v4l2-tracer(1)

V4L2-TRACER(1)

User Commands

V4L2-TRACER(1)

NAME

v4l2-tracer - 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.json

v4l2-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.

November 2022

v4l-utils 1.32.0