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+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+Boot-time tracing
+=================
+
+:Author: Masami Hiramatsu <mhiramat@kernel.org>
+
+Overview
+========
+
+Boot-time tracing allows users to trace boot-time process including
+device initialization with full features of ftrace including per-event
+filter and actions, histograms, kprobe-events and synthetic-events,
+and trace instances.
+Since kernel command line is not enough to control these complex features,
+this uses bootconfig file to describe tracing feature programming.
+
+Options in the Boot Config
+==========================
+
+Here is the list of available options list for boot time tracing in
+boot config file [1]_. All options are under "ftrace." or "kernel."
+prefix. See kernel parameters for the options which starts
+with "kernel." prefix [2]_.
+
+.. [1] See :ref:`Documentation/admin-guide/bootconfig.rst <bootconfig>`
+.. [2] See :ref:`Documentation/admin-guide/kernel-parameters.rst <kernelparameters>`
+
+Ftrace Global Options
+---------------------
+
+Ftrace global options have "kernel." prefix in boot config, which means
+these options are passed as a part of kernel legacy command line.
+
+kernel.tp_printk
+ Output trace-event data on printk buffer too.
+
+kernel.dump_on_oops [= MODE]
+ Dump ftrace on Oops. If MODE = 1 or omitted, dump trace buffer
+ on all CPUs. If MODE = 2, dump a buffer on a CPU which kicks Oops.
+
+kernel.traceoff_on_warning
+ Stop tracing if WARN_ON() occurs.
+
+kernel.fgraph_max_depth = MAX_DEPTH
+ Set MAX_DEPTH to maximum depth of fgraph tracer.
+
+kernel.fgraph_filters = FILTER[, FILTER2...]
+ Add fgraph tracing function filters.
+
+kernel.fgraph_notraces = FILTER[, FILTER2...]
+ Add fgraph non-tracing function filters.
+
+
+Ftrace Per-instance Options
+---------------------------
+
+These options can be used for each instance including global ftrace node.
+
+ftrace.[instance.INSTANCE.]options = OPT1[, OPT2[...]]
+ Enable given ftrace options.
+
+ftrace.[instance.INSTANCE.]tracing_on = 0|1
+ Enable/Disable tracing on this instance when starting boot-time tracing.
+ (you can enable it by the "traceon" event trigger action)
+
+ftrace.[instance.INSTANCE.]trace_clock = CLOCK
+ Set given CLOCK to ftrace's trace_clock.
+
+ftrace.[instance.INSTANCE.]buffer_size = SIZE
+ Configure ftrace buffer size to SIZE. You can use "KB" or "MB"
+ for that SIZE.
+
+ftrace.[instance.INSTANCE.]alloc_snapshot
+ Allocate snapshot buffer.
+
+ftrace.[instance.INSTANCE.]cpumask = CPUMASK
+ Set CPUMASK as trace cpu-mask.
+
+ftrace.[instance.INSTANCE.]events = EVENT[, EVENT2[...]]
+ Enable given events on boot. You can use a wild card in EVENT.
+
+ftrace.[instance.INSTANCE.]tracer = TRACER
+ Set TRACER to current tracer on boot. (e.g. function)
+
+ftrace.[instance.INSTANCE.]ftrace.filters
+ This will take an array of tracing function filter rules.
+
+ftrace.[instance.INSTANCE.]ftrace.notraces
+ This will take an array of NON-tracing function filter rules.
+
+
+Ftrace Per-Event Options
+------------------------
+
+These options are setting per-event options.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.enable
+ Enable GROUP:EVENT tracing.
+
+ftrace.[instance.INSTANCE.]event.GROUP.enable
+ Enable all event tracing within GROUP.
+
+ftrace.[instance.INSTANCE.]event.enable
+ Enable all event tracing.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.filter = FILTER
+ Set FILTER rule to the GROUP:EVENT.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.actions = ACTION[, ACTION2[...]]
+ Set ACTIONs to the GROUP:EVENT.
+
+ftrace.[instance.INSTANCE.]event.kprobes.EVENT.probes = PROBE[, PROBE2[...]]
+ Defines new kprobe event based on PROBEs. It is able to define
+ multiple probes on one event, but those must have same type of
+ arguments. This option is available only for the event which
+ group name is "kprobes".
+
+ftrace.[instance.INSTANCE.]event.synthetic.EVENT.fields = FIELD[, FIELD2[...]]
+ Defines new synthetic event with FIELDs. Each field should be
+ "type varname".
+
+Note that kprobe and synthetic event definitions can be written under
+instance node, but those are also visible from other instances. So please
+take care for event name conflict.
+
+Ftrace Histogram Options
+------------------------
+
+Since it is too long to write a histogram action as a string for per-event
+action option, there are tree-style options under per-event 'hist' subkey
+for the histogram actions. For the detail of the each parameter,
+please read the event histogram document (Documentation/trace/histogram.rst)
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]keys = KEY1[, KEY2[...]]
+ Set histogram key parameters. (Mandatory)
+ The 'N' is a digit string for the multiple histogram. You can omit it
+ if there is one histogram on the event.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]values = VAL1[, VAL2[...]]
+ Set histogram value parameters.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]sort = SORT1[, SORT2[...]]
+ Set histogram sort parameter options.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]size = NR_ENTRIES
+ Set histogram size (number of entries).
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]name = NAME
+ Set histogram name.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]var.VARIABLE = EXPR
+ Define a new VARIABLE by EXPR expression.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]<pause|continue|clear>
+ Set histogram control parameter. You can set one of them.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]onmatch.[M.]event = GROUP.EVENT
+ Set histogram 'onmatch' handler matching event parameter.
+ The 'M' is a digit string for the multiple 'onmatch' handler. You can omit it
+ if there is one 'onmatch' handler on this histogram.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]onmatch.[M.]trace = EVENT[, ARG1[...]]
+ Set histogram 'trace' action for 'onmatch'.
+ EVENT must be a synthetic event name, and ARG1... are parameters
+ for that event. Mandatory if 'onmatch.event' option is set.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]onmax.[M.]var = VAR
+ Set histogram 'onmax' handler variable parameter.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]onchange.[M.]var = VAR
+ Set histogram 'onchange' handler variable parameter.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]<onmax|onchange>.[M.]save = ARG1[, ARG2[...]]
+ Set histogram 'save' action parameters for 'onmax' or 'onchange' handler.
+ This option or below 'snapshot' option is mandatory if 'onmax.var' or
+ 'onchange.var' option is set.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]<onmax|onchange>.[M.]snapshot
+ Set histogram 'snapshot' action for 'onmax' or 'onchange' handler.
+ This option or above 'save' option is mandatory if 'onmax.var' or
+ 'onchange.var' option is set.
+
+ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.filter = FILTER_EXPR
+ Set histogram filter expression. You don't need 'if' in the FILTER_EXPR.
+
+Note that this 'hist' option can conflict with the per-event 'actions'
+option if the 'actions' option has a histogram action.
+
+
+When to Start
+=============
+
+All boot-time tracing options starting with ``ftrace`` will be enabled at the
+end of core_initcall. This means you can trace the events from postcore_initcall.
+Most of the subsystems and architecture dependent drivers will be initialized
+after that (arch_initcall or subsys_initcall). Thus, you can trace those with
+boot-time tracing.
+If you want to trace events before core_initcall, you can use the options
+starting with ``kernel``. Some of them will be enabled eariler than the initcall
+processing (for example,. ``kernel.ftrace=function`` and ``kernel.trace_event``
+will start before the initcall.)
+
+
+Examples
+========
+
+For example, to add filter and actions for each event, define kprobe
+events, and synthetic events with histogram, write a boot config like
+below::
+
+ ftrace.event {
+ task.task_newtask {
+ filter = "pid < 128"
+ enable
+ }
+ kprobes.vfs_read {
+ probes = "vfs_read $arg1 $arg2"
+ filter = "common_pid < 200"
+ enable
+ }
+ synthetic.initcall_latency {
+ fields = "unsigned long func", "u64 lat"
+ hist {
+ keys = func.sym, lat
+ values = lat
+ sort = lat
+ }
+ }
+ initcall.initcall_start.hist {
+ keys = func
+ var.ts0 = common_timestamp.usecs
+ }
+ initcall.initcall_finish.hist {
+ keys = func
+ var.lat = common_timestamp.usecs - $ts0
+ onmatch {
+ event = initcall.initcall_start
+ trace = initcall_latency, func, $lat
+ }
+ }
+ }
+
+Also, boot-time tracing supports "instance" node, which allows us to run
+several tracers for different purpose at once. For example, one tracer
+is for tracing functions starting with "user\_", and others tracing
+"kernel\_" functions, you can write boot config as below::
+
+ ftrace.instance {
+ foo {
+ tracer = "function"
+ ftrace.filters = "user_*"
+ }
+ bar {
+ tracer = "function"
+ ftrace.filters = "kernel_*"
+ }
+ }
+
+The instance node also accepts event nodes so that each instance
+can customize its event tracing.
+
+With the trigger action and kprobes, you can trace function-graph while
+a function is called. For example, this will trace all function calls in
+the pci_proc_init()::
+
+ ftrace {
+ tracing_on = 0
+ tracer = function_graph
+ event.kprobes {
+ start_event {
+ probes = "pci_proc_init"
+ actions = "traceon"
+ }
+ end_event {
+ probes = "pci_proc_init%return"
+ actions = "traceoff"
+ }
+ }
+ }
+
+
+This boot-time tracing also supports ftrace kernel parameters via boot
+config.
+For example, following kernel parameters::
+
+ trace_options=sym-addr trace_event=initcall:* tp_printk trace_buf_size=1M ftrace=function ftrace_filter="vfs*"
+
+This can be written in boot config like below::
+
+ kernel {
+ trace_options = sym-addr
+ trace_event = "initcall:*"
+ tp_printk
+ trace_buf_size = 1M
+ ftrace = function
+ ftrace_filter = "vfs*"
+ }
+
+Note that parameters start with "kernel" prefix instead of "ftrace".