#
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# Copyright (c) 2013 The Chromium OS Authors.
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#
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# SPDX-License-Identifier: GPL-2.0+
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#
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Tracing in U-Boot
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=================
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U-Boot supports a simple tracing feature which allows a record of excecution
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to be collected and sent to a host machine for analysis. At present the
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main use for this is to profile boot time.
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Overview
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--------
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The trace feature uses GCC's instrument-functions feature to trace all
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function entry/exit points. These are then recorded in a memory buffer.
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The memory buffer can be saved to the host over a network link using
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tftpput or by writing to an attached memory device such as MMC.
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On the host, the file is first converted with a tool called 'proftool',
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which extracts useful information from it. The resulting trace output
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resembles that emitted by Linux's ftrace feature, so can be visually
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displayed by pytimechart.
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Quick-start using Sandbox
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-------------------------
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Sandbox is a build of U-Boot that can run under Linux so it is a convenient
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way of trying out tracing before you use it on your actual board. To do
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this, follow these steps:
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Add the following to include/configs/sandbox.h (if not already there)
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#define CONFIG_TRACE
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#define CONFIG_CMD_TRACE
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#define CONFIG_TRACE_BUFFER_SIZE (16 << 20)
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#define CONFIG_TRACE_EARLY_SIZE (8 << 20)
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#define CONFIG_TRACE_EARLY
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#define CONFIG_TRACE_EARLY_ADDR 0x00100000
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Build sandbox U-Boot with tracing enabled:
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$ make FTRACE=1 O=sandbox sandbox_config
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$ make FTRACE=1 O=sandbox
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Run sandbox, wait for a bit of trace information to appear, and then capture
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a trace:
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$ ./sandbox/u-boot
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U-Boot 2013.04-rc2-00100-ga72fcef (Apr 17 2013 - 19:25:24)
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DRAM: 128 MiB
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trace: enabled
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Using default environment
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In: serial
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Out: serial
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Err: serial
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=>trace stats
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671,406 function sites
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69,712 function calls
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0 untracked function calls
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73,373 traced function calls
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16 maximum observed call depth
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15 call depth limit
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66,491 calls not traced due to depth
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=>trace stats
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671,406 function sites
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1,279,450 function calls
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0 untracked function calls
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950,490 traced function calls (333217 dropped due to overflow)
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16 maximum observed call depth
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15 call depth limit
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1,275,767 calls not traced due to depth
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=>trace calls 0 e00000
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Call list dumped to 00000000, size 0xae0a40
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=>print
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baudrate=115200
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profbase=0
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profoffset=ae0a40
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profsize=e00000
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stderr=serial
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stdin=serial
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stdout=serial
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Environment size: 117/8188 bytes
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=>sb save host 0 trace 0 ${profoffset}
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11405888 bytes written in 10 ms (1.1 GiB/s)
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=>reset
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Then run proftool to convert the trace information to ftrace format.
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$ ./sandbox/tools/proftool -m sandbox/System.map -p trace dump-ftrace >trace.txt
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Finally run pytimechart to display it:
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$ pytimechart trace.txt
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Using this tool you can zoom and pan across the trace, with the function
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calls on the left and little marks representing the start and end of each
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function.
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CONFIG Options
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--------------
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- CONFIG_TRACE
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Enables the trace feature in U-Boot.
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- CONFIG_CMD_TRACE
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Enables the trace command.
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- CONFIG_TRACE_BUFFER_SIZE
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Size of trace buffer to allocate for U-Boot. This buffer is
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used after relocation, as a place to put function tracing
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information. The address of the buffer is determined by
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the relocation code.
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- CONFIG_TRACE_EARLY
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Define this to start tracing early, before relocation.
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- CONFIG_TRACE_EARLY_SIZE
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Size of 'early' trace buffer. Before U-Boot has relocated
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it doesn't have a proper trace buffer. On many boards
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you can define an area of memory to use for the trace
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buffer until the 'real' trace buffer is available after
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relocation. The contents of this buffer are then copied to
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the real buffer.
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- CONFIG_TRACE_EARLY_ADDR
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Address of early trace buffer
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Building U-Boot with Tracing Enabled
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------------------------------------
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Pass 'FTRACE=1' to the U-Boot Makefile to actually instrument the code.
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This is kept as a separate option so that it is easy to enable/disable
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instrumenting from the command line instead of having to change board
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config files.
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Collecting Trace Data
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---------------------
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When you run U-Boot on your board it will collect trace data up to the
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limit of the trace buffer size you have specified. Once that is exhausted
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no more data will be collected.
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Collecting trace data has an affect on execution time/performance. You
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will notice this particularly with trvial functions - the overhead of
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recording their execution may even exceed their normal execution time.
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In practice this doesn't matter much so long as you are aware of the
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effect. Once you have done your optimisations, turn off tracing before
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doing end-to-end timing.
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The best time to start tracing is right at the beginning of U-Boot. The
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best time to stop tracing is right at the end. In practice it is hard
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to achieve these ideals.
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This implementation enables tracing early in board_init_f(). This means
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that it captures most of the board init process, missing only the
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early architecture-specific init. However, it also misses the entire
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SPL stage if there is one.
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U-Boot typically ends with a 'bootm' command which loads and runs an
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OS. There is useful trace data in the execution of that bootm
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command. Therefore this implementation provides a way to collect trace
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data after bootm has finished processing, but just before it jumps to
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the OS. In practical terms, U-Boot runs the 'fakegocmd' environment
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variable at this point. This variable should have a short script which
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collects the trace data and writes it somewhere.
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Trace data collection relies on a microsecond timer, accesed through
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timer_get_us(). So the first think you should do is make sure that
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this produces sensible results for your board. Suitable sources for
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this timer include high resolution timers, PWMs or profile timers if
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available. Most modern SOCs have a suitable timer for this. Make sure
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that you mark this timer (and anything it calls) with
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__attribute__((no_instrument_function)) so that the trace library can
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use it without causing an infinite loop.
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Commands
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--------
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The trace command has variable sub-commands:
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- stats
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Display tracing statistics
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- pause
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Pause tracing
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- resume
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Resume tracing
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- funclist [<addr> <size>]
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Dump a list of functions into the buffer
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- calls [<addr> <size>]
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Dump function call trace into buffer
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If the address and size are not given, these are obtained from environment
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variables (see below). In any case the environment variables are updated
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after the command runs.
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Environment Variables
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---------------------
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The following are used:
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- profbase
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Base address of trace output buffer
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- profoffset
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Offset of first unwritten byte in trace output buffer
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- profsize
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Size of trace output buffer
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All of these are set by the 'trace calls' command.
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These variables keep track of the amount of data written to the trace
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output buffer by the 'trace' command. The trace commands which write data
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to the output buffer can use these to specify the buffer to write to, and
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update profoffset each time. This allows successive commands to append data
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to the same buffer, for example:
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trace funclist 10000 e00000
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trace calls
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(the latter command appends more data to the buffer).
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- fakegocmd
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Specifies commands to run just before booting the OS. This
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is a useful time to write the trace data to the host for
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processing.
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Writing Out Trace Data
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----------------------
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Once the trace data is in an output buffer in memory there are various ways
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to transmit it to the host. Notably you can use tftput to send the data
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over a network link:
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fakegocmd=trace pause; usb start; set autoload n; bootp;
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trace calls 10000000 1000000;
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tftpput ${profbase} ${profoffset} 192.168.1.4:/tftpboot/calls
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This starts up USB (to talk to an attached USB Ethernet dongle), writes
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a trace log to address 10000000 and sends it to a host machine using
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TFTP. After this, U-Boot will boot the OS normally, albeit a little
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later.
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Converting Trace Output Data
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----------------------------
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The trace output data is kept in a binary format which is not documented
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here. To convert it into something useful, you can use proftool.
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This tool must be given the U-Boot map file and the trace data received
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from running that U-Boot. It produces a text output file.
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Options
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-m <map_file>
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Specify U-Boot map file
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-p <trace_file>
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Specifiy profile/trace file
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Commands:
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- dump-ftrace
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Write a text dump of the file in Linux ftrace format to stdout
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Viewing the Trace Data
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----------------------
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You can use pytimechart for this (sudo apt-get pytimechart might work on
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your Debian-style machine, and use your favourite search engine to obtain
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documentation). It expects the file to have a .txt extension. The program
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has terse user interface but is very convenient for viewing U-Boot
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profile information.
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Workflow Suggestions
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--------------------
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The following suggestions may be helpful if you are trying to reduce boot
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time:
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1. Enable CONFIG_BOOTSTAGE and CONFIG_BOOTSTAGE_REPORT. This should get
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you are helpful overall snapshot of the boot time.
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2. Build U-Boot with tracing and run it. Note the difference in boot time
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(it is common for tracing to add 10% to the time)
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3. Collect the trace information as descibed above. Use this to find where
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all the time is being spent.
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4. Take a look at that code and see if you can optimise it. Perhaps it is
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possible to speed up the initialisation of a device, or remove an unused
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feature.
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5. Rebuild, run and collect again. Compare your results.
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6. Keep going until you run out of steam, or your boot is fast enough.
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Configuring Trace
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-----------------
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There are a few parameters in the code that you may want to consider.
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There is a function call depth limit (set to 15 by default). When the
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stack depth goes above this then no tracing information is recorded.
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The maximum depth reached is recorded and displayed by the 'trace stats'
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command.
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Future Work
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-----------
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Tracing could be a little tidier in some areas, for example providing
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run-time configuration options for trace.
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Some other features that might be useful:
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- Trace filter to select which functions are recorded
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- Sample-based profiling using a timer interrupt
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- Better control over trace depth
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- Compression of trace information
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Simon Glass <sjg@chromium.org>
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April 2013
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