The Linux visl driver is a virtual V4L2 stateless-decoder test device. It lets a userspace client submit codec controls, queues and buffers so you can inspect API behavior without a physical decoder, but it does not decode pictures. Its capture buffers contain test-pattern diagnostics rather than reconstructed video frames.
What is the Linux visl driver?
visl (the Virtual Stateless Decoder Driver) is a Linux kernel media driver for developing and testing userspace implementations of the V4L2 stateless decoding API. The kernel presents it as a decoder-like video device, allowing an application to run its normal request, queue and dequeue loop even when the target machine has no supported decoder hardware. The authoritative description is in the Linux kernel visl documentation.
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The driver is useful because a stateless client has to do more than feed compressed bytes to a device. It must parse the bitstream, assemble codec-specific parameter structures, place those structures in V4L2 controls, queue the appropriate buffers and react to device events. visl gives that client a repeatable endpoint for exercising those interactions and for inspecting what the kernel receives.
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How stateful and stateless V4L2 decoding differ
Understanding the two V4L2 models explains why visl exists. Both models normally use a memory-to-memory device with two queues, but the client’s responsibilities are different.
| Aspect | Stateful decoder | Stateless decoder |
|---|---|---|
| Input supplied by the application | Complete chunks of the compressed bytestream | Parsed codec data plus codec-specific controls accompanying the decode work |
| Who maintains codec parsing state | The decoder is expected to derive the needed state from the bytestream | The userspace client parses the stream and supplies the relevant metadata and parameters |
| V4L2 queue names | OUTPUT for encoded input and CAPTURE for decoded output |
The same queue convention, with controls describing each decode operation |
| Typical implementation described by the kernel API | Memory-to-memory decoder workflow | Most often a hardware-accelerator workflow |
| What visl targets | Not a stateful-decoder compatibility test | A virtual device specifically for stateless API development and testing |
The V4L2 stateful decoder interface documentation describes the stateful workflow in which the application supplies encoded buffers and receives decoded frames. The stateless codec control reference documents the controls used to carry parsed information.
What a stateless client must manage
A stateless client generally has to identify codec boundaries and parse syntax elements before submitting a decode request. Depending on the codec, that can include sequence, picture and slice information. For H.264, for example, the control API represents structures such as sequence parameter sets, picture parameter sets and slice parameters. The exact structures and flags are codec-specific and are defined by the kernel’s stateless control reference.
The application also has to maintain the relationship between the controls and the buffers they describe. A control set for one picture must be associated with the correct encoded data and destination surface. This explicit association is the reason a stateless client can expose bugs that a stateful client would hide inside the decoder.
Does visl actually decode video?
No. visl never reconstructs a video frame from compressed data. Its capture queue receives diagnostic output generated by the virtual test-pattern generator. The output can show that a request reached the driver, that controls were accepted and that queue processing occurred, but it is not a viewable rendition of the source movie.
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Do not use a visl capture buffer to judge:
- Pixel correctness, image quality or reference-frame handling.
- Whether a physical decoder can decode the stream.
- Decode throughput, latency, power use or thermal behavior.
- Compatibility with a particular GPU, media accelerator or vendor kernel.
Those questions require a real decoder implementation and, for image-quality checks, a way to compare its reconstructed frames with a known reference. visl can complement that testing by isolating userspace/API behavior before hardware is available, but it cannot replace hardware validation.
Which codecs does visl support?
The current Linux kernel documentation under the latest path lists seven codec families. The list is documentation for the current kernel tree consulted on October 7, 2026, not a promise that every released or vendor kernel exposes all of them.
| Codec family listed by current visl documentation | What to verify on a target system |
|---|---|
| FWHT | That the target kernel’s visl build includes the FWHT format and controls |
| MPEG-2 | The MPEG-2 control set and pixel formats exposed by that kernel |
| VP8 | VP8 controls and the device’s advertised formats |
| VP9 | VP9 controls, profile handling and formats |
| H.264 | H.264 parameter controls and the stream features your client submits |
| HEVC | HEVC controls, including the sequence-parameter information used in tracing examples |
| AV1 | AV1 controls and any kernel-version-specific requirements |
Before writing a version-specific claim into a test plan, check the documentation and media-device capabilities for the exact kernel you will run. The latest page can describe a newer API surface than an LTS or vendor kernel.
How to test a stateless client with visl
Use visl to validate the mechanics of your client, not its decoded pixels. A practical workflow is:
- Build or obtain a kernel with visl enabled. The virtual device must be present in the running kernel, and its media functionality must be available to the userspace environment. Confirm that the device appears through the normal V4L2/media-device discovery used by your application.
- Choose a codec supported by that kernel. Start with the codec whose controls your client already handles. Compare the target kernel’s advertised controls and formats with the definitions in the stateless control reference.
- Point the client at the virtual device. Run the same stateless decode loop you intend to use with hardware. The application should configure the
OUTPUTqueue for encoded input, configure theCAPTUREqueue for the destination format, submit codec controls and queue buffers in its normal order. - Feed a known bitstream. Use a small, reproducible stream that your parser can identify reliably. The purpose is to make control submissions and buffer ordering easy to compare between runs, not to obtain a playable file from visl.
- Observe events and completions. Record which controls are submitted for each buffer, which buffers are returned and how the client handles format or end-of-stream events. A client that only works with one physical driver often reveals assumptions here.
- Enable the relevant visl trace events. The driver documents codec-specific trace groups. Enable the group for the codec under test and inspect the resulting records to see the controls and values received by the virtual device. The documentation includes a HEVC sequence-parameter-set tracing example.
- Inspect debugfs when buffer contents matter. visl provides a debugfs interface for examining
OUTPUTbuffer data. Driver parameters determine whether bitstream buffers are retained and which frames are dumped; select those settings deliberately because retaining every buffer can produce substantial diagnostic data. - Compare implementations. Run a known-good client and your developing client with the same stream and tracing configuration. Differences in control presence, array sizes, buffer references or queue sequencing are often easier to spot in visl logs than on hardware.
Keep the generated capture data as diagnostic reference material. It is not decoded video, and it should not be passed to a player as though it were the source stream’s picture output.
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Tracing V4L2 codec controls
Tracing is visl’s most valuable capability for a userspace developer. A stateless request can contain many fields that are difficult to inspect from application logs alone. The driver’s per-codec trace events expose what reached the kernel, letting you check whether your parser’s result matches the control payload the API expects.
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Select the trace group that corresponds to the codec being tested. For HEVC, the kernel guide demonstrates tracing a sequence parameter set; equivalent codec-specific groups are available for the other supported families where documented. Capture the trace while submitting one or a few frames, then correlate each record with your application’s request identifier or buffer index.
Interpret differences carefully
- A missing trace record can indicate that the request was never queued, that tracing was enabled for the wrong codec group or that the device rejected the operation earlier.
- A field mismatch usually points to parser or structure-population logic, but confirm the kernel version before assuming the layout is identical.
- Values that look unstable may be pointers, queue addresses or other diagnostic details rather than codec semantics.
When tpg_verbose is enabled, visl adds details such as pointers or queue status. Those details make the generated output less stable for the same input, so use verbose mode when investigating a specific failure and disable it when you need compact, repeatable logs.
What visl can prove—and what it cannot
A passing visl run can establish that your application can discover the virtual device, negotiate a supported format, submit the expected stateless controls, queue buffers and handle the resulting V4L2 events. It is especially useful for continuous integration, regression tests and development on machines without decoder hardware.
It cannot establish decode correctness, image quality, performance, power consumption or physical-hardware compatibility. Since no frame decoding occurs, a client can pass visl while still containing a bad motion-vector interpretation, an incorrect reference-frame lifetime or a hardware-specific synchronization bug. Conversely, a failure on visl may expose an API-contract problem even if a permissive hardware driver happened to accept the same client.
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Kernel-version and deployment cautions
Documentation and driver behavior are version-sensitive. The codec list above comes from the current latest visl guide, while the stateful decoder reference cited here is explicitly for Linux v6.1. A vendor kernel may backport controls, omit a codec, rename a trace event or carry local changes. Record the kernel release, configuration and media-device capabilities alongside test results.
Do not infer support merely from a header file on the build machine. The running kernel must expose the device and controls, and the application must negotiate formats actually reported by that device. Treat a capability query and a short traced run as the authoritative check for the environment in which your client will ship.
Common failure modes and useful checks
The application cannot find a decoder device
Check that visl is enabled and loaded in the running kernel, then inspect the media/V4L2 device inventory used by your discovery code. A userspace binary built with stateless support cannot create the virtual device if the kernel configuration does not include it.
Controls are rejected
Verify that the controls belong to the codec negotiated on the device and that their dimensions, flags and referenced buffers match the target kernel’s stateless control definitions. Trace the request to distinguish a parser-population error from a queue-ordering error.
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Capture buffers contain unfamiliar patterns
This is expected. visl writes diagnostic test-pattern information rather than decoded pictures. Use traces and buffer metadata to assess the API transaction; use a real decoder for visual output.
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Two runs produce different verbose output
Disable tpg_verbose or compare only stable fields. The documentation warns that extra pointers and queue-status details can vary even for identical input.
A client works on visl but fails on hardware
Move to hardware-specific testing. Check synchronization, supported profiles, memory layout, reference-frame management and timing behavior, none of which visl’s no-decode path can validate.
Where visl fits in a development strategy
Use visl early, when the primary risk is an incorrect userspace interpretation of the V4L2 stateless API. It is a low-dependency environment for testing control construction, queue transitions, event handling and diagnostic logging. Once those interactions are stable, run the same client against each physical decoder and add tests for reconstructed-frame correctness, format conversion, throughput and power.
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