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![Tom Rini](/assets/img/avatar_default.png)
When U-Boot started using SPDX tags we were among the early adopters and there weren't a lot of other examples to borrow from. So we picked the area of the file that usually had a full license text and replaced it with an appropriate SPDX-License-Identifier: entry. Since then, the Linux Kernel has adopted SPDX tags and they place it as the very first line in a file (except where shebangs are used, then it's second line) and with slightly different comment styles than us. In part due to community overlap, in part due to better tag visibility and in part for other minor reasons, switch over to that style. This commit changes all instances where we have a single declared license in the tag as both the before and after are identical in tag contents. There's also a few places where I found we did not have a tag and have introduced one. Signed-off-by: Tom Rini <trini@konsulko.com>
347 lines
10 KiB
Plaintext
347 lines
10 KiB
Plaintext
# SPDX-License-Identifier: GPL-2.0+
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#
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# Copyright (c) 2013 The Chromium OS Authors.
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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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