Maintaining the module codebase

Updates in the zephyr main tree, for example, in public Zephyr APIs, may require patching a module’s codebase. The responsibility for keeping the module codebase up to date is shared between the contributor of such updates in Zephyr and the module owner. In particular:

the contributor of the original changes in Zephyr is required to submit the corresponding changes that are required in module repositories, to ensure that Zephyr CI on the pull request with the original changes, as well as the module integration testing are successful.
the module owner has the overall responsibility for synchronizing and testing the module codebase with the zephyr main tree. This includes occasional advanced testing of the module’s codebase in addition to the testing performed by Zephyr’s CI. The module owner is required to fix issues in the module’s codebase that have not been caught by Zephyr pull request CI runs.

Contributing changes to modules

Submitting and merging changes directly to a module’s codebase, that is, before they have been merged in the corresponding external project repository, should be limited to:

changes required due to updates in the zephyr main tree
urgent changes that should not wait to be merged in the external project first, such as fixes to security vulnerabilities.

Non-trivial changes to a module’s codebase, including changes in the module design or functionality should be discouraged, if the module has an upstream project repository. In that case, such changes shall be submitted to the upstream project, directly.

Submitting changes to modules describes in detail the process of contributing changes to module repositories.

Contribution guidelines

Contributing to Zephyr modules shall follow the generic project Contribution guidelines.

Pull Requests: may be merged with minimum of 2 approvals, including an approval by the PR assignee. In addition to this, pull requests in module repositories may only be merged if the introduced changes are verified with Zephyr CI tools, as described in more detail in other sections on this page.

The merging of pull requests in the main branch of a module repository must be coupled with the corresponding manifest file update in the zephyr main tree.

Issue Reporting: GitHub issues are intentionally disabled in module repositories, in favor of a centralized policy for issue reporting. Tickets concerning, for example, bugs or enhancements in modules shall be opened in the main zephyr repository. Issues should be appropriately labeled using GitHub labels corresponding to each module, where applicable.

Note

It is allowed to file bug reports for zephyr modules to track the corresponding upstream project bugs in Zephyr. These bug reports shall not affect the Release Quality Criteria.

Licensing requirements and policies

All source files in a module’s codebase shall include a license header, unless the module repository has main license file that covers source files that do not include license headers.

Main license files shall be added in the module’s codebase by Zephyr developers, only if they exist as part of the external project, and they contain a permissive OSI-compliant license. Main license files should preferably contain the full license text instead of including an SPDX license identifier. If multiple main license files are present it shall be made clear which license applies to each source file in a module’s codebase.

Individual license headers in module source files supersede the main license.

Any new content to be added in a module repository will require to have license coverage.

Note

Zephyr recommends conveying module licensing via individual license headers and main license files. This not a hard requirement; should an external project have its own practice of conveying how licensing applies in the module’s codebase (for example, by having a single or multiple main license files), this practice may be accepted by and be referred to in the Zephyr module, as long as licensing requirements, for example OSI compliance, are satisfied.

License policies

When creating a module repository a developer shall:

import the main license files, if they exist in the external project, and
document (for example in the module README or .yml file) the default license that covers the module’s codebase.

License checks

License checks (via CI tools) shall be enabled on every pull request that adds new content in module repositories.

Documentation requirements

All Zephyr module repositories shall include an .rst file documenting:

the scope and the purpose of the module
how the module integrates with Zephyr
the owner of the module repository
synchronization information with the external project (commit, SHA, version etc.)
licensing information as described in Licensing requirements and policies.

The file shall be required for the inclusion of the module and the contained information should be kept up to date.

Testing requirements

All Zephyr modules should provide some level of integration testing, ensuring that the integration with Zephyr works correctly. Integration tests:

may be in the form of a minimal set of samples and tests that reside in the zephyr main tree
should verify basic usage of the module (configuration, functional APIs, etc.) that is integrated with Zephyr.
shall be built and executed (for example in QEMU) as part of twister runs in pull requests that introduce changes in module repositories.

Note

New modules, that are candidates for being included in the Zephyr default manifest, shall provide some level of integration testing.

Note

Vendor HALs are implicitly tested via Zephyr tests built or executed on target platforms, so they do not need to provide integration tests.

The purpose of integration testing is not to provide functional verification of the module; this should be part of the testing framework of the external project.

Certain external projects provide test suites that reside in the upstream testing infrastructure but are written explicitly for Zephyr. These tests may (but are not required to) be part of the Zephyr test framework.

Deprecating and removing modules

Modules may be deprecated for reasons including, but not limited to:

Lack of maintainership in the module
Licensing changes in the external project
Codebase becoming obsolete

The module information shall indicate whether a module is deprecated and the build system shall issue a warning when trying to build Zephyr using a deprecated module.

Deprecated modules may be removed from the Zephyr default manifest after 2 Zephyr releases.

Note

Repositories of removed modules shall remain accessible via their original URL, as they are required by older Zephyr versions.

Integrate modules in Zephyr build system

The build system variable ZEPHYR_MODULES is a CMake list of absolute paths to the directories containing Zephyr modules. These modules contain CMakeLists.txt and Kconfig files describing how to build and configure them, respectively. Module CMakeLists.txt files are added to the build using CMake’s add_subdirectory() command, and the Kconfig files are included in the build’s Kconfig menu tree.

If you have west installed, you don’t need to worry about how this variable is defined unless you are adding a new module. The build system knows how to use west to set ZEPHYR_MODULES. You can add additional modules to this list by setting the ZEPHYR_EXTRA_MODULES CMake variable or by adding a ZEPHYR_EXTRA_MODULES line to .zephyrrc (See the section on Environment Variables for more details). This can be useful if you want to keep the list of modules found with west and also add your own.

Note

If the module FOO is provided by west but also given with -DZEPHYR_EXTRA_MODULES=/<path>/foo then the module given by the command line variable ZEPHYR_EXTRA_MODULES will take precedence. This allows you to use a custom version of FOO when building and still use other Zephyr modules provided by west. This can for example be useful for special test purposes.

If you want to permanently add modules to the zephyr workspace and you are using zephyr as your manifest repository, you can also add a west manifest file into the submanifests directory. See submanifests/README.txt for more details.

See Basics for more on west workspaces.

Finally, you can also specify the list of modules yourself in various ways, or not use modules at all if your application doesn’t need them.

Module yaml file description

A module can be described using a file named zephyr/module.yml. The format of zephyr/module.yml is described in the following:

Module name

Each Zephyr module is given a name by which it can be referred to in the build system.

The name should be specified in the zephyr/module.yml file. This will ensure the module name is not changeable through user-defined directory names or west manifest files:

name: <name>

In CMake the location of the Zephyr module can then be referred to using the CMake variable ZEPHYR_<MODULE_NAME>_MODULE_DIR and the variable ZEPHYR_<MODULE_NAME>_CMAKE_DIR holds the location of the directory containing the module’s CMakeLists.txt file.

Note

When used for CMake and Kconfig variables, all letters in module names are converted to uppercase and all non-alphanumeric characters are converted to underscores (_). As example, the module foo-bar must be referred to as ZEPHYR_FOO_BAR_MODULE_DIR in CMake and Kconfig.

Here is an example for the Zephyr module foo:

name: foo

Note

If the name field is not specified then the Zephyr module name will be set to the name of the module folder. As example, the Zephyr module located in <workspace>/modules/bar will use bar as its module name if nothing is specified in zephyr/module.yml.

Module integration files (in-module)

Inclusion of build files, CMakeLists.txt and Kconfig, can be described as:

build:
  cmake: <cmake-directory>
  kconfig: <directory>/Kconfig

The cmake: <cmake-directory> part specifies that <cmake-directory> contains the CMakeLists.txt to use. The kconfig: <directory>/Kconfig part specifies the Kconfig file to use. Neither is required: cmake defaults to zephyr, and kconfig defaults to zephyr/Kconfig.

Here is an example module.yml file referring to CMakeLists.txt and Kconfig files in the root directory of the module:

build:
  cmake: .
  kconfig: Kconfig

Build system integration

When a module has a module.yml file, it will automatically be included into the Zephyr build system. The path to the module is then accessible through Kconfig and CMake variables.

In both Kconfig and CMake, the variable ZEPHYR_<MODULE_NAME>_MODULE_DIR contains the absolute path to the module.

In CMake, ZEPHYR_<MODULE_NAME>_CMAKE_DIR contains the absolute path to the directory containing the CMakeLists.txt file that is included into CMake build system. This variable’s value is empty if the module.yml file does not specify a CMakeLists.txt.

To read these variables for a Zephyr module named foo:

In CMake: use ${ZEPHYR_FOO_MODULE_DIR} for the module’s top level directory, and ${ZEPHYR_FOO_CMAKE_DIR} for the directory containing its CMakeLists.txt
In Kconfig: use $(ZEPHYR_FOO_MODULE_DIR) for the module’s top level directory

Notice how a lowercase module name foo is capitalized to FOO in both CMake and Kconfig.

These variables can also be used to test whether a given module exists. For example, to verify that foo is the name of a Zephyr module:

if(ZEPHYR_FOO_MODULE_DIR)
  # Do something if FOO exists.
endif()

In Kconfig, the variable may be used to find additional files to include. For example, to include the file some/Kconfig in module foo:

source "$(ZEPHYR_FOO_MODULE_DIR)/some/Kconfig"

During CMake processing of each Zephyr module, the following two variables are also available:

the current module’s top level directory: ${ZEPHYR_CURRENT_MODULE_DIR}
the current module’s CMakeLists.txt directory: ${ZEPHYR_CURRENT_CMAKE_DIR}

This removes the need for a Zephyr module to know its own name during CMake processing. The module can source additional CMake files using these CURRENT variables. For example:

include(${ZEPHYR_CURRENT_MODULE_DIR}/cmake/code.cmake)

It is possible to append values to a Zephyr CMake list variable from the module’s first CMakeLists.txt file. To do so, append the value to the list and then set the list in the PARENT_SCOPE of the CMakeLists.txt file. For example, to append bar to the FOO_LIST variable in the Zephyr CMakeLists.txt scope:

list(APPEND FOO_LIST bar)
set(FOO_LIST ${FOO_LIST} PARENT_SCOPE)

An example of a Zephyr list where this is useful is when adding additional directories to the SYSCALL_INCLUDE_DIRS list.

Zephyr module dependencies

A Zephyr module may be dependent on other Zephyr modules to be present in order to function correctly. Or it might be that a given Zephyr module must be processed after another Zephyr module, due to dependencies of certain CMake targets.

Such a dependency can be described using the depends field.

build:
  depends:
    - <module>

Here is an example for the Zephyr module foo that is dependent on the Zephyr module bar to be present in the build system:

name: foo
build:
  depends:
  - bar

This example will ensure that bar is present when foo is included into the build system, and it will also ensure that bar is processed before foo.

Module integration files (external)

Module integration files can be located externally to the Zephyr module itself. The MODULE_EXT_ROOT variable holds a list of roots containing integration files located externally to Zephyr modules.

Module integration files in Zephyr

The Zephyr repository contain CMakeLists.txt and Kconfig build files for certain known Zephyr modules.

Those files are located under

<ZEPHYR_BASE>
└── modules
    └── <module_name>
        ├── CMakeLists.txt
        └── Kconfig

Module integration files in a custom location

You can create a similar MODULE_EXT_ROOT for additional modules, and make those modules known to Zephyr build system.

Create a MODULE_EXT_ROOT with the following structure

<MODULE_EXT_ROOT>
└── modules
    ├── modules.cmake
    └── <module_name>
        ├── CMakeLists.txt
        └── Kconfig

and then build your application by specifying -DMODULE_EXT_ROOT parameter to the CMake build system. The MODULE_EXT_ROOT accepts a CMake list of roots as argument.

A Zephyr module can automatically be added to the MODULE_EXT_ROOT list using the module description file zephyr/module.yml, see Build settings.

Note

ZEPHYR_BASE is always added as a MODULE_EXT_ROOT with the lowest priority. This allows you to overrule any integration files under <ZEPHYR_BASE>/modules/<module_name> with your own implementation your own MODULE_EXT_ROOT.

The modules.cmake file must contain the logic that specifies the integration files for Zephyr modules via specifically named CMake variables.

To include a module’s CMake file, set the variable ZEPHYR_<MODULE_NAME>_CMAKE_DIR to the path containing the CMake file.

To include a module’s Kconfig file, set the variable ZEPHYR_<MODULE_NAME>_KCONFIG to the path to the Kconfig file.

The following is an example on how to add support the the FOO module.

Create the following structure

<MODULE_EXT_ROOT>
└── modules
    ├── modules.cmake
    └── foo
        ├── CMakeLists.txt
        └── Kconfig

and inside the modules.cmake file, add the following content

set(ZEPHYR_FOO_CMAKE_DIR ${CMAKE_CURRENT_LIST_DIR}/foo)
set(ZEPHYR_FOO_KCONFIG   ${CMAKE_CURRENT_LIST_DIR}/foo/Kconfig)

Module integration files (zephyr/module.yml)

The module description file zephyr/module.yml can be used to specify that the build files, CMakeLists.txt and Kconfig, are located in a Module integration files (external).

Build files located in a MODULE_EXT_ROOT can be described as:

build:
  cmake-ext: True
  kconfig-ext: True

This allows control of the build inclusion to be described externally to the Zephyr module.

The Zephyr repository itself is always added as a Zephyr module ext root.

Build settings

It is possible to specify additional build settings that must be used when including the module into the build system.

All root settings are relative to the root of the module.

Build settings supported in the module.yml file are:

board_root: Contains additional boards that are available to the build system. Additional boards must be located in a <board_root>/boards folder.
dts_root: Contains additional dts files related to the architecture/soc families. Additional dts files must be located in a <dts_root>/dts folder.
soc_root: Contains additional SoCs that are available to the build system. Additional SoCs must be located in a <soc_root>/soc folder.
arch_root: Contains additional architectures that are available to the build system. Additional architectures must be located in a <arch_root>/arch folder.
module_ext_root: Contains CMakeLists.txt and Kconfig files for Zephyr modules, see also Module integration files (external).

Example of a module.yaml file containing additional roots, and the corresponding file system layout.

build:
  settings:
    board_root: .
    dts_root: .
    soc_root: .
    arch_root: .
    module_ext_root: .

requires the following folder structure:

<zephyr-module-root>
├── arch
├── boards
├── dts
├── modules
└── soc

Twister (Test Runner)

To execute both tests and samples available in modules, the Zephyr test runner (twister) should be pointed to the directories containing those samples and tests. This can be done by specifying the path to both samples and tests in the zephyr/module.yml file. Additionally, if a module defines out of tree boards, the module file can point twister to the path where those files are maintained in the module. For example:

build:
  cmake: .
samples:
  - samples
tests:
  - tests
boards:
  - boards

Binary Blobs

Zephyr supports fetching and using binary blobs, and their metadata is contained entirely in zephyr/module.yml. This is because a binary blob must always be associated with a Zephyr module, and thus the blob metadata belongs in the module’s description itself.

Binary blobs are fetched using west blobs. If west is not used, they must be downloaded and verified manually.

The blobs section in zephyr/module.yml consists of a sequence of maps, each of which has the following entries:

path: The path to the binary blob, relative to the zephyr/blobs/ folder in the module repository
sha256: SHA-256 checksum of the binary blob file
type: The type of binary blob. Currently limited to img or lib
version: A version string
license-path: Path to the license file for this blob, relative to the root of the module repository
url: URL that identifies the location the blob will be fetched from, as well as the fetching scheme to use
description: Human-readable description of the binary blob
doc-url: A URL pointing to the location of the official documentation for this blob

Module Inclusion

Using West

If west is installed and ZEPHYR_MODULES is not already set, the build system finds all the modules in your west installation and uses those. It does this by running west list to get the paths of all the projects in the installation, then filters the results to just those projects which have the necessary module metadata files.

Each project in the west list output is tested like this:

If the project contains a file named zephyr/module.yml, then the content of that file will be used to determine which files should be added to the build, as described in the previous section.
Otherwise (i.e. if the project has no zephyr/module.yml), the build system looks for zephyr/CMakeLists.txt and zephyr/Kconfig files in the project. If both are present, the project is considered a module, and those files will be added to the build.
If neither of those checks succeed, the project is not considered a module, and is not added to ZEPHYR_MODULES.

Without West

If you don’t have west installed or don’t want the build system to use it to find Zephyr modules, you can set ZEPHYR_MODULES yourself using one of the following options. Each of the directories in the list must contain either a zephyr/module.yml file or the files zephyr/CMakeLists.txt and Kconfig, as described in the previous section.

At the CMake command line, like this:

cmake -DZEPHYR_MODULES=<path-to-module1>[;<path-to-module2>[...]] ...

At the top of your application’s top level CMakeLists.txt, like this:
```
set(ZEPHYR_MODULES <path-to-module1> <path-to-module2> [...])
find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
```
If you choose this option, make sure to set the variable before calling find_package(Zephyr ...), as shown above.
In a separate CMake script which is pre-loaded to populate the CMake cache, like this:
```
# Put this in a file with a name like "zephyr-modules.cmake"
set(ZEPHYR_MODULES <path-to-module1> <path-to-module2>
  CACHE STRING "pre-cached modules")
```
You can tell the build system to use this file by adding -C zephyr-modules.cmake to your CMake command line.

Not using modules

If you don’t have west installed and don’t specify ZEPHYR_MODULES yourself, then no additional modules are added to the build. You will still be able to build any applications that don’t require code or Kconfig options defined in an external repository.

Submitting changes to modules

When submitting new or making changes to existing modules the main repository Zephyr needs a reference to the changes to be able to verify the changes. In the main tree this is done using revisions. For code that is already merged and part of the tree we use the commit hash, a tag, or a branch name. For pull requests however, we require specifying the pull request number in the revision field to allow building the zephyr main tree with the changes submitted to the module.

To avoid merging changes to master with pull request information, the pull request should be marked as DNM (Do Not Merge) or preferably a draft pull request to make sure it is not merged by mistake and to allow for the module to be merged first and be assigned a permanent commit hash. Once the module is merged, the revision will need to be changed either by the submitter or by the maintainer to the commit hash of the module which reflects the changes.

Note that multiple and dependent changes to different modules can be submitted using exactly the same process. In this case you will change multiple entries of all modules that have a pull request against them.

Process for submitting a new module

Please follow the process in Submission and review process and obtain the TSC approval to integrate the external source code as a module

If the request is approved, a new repository will created by the project team and initialized with basic information that would allow submitting code to the module project following the project contribution guidelines.

If a module is maintained as a fork of another project on Github, the Zephyr module related files and changes in relation to upstream need to be maintained in a special branch named zephyr.

Maintainers from the Zephyr project will create the repository and initialize it. You will be added as a collaborator in the new repository. Submit the module content (code) to the new repository following the guidelines described here, and then add a new entry to the west.yml with the following information:

- name: <name of repository>
  path: <path to where the repository should be cloned>
  revision: <ref pointer to module pull request>

For example, to add my_module to the manifest:

- name: my_module
  path: modules/lib/my_module
  revision: pull/23/head

Where 23 in the example above indicated the pull request number submitted to the my_module repository. Once the module changes are reviewed and merged, the revision needs to be changed to the commit hash from the module repository.

Process for submitting changes to existing modules

Submit the changes using a pull request to an existing repository following the contribution guidelines.

Submit a pull request changing the entry referencing the module into the west.yml of the main Zephyr tree with the following information:

- name: <name of repository>
  path: <path to where the repository should be cloned>
  revision: <ref pointer to module pull request>

For example, to add my_module to the manifest:

- name: my_module
  path: modules/lib/my_module
  revision: pull/23/head

Where 23 in the example above indicated the pull request number submitted to the my_module repository. Once the module changes are reviewed and merged, the revision needs to be changed to the commit hash from the module repository.