Tracing in U-Boot

U-Boot supports a simple tracing feature which allows a record of execution to be collected and sent to a host machine for analysis. At present the main use for this is to profile boot time.

Overview

The trace feature uses GCC’s instrument-functions feature to trace all function entry/exit points. These are then recorded in a memory buffer. The memory buffer can be saved to the host over a network link using tftpput or by writing to an attached memory device such as MMC.

On the host, the file is first converted with a tool called ‘proftool’, which extracts useful information from it. The resulting trace output resembles that emitted by Linux’s ftrace feature, so can be visually displayed by pytimechart.

Quick-start using Sandbox

Sandbox is a build of U-Boot that can run under Linux so it is a convenient way of trying out tracing before you use it on your actual board. To do this, follow these steps:

Add the following to config/sandbox_defconfig

CONFIG_TRACE=y

Build sandbox U-Boot with tracing enabled:

$ make FTRACE=1 O=sandbox sandbox_config
$ make FTRACE=1 O=sandbox

Run sandbox, wait for a bit of trace information to appear, and then capture a trace:

$ ./sandbox/u-boot

U-Boot 2013.04-rc2-00100-ga72fcef (Apr 17 2013 - 19:25:24)

DRAM:  128 MiB
trace: enabled
Using default environment

In:    serial
Out:   serial
Err:   serial
=>trace stats
    671,406 function sites
     69,712 function calls
          0 untracked function calls
     73,373 traced function calls
         16 maximum observed call depth
         15 call depth limit
     66,491 calls not traced due to depth
=>trace stats
    671,406 function sites
  1,279,450 function calls
          0 untracked function calls
    950,490 traced function calls (333217 dropped due to overflow)
         16 maximum observed call depth
         15 call depth limit
      1,275,767 calls not traced due to depth
=>trace calls 0 e00000
Call list dumped to 00000000, size 0xae0a40
=>print
baudrate=115200
profbase=0
profoffset=ae0a40
profsize=e00000
stderr=serial
stdin=serial
stdout=serial

Environment size: 117/8188 bytes
=>host save host 0 trace 0 ${profoffset}
11405888 bytes written in 10 ms (1.1 GiB/s)
=>reset

Then run proftool to convert the trace information to ftrace format

$ ./sandbox/tools/proftool -m sandbox/System.map -p trace dump-ftrace >trace.txt

Finally run pytimechart to display it

$ pytimechart trace.txt

Using this tool you can zoom and pan across the trace, with the function calls on the left and little marks representing the start and end of each function.

CONFIG Options

CONFIG_TRACE

Enables the trace feature in U-Boot.

CONFIG_CMD_TRACE

Enables the trace command.

CONFIG_TRACE_BUFFER_SIZE

Size of trace buffer to allocate for U-Boot. This buffer is used after relocation, as a place to put function tracing information. The address of the buffer is determined by the relocation code.

CONFIG_TRACE_EARLY

Define this to start tracing early, before relocation.

CONFIG_TRACE_EARLY_SIZE

Size of ‘early’ trace buffer. Before U-Boot has relocated it doesn’t have a proper trace buffer. On many boards you can define an area of memory to use for the trace buffer until the ‘real’ trace buffer is available after relocation. The contents of this buffer are then copied to the real buffer.

CONFIG_TRACE_EARLY_ADDR

Address of early trace buffer

Building U-Boot with Tracing Enabled

Pass ‘FTRACE=1’ to the U-Boot Makefile to actually instrument the code. This is kept as a separate option so that it is easy to enable/disable instrumenting from the command line instead of having to change board config files.

Collecting Trace Data

When you run U-Boot on your board it will collect trace data up to the limit of the trace buffer size you have specified. Once that is exhausted no more data will be collected.

Collecting trace data has an affect on execution time/performance. You will notice this particularly with trivial functions - the overhead of recording their execution may even exceed their normal execution time. In practice this doesn’t matter much so long as you are aware of the effect. Once you have done your optimizations, turn off tracing before doing end-to-end timing.

The best time to start tracing is right at the beginning of U-Boot. The best time to stop tracing is right at the end. In practice it is hard to achieve these ideals.

This implementation enables tracing early in board_init_f(). This means that it captures most of the board init process, missing only the early architecture-specific init. However, it also misses the entire SPL stage if there is one.

U-Boot typically ends with a ‘bootm’ command which loads and runs an OS. There is useful trace data in the execution of that bootm command. Therefore this implementation provides a way to collect trace data after bootm has finished processing, but just before it jumps to the OS. In practical terms, U-Boot runs the ‘fakegocmd’ environment variable at this point. This variable should have a short script which collects the trace data and writes it somewhere.

Trace data collection relies on a microsecond timer, accessed through timer_get_us(). So the first think you should do is make sure that this produces sensible results for your board. Suitable sources for this timer include high resolution timers, PWMs or profile timers if available. Most modern SOCs have a suitable timer for this. Make sure that you mark this timer (and anything it calls) with notrace so that the trace library can use it without causing an infinite loop.

Commands

The trace command has variable sub-commands:

stats

Display tracing statistics

pause

Pause tracing

resume

Resume tracing

funclist [<addr> <size>]

Dump a list of functions into the buffer

calls [<addr> <size>]

Dump function call trace into buffer

If the address and size are not given, these are obtained from environment variables (see below). In any case the environment variables are updated after the command runs.

Environment Variables

The following are used:

profbase

Base address of trace output buffer

profoffset

Offset of first unwritten byte in trace output buffer

profsize

Size of trace output buffer

All of these are set by the ‘trace calls’ command.

These variables keep track of the amount of data written to the trace output buffer by the ‘trace’ command. The trace commands which write data to the output buffer can use these to specify the buffer to write to, and update profoffset each time. This allows successive commands to append data to the same buffer, for example:

=> trace funclist 10000 e00000
=> trace calls

(the latter command appends more data to the buffer).

fakegocmd

Specifies commands to run just before booting the OS. This is a useful time to write the trace data to the host for processing.

Writing Out Trace Data

Once the trace data is in an output buffer in memory there are various ways to transmit it to the host. Notably you can use tftput to send the data over a network link:

fakegocmd=trace pause; usb start; set autoload n; bootp;
trace calls 10000000 1000000;
tftpput ${profbase} ${profoffset} 192.168.1.4:/tftpboot/calls

This starts up USB (to talk to an attached USB Ethernet dongle), writes a trace log to address 10000000 and sends it to a host machine using TFTP. After this, U-Boot will boot the OS normally, albeit a little later.

Converting Trace Output Data

The trace output data is kept in a binary format which is not documented here. To convert it into something useful, you can use proftool.

This tool must be given the U-Boot map file and the trace data received from running that U-Boot. It produces a text output file.

Options

-m <map_file>

Specify U-Boot map file

-p <trace_file>

Specify profile/trace file

Commands:

dump-ftrace

Write a text dump of the file in Linux ftrace format to stdout

Viewing the Trace Data

You can use pytimechart for this (sudo apt-get pytimechart might work on your Debian-style machine, and use your favourite search engine to obtain documentation). It expects the file to have a .txt extension. The program has terse user interface but is very convenient for viewing U-Boot profile information.

Workflow Suggestions

The following suggestions may be helpful if you are trying to reduce boot time:

  1. Enable CONFIG_BOOTSTAGE and CONFIG_BOOTSTAGE_REPORT. This should get you are helpful overall snapshot of the boot time.

  2. Build U-Boot with tracing and run it. Note the difference in boot time (it is common for tracing to add 10% to the time)

  3. Collect the trace information as described above. Use this to find where all the time is being spent.

  4. Take a look at that code and see if you can optimize it. Perhaps it is possible to speed up the initialization of a device, or remove an unused feature.

  5. Rebuild, run and collect again. Compare your results.

  6. Keep going until you run out of steam, or your boot is fast enough.

Configuring Trace

There are a few parameters in the code that you may want to consider. There is a function call depth limit (set to 15 by default). When the stack depth goes above this then no tracing information is recorded. The maximum depth reached is recorded and displayed by the ‘trace stats’ command.

Future Work

Tracing could be a little tidier in some areas, for example providing run-time configuration options for trace.

Some other features that might be useful:

  • Trace filter to select which functions are recorded

  • Sample-based profiling using a timer interrupt

  • Better control over trace depth

  • Compression of trace information

Simon Glass <sjg@chromium.org> April 2013