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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 cmdline is not enough to control these complex features,
+this uses supplemental kernel cmdline (SKC) to describe tracing
+feature programming.
+
+Options in Supplemental Kernel Cmdline
+======================================
+
+Here is the list of available options list for boot time tracing in
+supplemental kenrel cmdline file [1]_. All options are under "ftrace."
+prefix to isolate from other subsystems.
+
+.. [1] See Documentation/admin-guide/skc.rst for details.
+
+Ftrace Global Options
+---------------------
+
+These options are only for global ftrace node since these are globally
+applied.
+
+ftrace.tp_printk;
+   Output trace-event data on printk buffer too.
+
+ftrace.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.
+
+ftrace.traceoff_on_warning;
+   Stop tracing if WARN_ON() occurs.
+
+ftrace.fgraph.filters = FILTER[, FILTER2...];
+   Add fgraph tracing function filters.
+
+ftrace.fgraph.notraces = FILTER[, FILTER2...];
+   Add fgraph non tracing function filters.
+
+ftrace.fgraph.max_depth = MAX_DEPTH;
+   Set MAX_DEPTH to maximum depth of fgraph tracer.
+
+
+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.]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.]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;
+   Enables GROUP: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.
+
+Examples
+========
+
+For example, to add filter and actions for each event, define kprobe
+events, and synthetic events with histogram, write SKC like below.
+
+::
+
+  ftrace.event {
+        task.task_newtask {
+                filter = "pid < 128";
+                enable;
+        }
+        kprobes.vfs_read {
+                probes = "vfs_read $arg1 $arg2";
+                filter = "common_pid < 200";
+                enable;
+        }
+        synthetic.initcall_latency {
+                fields = "unsigned long func", "u64 lat";
+                actions = "hist:keys=func.sym,lat:vals=lat:sort=lat";
+        }
+        initcall.initcall_start {
+                actions = "hist:keys=func:ts0=common_timestamp.usecs";
+        }
+        initcall.initcall_finish {
+                actions = "hist:keys=func:lat=common_timestamp.usecs-$ts0:onmatch(initcall.initcall_start).initcall_latency(func,$lat)";
+        }
+  }
+
+Also, boottime tracing supports "instance" node, which allows us to run
+several tracers for different purpose at once. For example, one tracer
+is for tracing functions in module alpha, and others tracing module beta,
+you can write as below.
+
+::
+
+  ftrace.instance {
+        foo {
+                tracer = "function";
+                ftrace-filters = "*:mod:alpha";
+        }
+        bar {
+                tracer = "function";
+                ftrace-filters = "*:mod:beta";
+        }
+  }
+
+The instance node also accepts event nodes so that each instance
+can customize its event tracing.
+
+This boot-time trace also supports ftrace kernel parameters.
+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 SKC like below.
+
+::
+
+  ftrace {
+        options = sym-addr;
+        events = "initcall:*";
+        tp-printk;
+        buffer-size = 1MB;
+        ftrace-filters = "vfs*";
+  }
+
+However, since the initialization timing is different, if you need
+to trace very early boot, please use normal kernel parameters.
