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v4l2-tracer - Man Page

An application to trace and replay stateless video decoding.


v4l2-tracer [options] trace <tracee>

v4l2-tracer [options] retrace  <trace_file>.json

v4l2-tracer clean  <file>.json


The v4l2-tracer utility traces, records and replays userspace applications that implement the v4l2 memory-to-memory stateless video decoder interface.


Trace system calls and video frame data passed by userspace application <tracee> to kernel driver. All stateless codec controls in user-space API can be traced. Outputs a JSON-formatted trace file.


Read the JSON-formatted <trace_file>.json. Replay the same system calls and pass the same video frame data to kernel driver. Outputs a JSON-formatted retrace file.


Remove lines with irrelevant differences (e.g. file descriptors and memory addresses) from JSON files. Outputs a clean copy, not necessarily still in JSON-format.


Common Options

-c,  --compact

Write minimal whitespace in JSON file.

-g,  --debug

Turn on verbose reporting plus additional debug info.

-h,  --help

Display this message.

-r,  --raw

Write decoded video frame data to JSON file.

-v,  --verbose

Turn on verbose reporting.

-y,  --yuv

Write decoded video frame data to yuv file.

Retrace Options

-d,  --device <dev>

Use a different video device than specified in the trace file.
<dev> must be a digit corresponding to an existing /dev/video<dev>

-m,  --media <dev>

Use a different media device than specified in the trace file.
<dev> must be a digit corresponding to an existing /dev/media<dev>

Exit Status

On success, it returns 0. Otherwise, it will return 1 or an error code.


Trace an application decoding VP8 video:
v4l2-tracer trace gst-launch-1.0 -- filesrc location=test-25fps.vp8 ! parsebin ! v4l2slvp8dec ! videocodectestsink
A trace file is generated:


Retrace the trace file:
v4l2-tracer retrace 71827_trace.json
Specify device nodes if retracing on a different driver:
v4l2-tracer -d0 -m0 retrace 71827_trace.json
A retrace file is generated:


    Remove file descriptors and addresses (optional):
        v4l2-tracer clean 71827_trace.json
v4l2-tracer clean 71827_trace_retrace.json
Clean files are generated for comparison:



Bug reports or questions about this utility should be sent to the linux-media@vger.kernel.org mailinglist.


November 2022 v4l-utils 1.26.1""