Add tracing to a driver

This document describes how to add tracing to a driver in a Fuchsia system.

Overview

For an overview of tracing, see Fuchsia tracing system. However, drivers don't need to specify a trace provider. The driver host provides one for its drivers.

For a tutorial on tracing, see Add tracing in your code. And for the tracing API reference, see Tracing: C and C++ macros.

Add trace records

To add tracing a driver, you need to update the driver's source code and BUILD.gn file.

Source code update

To add trace records in a driver component, update the source code to invoke the TRACE_*() macros from lib/trace/event.h, for example:

#include <lib/trace/event.h>

void DoSomething(int a, std::string b) {
  TRACE_DURATION("example", "DoSomething", "a", a, "b", b);

  // Do something
}

The first two arguments to most macros are the trace category and the event name. In this example, they are example and DoSomething respectively.

Trace categories let you specify what types of data the tracing system collects. If a category is not requested, no data is collected. Categories don't need to be unique across the driver. One typically groups several events under the same category.

An event name is included in the trace to describe the event. It is typically unique for each event.

BUILD.gn update

To pick up tracing support, add the following target to your driver's BUILD.gn:

fuchsia_driver("my_driver") {
  deps = [
    ...
    "//zircon/system/ulib/trace",
  ]
}

Boot with tracing

Fuchsia uses a kernel command-line flag to enable tracing in drivers during boot:

driver.tracing.enable=1

This is the default setting in Fuchsia devices.

To disable participation of drivers in Fuchsia tracing, boot the kernel with the following command-line flag:

driver.tracing.enable=0

For instructions on booting a specific Fuchsia device, see documentation for your hardware or QEMU. Tracing doesn't require anything special during boot.

Use tracing

Use ffx trace to record a trace and view the result with the Perfetto viewer.

The example command below uses the example category described in the Source code update section above:

$ ffx trace start --categories "example,kernel:sched,kernel:meta"

The kernel:sched,kernel:meta categories need to be present if you want to visualize the result. The visualizer wants to associate trace data with threads and processes, and it needs the data provided by the kernel through these categories.

For additional details, as well as instructions on tracing directly on a Fuchsia device without ffx, see Record traces for performance analysis.