[PATCH v2 2/2] trace: allocate space from temparary trace sequence buffer

[Linyu Yuan]

there is one dwc3 trace event declare as below,
DECLARE_EVENT_CLASS(dwc3_log_event,
	TP_PROTO(u32 event, struct dwc3 *dwc),
	TP_ARGS(event, dwc),
	TP_STRUCT__entry(
		__field(u32, event)
		__field(u32, ep0state)
		__dynamic_array(char, str, DWC3_MSG_MAX)
	),
	TP_fast_assign(
		__entry->event = event;
		__entry->ep0state = dwc->ep0state;
	),
	TP_printk("event (%08x): %s", __entry->event,
			dwc3_decode_event(__get_str(str), DWC3_MSG_MAX,
				__entry->event, __entry->ep0state))
);
the problem is when trace function called, it will allocate up to
DWC3_MSG_MAX bytes from trace event buffer, but never fill the buffer
during fast assignment, it only fill the buffer when output function are
called, so this means if output function are not called, the buffer will
never used.

add __get_buf(len) which allocate space from iter->tmp_seq when trace
output function called, it allow user write any data to allocatd space.

the mentioned dwc3 trace event will changed as below,
DECLARE_EVENT_CLASS(dwc3_log_event,
	TP_PROTO(u32 event, struct dwc3 *dwc),
	TP_ARGS(event, dwc),
	TP_STRUCT__entry(
		__field(u32, event)
		__field(u32, ep0state)
	),
	TP_fast_assign(
		__entry->event = event;
		__entry->ep0state = dwc->ep0state;
	),
	TP_printk("event (%08x): %s", __entry->event,
		dwc3_decode_event(__get_buf(DWC3_MSG_MAX), DWC3_MSG_MAX,
				__entry->event, __entry->ep0state))
);.

Signed-off-by: Linyu Yuan <quic_linyyuan@xxxxxxxxxxx>
---
 include/linux/trace_seq.h                  |  6 ++++++
 include/trace/stages/stage3_trace_output.h |  4 ++++
 include/trace/stages/stage7_class_define.h |  1 +
 kernel/trace/trace_seq.c                   | 31 ++++++++++++++++++++++++++++++
 4 files changed, 42 insertions(+)

diff --git a/include/linux/trace_seq.h b/include/linux/trace_seq.h
index 5a2c650..9703d03 100644
--- a/include/linux/trace_seq.h
+++ b/include/linux/trace_seq.h
@@ -95,6 +95,7 @@ extern void trace_seq_bitmask(struct trace_seq *s, const unsigned long *maskp,
 extern int trace_seq_hex_dump(struct trace_seq *s, const char *prefix_str,
 			      int prefix_type, int rowsize, int groupsize,
 			      const void *buf, size_t len, bool ascii);
+void *trace_seq_alloc_buffer(struct trace_seq *s, int len);
 
 #else /* CONFIG_TRACING */
 static inline void trace_seq_printf(struct trace_seq *s, const char *fmt, ...)
@@ -138,6 +139,11 @@ static inline int trace_seq_path(struct trace_seq *s, const struct path *path)
 {
 	return 0;
 }
+
+static inline void *trace_seq_alloc_buffer(struct trace_seq *s, int len)
+{
+	return NULL;
+}
 #endif /* CONFIG_TRACING */
 
 #endif /* _LINUX_TRACE_SEQ_H */
diff --git a/include/trace/stages/stage3_trace_output.h b/include/trace/stages/stage3_trace_output.h
index 66374df..a2921bd 100644
--- a/include/trace/stages/stage3_trace_output.h
+++ b/include/trace/stages/stage3_trace_output.h
@@ -139,3 +139,7 @@
 		u64 ____val = (u64)(value);		\
 		(u32) do_div(____val, NSEC_PER_SEC);	\
 	})
+
+#undef __get_buf
+#define __get_buf(len)							\
+	trace_seq_alloc_buffer(p, len)
diff --git a/include/trace/stages/stage7_class_define.h b/include/trace/stages/stage7_class_define.h
index 8795429..bcb960d 100644
--- a/include/trace/stages/stage7_class_define.h
+++ b/include/trace/stages/stage7_class_define.h
@@ -23,6 +23,7 @@
 #undef __get_rel_sockaddr
 #undef __print_array
 #undef __print_hex_dump
+#undef __get_buf
 
 /*
  * The below is not executed in the kernel. It is only what is
diff --git a/kernel/trace/trace_seq.c b/kernel/trace/trace_seq.c
index 9c90b3a..1501e3b 100644
--- a/kernel/trace/trace_seq.c
+++ b/kernel/trace/trace_seq.c
@@ -403,3 +403,34 @@ int trace_seq_hex_dump(struct trace_seq *s, const char *prefix_str,
 	return 1;
 }
 EXPORT_SYMBOL(trace_seq_hex_dump);
+
+/**
+ * trace_seq_alloc_buffer - allocate seq buffer with size len
+ * @s: trace sequence descriptor
+ * @len: size of buffer to be allocated
+ *
+ * allocate space with size of @len from seq buffer for output usage,
+ * On success, it returns start address of the allocated buffer,
+ * user can fill data start from the address, user should make sure the
+ * data length not exceed the @len, if it exceed, behavior is undefined.
+ *
+ * Returns NULL if no buffer can be allocated, it also means system will
+ * crash, it is user responsiblity to make sure total buffer used will
+ * not exceed PAGE_SIZE.
+ *
+ * it allow multiple usage in one trace output function call.
+ */
+void *trace_seq_alloc_buffer(struct trace_seq *s, int len)
+{
+	char *buf = trace_seq_buffer_ptr(s);
+
+	seq_buf_commit(&s->seq, len);
+
+	if (unlikely(seq_buf_has_overflowed(&s->seq))) {
+		s->full = 1;
+		return NULL;
+	}
+
+	return (void *)buf;
+}
+EXPORT_SYMBOL(trace_seq_alloc_buffer);
-- 
2.7.4