2 files changed, 31 insertions, 2 deletions
diff --git a/Documentation/trace/events.rst b/Documentation/trace/events.rst
index f792b1959a33..2a5aa48eff6c 100644
--- a/Documentation/trace/events.rst
+++ b/Documentation/trace/events.rst
@@ -589,8 +589,19 @@ name::
         { .type = "int",                .name = "my_int_field" },
   };
 
-See synth_field_size() for available types. If field_name contains [n]
-the field is considered to be an array.
+See synth_field_size() for available types.
+
+If field_name contains [n], the field is considered to be a static array.
+
+If field_names contains[] (no subscript), the field is considered to
+be a dynamic array, which will only take as much space in the event as
+is required to hold the array.
+
+Because space for an event is reserved before assigning field values
+to the event, using dynamic arrays implies that the piecewise
+in-kernel API described below can't be used with dynamic arrays.  The
+other non-piecewise in-kernel APIs can, however, be used with dynamic
+arrays.
 
 If the event is created from within a module, a pointer to the module
 must be passed to synth_event_create().  This will ensure that the
diff --git a/Documentation/trace/histogram.rst b/Documentation/trace/histogram.rst
index 8408670d0328..b573604deabd 100644
--- a/Documentation/trace/histogram.rst
+++ b/Documentation/trace/histogram.rst
@@ -1776,6 +1776,24 @@ consisting of the name of the new event along with one or more
 variables and their types, which can be any valid field type,
 separated by semicolons, to the tracing/synthetic_events file.
 
+See synth_field_size() for available types.
+
+If field_name contains [n], the field is considered to be a static array.
+
+If field_names contains[] (no subscript), the field is considered to
+be a dynamic array, which will only take as much space in the event as
+is required to hold the array.
+
+A string field can be specified using either the static notation:
+
+  char name[32];
+
+Or the dynamic:
+
+  char name[];
+
+The size limit for either is 256.
+
 For instance, the following creates a new event named 'wakeup_latency'
 with 3 fields: lat, pid, and prio.  Each of those fields is simply a
 variable reference to a variable on another event::