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visl is a virtual Linux V4L2 stateless-decoder driver for developing and diagnosing userspace clients, not a video decoder. It accepts the codec controls and parsed data that a stateless client would submit, then writes test-pattern diagnostics to capture buffers. That makes it useful for checking queue handling, control construction, buffer lifetime, and tracing when a physical accelerator is unavailable. It cannot show whether a compressed stream produces correct pictures, how fast a decoder runs, or whether a particular hardware block will accept the same workload.
The Linux kernel documentation lists support in the current latest guide for FWHT, MPEG-2, VP8, VP9, H.264, HEVC, and AV1. The list is documentation for the current kernel tree, not a promise that every released or vendor kernel has all of those implementations. Always check the documentation and driver in the kernel you are targeting.
What is the Linux visl driver?
Video4Linux2 (V4L2) exposes userspace interfaces for video devices. The virtual stateless decoder driver, commonly called visl, implements a software device whose purpose is API development and testing. A media application can run its normal stateless-decoder loop against visl, submit codec controls and buffers, and observe how the application interacts with the kernel without first obtaining a decoder card or system-on-chip.
The driver is described in the Linux kernel’s Virtual Stateless Decoder Driver (visl) guide. Its capture side contains a V4L2 test-pattern generator rather than decoded pictures. As the official documentation states: “No actual decoding of video frames is performed by visl.” The generated data is therefore a diagnostic artifact: it can identify which request, buffer, or control reached the driver, but it is not a viewable reconstruction of the input movie.
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This distinction determines the right use case. visl is a virtual development and test endpoint for a userspace implementation of the stateless API. It is not a software codec, a hardware-acceleration layer, a playback device, or evidence that a physical decoder supports a format.
How V4L2 stateless decoding represents codec work
A compressed video stream contains syntax that must be parsed into codec-specific structures before a stateless decode operation can be scheduled. In V4L2’s stateless model, userspace supplies that parsed information through controls alongside the relevant bitstream data. Controls can represent items such as H.264 sequence parameter sets, picture parameter sets, and slice parameters. The kernel’s Stateless Codec Control Reference documents those controls and describes them as interfaces intended for stateless decoders and encoders, typically hardware accelerators.
The application consequently owns more of the codec conversation. It parses enough of the bitstream to populate the controls, keeps references and ordering information consistent, queues the compressed data, and associates the controls with the decode work expected by the device. The device consumes those explicit parameters instead of inferring every piece of state from an opaque stream.
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V4L2’s memory-to-memory decoder convention names the encoded-input queue OUTPUT and the decoded-destination queue CAPTURE. A stateless client places compressed data in OUTPUT buffers and supplies the codec controls required for that operation. The driver returns its result through CAPTURE buffers. With visl, the CAPTURE payload is test-pattern debugging information, while the queue choreography and control submission are still the interactions your client needs to exercise.
Stateful versus stateless V4L2 decoders
The two interfaces differ in where decoder state is assembled and who supplies it. The following comparison follows the kernel’s stateful and stateless interface descriptions rather than comparing consumer products.
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| Aspect | Stateful decoder interface | Stateless decoder interface |
|---|---|---|
| Input expected from the client | Complete chunks of the compressed bytestream | Parsed codec data and codec-specific controls accompany the decode work |
| Client responsibility | The decoder is not expected to need additional per-buffer information to process a complete input buffer | The client parses codec syntax and manages the metadata and controls required for each operation |
| Typical model in the documentation | Memory-to-memory decoder workflow | Usually associated with hardware accelerators that expose explicit codec controls |
| What visl exercises | Not the target of this virtual stateless device | The userspace control, queue, and buffer behavior of a stateless API client |
The stateful workflow is often simpler for an application: it submits complete compressed chunks and lets the decoder maintain codec state internally. A stateless workflow can expose more hardware capability and scheduling control, but a client must track parameter sets, references, slices, and other codec-specific details. A client written for the stateful interface cannot be made stateless merely by changing the device node; its control and buffer logic must match the stateless API.
Which codecs does visl support?
The current kernel documentation page served under the latest path lists these codec families:
| Codec family | Availability statement | What to verify |
|---|---|---|
| FWHT | Listed by the current visl guide | Confirm the target kernel’s visl implementation and control definitions |
| MPEG-2 | Listed by the current visl guide | Confirm the target kernel version and userspace control support |
| VP8 | Listed by the current visl guide | Confirm the target kernel version and userspace control support |
| VP9 | Listed by the current visl guide | Confirm the target kernel version and userspace control support |
| H.264 | Listed by the current visl guide | Confirm the target kernel version and the H.264 stateless controls your client uses |
| HEVC | Listed by the current visl guide | Confirm the target kernel version and HEVC control coverage |
| AV1 | Listed by the current visl guide | Confirm the target kernel version and AV1 control coverage |
This list reflects the latest documentation consulted on 7 October 2026. It should not be read as a compatibility guarantee for an older distribution kernel, a vendor tree, or a build in which visl is unavailable. Codec names alone also do not prove that a particular stream’s profile, level, bit depth, or feature combination is accepted; the relevant controls and the target kernel implementation still determine that.
How to test a stateless client with visl
Use visl to validate the API path, not the decoded image. A practical test proceeds as follows.
- Identify the target kernel. Record the exact kernel release or vendor revision on which the client must run. Compare its visl documentation and stateless control definitions with the current guides; the stateful interface reference used for the comparison here is explicitly for Linux v6.1, while the visl and stateless-control pages are served from current documentation paths.
- Make sure the test device is available. Use a kernel build that includes the visl virtual stateless decoder and expose its V4L2 device to the test environment. Do not assume that a distribution’s older kernel contains every codec listed in the current guide.
- Run the client’s ordinary stateless decode loop. Feed it the same parsed codec structures and compressed OUTPUT data that you intend to use with a real stateless decoder. The goal is to exercise format negotiation, control creation, queueing, dequeueing, synchronization, and error handling without inserting a special “test mode” that bypasses those paths.
- Inspect the CAPTURE buffers. Treat their contents as the driver’s diagnostic test pattern. They can help correlate a completed operation with submitted controls and buffer state, but they must not be passed to an image-quality comparison as if they were decoded frames.
- Compare API behavior. Run a known-working client and the implementation under development with equivalent stream segments and controls. Differences in queue order, missing controls, buffer ownership, or request completion become visible even though neither run produces a reconstructed picture.
When a client fails, first separate an API failure from a codec-decoding failure. A format-negotiation error, rejected control, stalled queue, or bad buffer lifecycle is within visl’s testing scope. A wrong pixel, reference-frame error, concealed block, or throughput regression is not demonstrated by this driver.
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Tracing controls and inspecting buffers
Codec-specific trace events
The visl documentation describes per-codec trace-event groups. Enable the group relevant to the stream you are submitting, then inspect the events to see the controls delivered by the application. The guide includes a HEVC sequence-parameter-set (SPS) trace example; use that pattern as a diagnostic reference while selecting the corresponding event group for another codec. Trace output is particularly useful when a client appears to submit a structure but the driver receives a default, truncated, or differently ordered value.
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Debugfs inspection of OUTPUT data
visl provides a debugfs interface for inspecting OUTPUT buffer data when a raw bitstream comparison is needed. The driver has parameters that control whether bitstream buffers are retained and which frames are dumped. Consult the guide and the target kernel’s parameter names before configuring them; parameter availability can change with the kernel implementation.
Retaining or dumping buffers can consume storage and make a test run harder to interpret, so enable it for a focused comparison. Check that the captured data corresponds to the intended buffer and frame, then compare it with the bytes your parser queued. This can expose accidental offsets, incomplete slices, or a mismatch between the parser’s notion of a frame and the queue’s buffer boundaries.
ftrace and verbose test-pattern output
Kernel ftrace output can be combined with visl trace events to align userspace actions, control submission, and driver-side processing. Keep timestamps and buffer identifiers together when comparing runs. If tpg_verbose is enabled, the test-pattern generator includes extra details such as pointers or queue status. Those values can change between runs for identical input, so verbose output is useful for diagnosing a particular execution but is not a stable golden file for byte-for-byte regression tests.
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What visl can and cannot prove
visl can provide evidence about the software contract around a stateless decoder:
- Whether a client negotiates formats and queue roles as expected.
- Whether codec-specific controls are constructed, submitted, and associated with decode work.
- Whether OUTPUT and CAPTURE buffers are queued, completed, and recycled without an obvious lifecycle error.
- Whether two client implementations send materially different controls or bitstream buffers.
- Whether trace and debug instrumentation can capture the information needed to investigate a kernel/userspace mismatch.
It cannot establish the properties that require actual frame decoding:
- Picture correctness, reference-frame handling, error concealment, or conformance to a bitstream’s visual result.
- Image quality, decoded-output fidelity, or pixel-level differences between implementations.
- Decode throughput, latency under load, power consumption, thermal behavior, or memory-bandwidth use.
- Compatibility with a physical decoder, including vendor-specific profile, level, bit-depth, or performance limits.
Those questions require a real decoder path and an appropriate stream and measurement method. A visl test that passes is evidence that the client exercised the virtual API successfully; it is not a substitute for hardware validation.
Common failure modes and a diagnostic path
The client expects decoded pictures
Symptoms include a successful queue cycle followed by unusable or unfamiliar CAPTURE data. Check the device identity and read the visl documentation: the capture payload is generated diagnostic output. Keep the run for API assertions, and use a real decoder for pixel validation.
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A control is rejected or a trace field is missing
Verify that the control belongs to the codec and kernel version selected for the test, that its structure is fully populated according to the stateless control reference, and that it is submitted at the point in the queueing sequence expected by the client. Compare the rejected submission with a working implementation while tracing the relevant codec event group.
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OUTPUT data does not match the parser’s input
Enable the visl debugfs buffer-retention or frame-dump facility described by the target guide, then compare the retained bytes with the application’s source buffer. Look for offset, length, slice-boundary, and frame-association mistakes before investigating codec semantics.
Two identical runs produce different verbose output
Check whether tpg_verbose is enabled. Pointers and queue-status details are intentionally run-specific, so compare stable fields and event ordering rather than requiring identical verbose text. Disable verbose details when producing a compact regression artifact.
The device is missing on a distribution kernel
Do not infer that the current codec list applies to that build. Confirm the exact kernel revision, whether the visl driver was included, and which stateless controls it exposes. If the virtual device is absent, the client must be tested against another kernel containing visl or against an actual stateless decoder; no userspace setting can create support that the kernel does not provide.
A practical decision framework
Choose visl when the question is about the interface transaction: control contents, queue sequencing, buffer ownership, tracing, or portability of a userspace client across development environments. Choose a real decoder when the question is about the decoded result: visual correctness, performance, power, or hardware-specific acceptance.
For a new client, start with visl to make API failures observable, then repeat the same test matrix on each target accelerator. Keep the two result classes separate in reports: “virtual API test passed” describes the controls and queues, while “hardware decode validated” requires evidence from actual decoded frames and measurements. This wording prevents a diagnostic test from being mistaken for codec or hardware certification.
Quick Recap
Authoritative references
- Linux kernel: The Virtual Stateless Decoder Driver (visl) — driver purpose, codec list, trace guidance, debugfs behavior, and test-pattern limitations.
- Linux kernel: Stateless Codec Control Reference — codec-specific V4L2 controls and their stateless-decoder use.
- Linux kernel v6.1: Memory-to-Memory Stateful Video Decoder Interface — the stateful OUTPUT/CAPTURE decoder model used for comparison.
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