v4l2-tracer - Man Page
An application to trace and replay stateless video decoding.
Synopsis
v4l2-tracer [options] trace <tracee>
v4l2-tracer [options] retrace <trace_file>.json
v4l2-tracer clean <file>.json
Description
The v4l2-tracer utility traces, records and replays userspace applications that implement the v4l2 memory-to-memory stateless video decoder interface.
Trace
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.
Retrace
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.
Clean
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.
Options
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.
Example
- 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:
71827_trace.json
- 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:
71827_trace_retrace.json
Remove file descriptors and addresses (optional): v4l2-tracer clean 71827_trace.jsonv4l2-tracer clean 71827_trace_retrace.json
- Clean files are generated for comparison:
clean_71827_trace.json
clean_71827_trace_retrace.json
Bugs
Bug reports or questions about this utility should be sent to the linux-media@vger.kernel.org mailinglist.