For a Linux driver meant for broad use and long-term support, aim to develop it in-tree if its purpose, license and maintainers meet kernel contribution requirements. Choose an out-of-tree module when the driver is experimental, product-specific or tied to a controlled kernel—and only if someone is responsible for keeping it compatible, building and signing it, and supporting its separate release path.
What in-tree and out-of-tree mean
An in-tree driver is part of the Linux kernel source. An out-of-tree driver is maintained separately and built as an external module, typically against the configured kernel build tree for the system where it will run. Linux supports this workflow through kbuild; it is a legitimate development and deployment route, not a shortcut around compatibility work. The kernel’s external-module documentation explains the required build process.
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The distinction is about who owns ongoing integration and compatibility work. Mainline code can be updated alongside kernel changes; a separate module’s owner must track those changes and adapt the module. Neither placement guarantees that a driver will work with every distribution or kernel release.
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| Decision factor | In-tree is a stronger fit when… | Out-of-tree is a stronger fit when… |
|---|---|---|
| Audience | You are targeting general Linux users and want distributions that enable the driver to ship it with their kernels. | The driver serves a narrow product, internal deployment or limited set of target systems. |
| Compatibility ownership | You want kernel maintainers and contributors to adapt the code as internal interfaces change. | Your team can fund and own compatibility work for the kernel versions it supports. |
| Release control | You can work through kernel contribution and review processes rather than dictate an independent release schedule. | You need to control the target kernel, module packaging and release cadence. |
| Contribution eligibility | The implementation can meet the kernel’s licensing and contribution requirements. | The code cannot currently be upstreamed, or its constraints require a separate delivery path. |
| Deployment operations | You want to avoid requiring each user to obtain and manage a separate driver build. | You can handle separate installation, kernel matching, signing and user support. |
These are decision axes, not guarantees. Upstream acceptance is not automatic, and distributions choose which drivers to enable. An out-of-tree module can be a sound choice when its scope is deliberately limited and its maintenance owner is clear.
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What you take on with an external module
Build against the target kernel
Build an external module with kbuild against the relevant configured kernel tree. During MODPOST, kbuild uses Module.symvers to validate references to exported symbols. The module’s Makefile can conceal much of the build detail from users, but it cannot remove the need to build for the intended kernel. The kernel documentation states: “Modules must use kbuild to stay compatible with changes in the build infrastructure and to pick up the right flags for the compiler.” See Building External Modules.
Plan for kernel changes
A successful build or load on one kernel does not establish compatibility with later releases. Linux does not promise a stable internal driver API: internal interfaces change, so separately maintained code needs continuing updates. The kernel documentation calls this out in The Linux Kernel Driver Interface and its development introduction. The latter states, “The internal kernel API is in constant flux.”
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In-tree placement does not make maintenance disappear. Its practical advantage is that code can be adapted as kernel changes land, rather than leaving each external module owner to discover and resolve incompatibilities independently.
Check module-signing policy before deployment
Whether an external module can load depends in part on the target kernel’s signing configuration. With restrictive signature enforcement, the module must carry a valid signature from a key that kernel trusts. In permissive configurations, unsigned modules or modules signed by an unknown key may load, but the kernel records an unsigned-module taint. Review the module-signing facility documentation and the target system’s actual policy before choosing a delivery process.
Understand what kernel taint records
Loading an externally built module sets the kernel’s O taint flag. Taint is diagnostic metadata intended to preserve context when investigating kernel problems; it does not prove that the module caused a fault. Other flags record different conditions: P indicates a proprietary module, while E indicates an unsigned module. The flags are distinct and may coexist. See Tainted kernels.
Upstreaming depends on purpose, licensing and contribution rules
The kernel’s licensing rules identify GPL-2.0 as the kernel-wide license and require source files to identify their license. The MODULE_LICENSE tag is loader metadata that tells the kernel and userspace tools whether a module is treated as free software or proprietary; it does not set or replace the source’s legal license. A proprietary-tagged module receives the P taint and cannot link against symbols exported with EXPORT_SYMBOL_GPL. Consult Linux kernel licensing rules.
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Kernel contributions must use a license compatible with GPLv2; contributions without a compatible license are not accepted. That requirement is separate from the technical ability to compile an external module. Read the kernel development introduction when assessing an upstream contribution.
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Upstreaming is most valuable when the driver has a broader user base and a realistic path to community review and long-term maintenance. If the driver is intentionally private or cannot meet contribution requirements, a separate module may be necessary—but the project should name who owns compatibility and deployment support.
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Rust drivers follow the same placement trade-off
Rust modules can also be developed out of tree. Rust for Linux warns that Rust internal APIs may change at any time and describes its focus as getting code into mainline. Do not treat a project branch or current external build as a promise of a stable external Rust-module interface. See Rust for Linux: Out-of-tree modules.
A practical placement checklist
- Target users: Is this for general Linux users, or a controlled set of product systems?
- Maintenance owner: Who will update the code when kernel internals or build infrastructure change?
- Kernel range: Can you limit supported kernels and test each target, or do you need reach across distributions?
- Contribution fit: Can the project use a compatible license and follow kernel review and contribution requirements?
- Deployment readiness: Can your release process build against the target kernel, satisfy signing policy and handle support requests?
- Distribution reach: Is it acceptable for each distribution to decide whether to enable and ship the driver?
If broad distribution and shared maintenance matter most, pursue in-tree development. If product constraints require separate delivery, choose out-of-tree only with an explicit compatibility and deployment plan.
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