diff --git a/tools/perf/design.txt b/tools/perf/design.txt
index a58cc9a..d68515f 100644
--- a/tools/perf/design.txt
+++ b/tools/perf/design.txt
@@ -43,7 +43,11 @@ struct perf_event_attr {
          */
         __u64                   config;
 
-        __u64                   irq_period;
+        union {
+            __u64               sample_period;
+            __u64               sample_freq;
+        };
+
         __u32                   record_type;
         __u32                   read_format;
 
@@ -147,7 +151,7 @@ tracer is available, and event_id values can be obtained from
 Counters come in two flavours: counting counters and sampling
 counters.  A "counting" counter is one that is used for counting the
 number of events that occur, and is characterised by having
-irq_period = 0.
+sample_period = 0.
 
 
 A read() on a counter returns the current value of the counter and possible
@@ -170,8 +174,8 @@ into account.
 
 
 A "sampling" counter is one that is set up to generate an interrupt
-every N events, where N is given by 'irq_period'.  A sampling counter
-has irq_period > 0. The record_type controls what data is recorded on each
+every N events, where N is given by 'sample_period'.  A sampling counter
+has sample_period > 0. The record_type controls what data is recorded on each
 interrupt:
 
 /*