tracing: probe-events: Add comments about entry data storing code

Add comments about entry data storing code to __store_entry_arg() and
traceprobe_get_entry_data_size(). These are a bit complicated because of
building the entry data storing code and scanning it.

This just add comments, no behavior change.

Link: https://lore.kernel.org/all/174061715004.501424.333819546601401102.stgit@devnote2/

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Closes: https://lore.kernel.org/all/20250226102223.586d7119@gandalf.local.home/
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Author: mhiramat

kernel/trace/trace_probe.c

@@ -770,6 +770,10 @@ static int check_prepare_btf_string_fetch(char *typename,
 
 #ifdef CONFIG_HAVE_FUNCTION_ARG_ACCESS_API
 
+/*
+ * Add the entry code to store the 'argnum'th parameter and return the offset
+ * in the entry data buffer where the data will be stored.
+ */
 static int __store_entry_arg(struct trace_probe *tp, int argnum)
 {
 	struct probe_entry_arg *earg = tp->entry_arg;
@@ -793,6 +797,20 @@ static int __store_entry_arg(struct trace_probe *tp, int argnum)
 		tp->entry_arg = earg;
 	}
 
+	/*
+	 * The entry code array is repeating the pair of
+	 * [FETCH_OP_ARG(argnum)][FETCH_OP_ST_EDATA(offset of entry data buffer)]
+	 * and the rest of entries are filled with [FETCH_OP_END].
+	 *
+	 * To reduce the redundant function parameter fetching, we scan the entry
+	 * code array to find the FETCH_OP_ARG which already fetches the 'argnum'
+	 * parameter. If it doesn't match, update 'offset' to find the last
+	 * offset.
+	 * If we find the FETCH_OP_END without matching FETCH_OP_ARG entry, we
+	 * will save the entry with FETCH_OP_ARG and FETCH_OP_ST_EDATA, and
+	 * return data offset so that caller can find the data offset in the entry
+	 * data buffer.
+	 */
 	offset = 0;
 	for (i = 0; i < earg->size - 1; i++) {
 		switch (earg->code[i].op) {
@@ -826,6 +844,16 @@ int traceprobe_get_entry_data_size(struct trace_probe *tp)
 	if (!earg)
 		return 0;
 
+	/*
+	 * earg->code[] array has an operation sequence which is run in
+	 * the entry handler.
+	 * The sequence stopped by FETCH_OP_END and each data stored in
+	 * the entry data buffer by FETCH_OP_ST_EDATA. The FETCH_OP_ST_EDATA
+	 * stores the data at the data buffer + its offset, and all data are
+	 * "unsigned long" size. The offset must be increased when a data is
+	 * stored. Thus we need to find the last FETCH_OP_ST_EDATA in the
+	 * code array.
+	 */
 	for (i = 0; i < earg->size; i++) {
 		switch (earg->code[i].op) {
 		case FETCH_OP_END: