media: v4l2-ctrl: Add a comment on why we zero out compound controls fields

Since it's not necessarily very explicit why we need to zero some fields
in std_validate_compound, add a comment before the function to explain
why, which should help as a reminder.
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/v4l2-core/v4l2-ctrls.c

@@ -1645,6 +1645,10 @@ static void std_log(const struct v4l2_ctrl *ctrl)
 #define zero_padding(s) \
 	memset(&(s).padding, 0, sizeof((s).padding))
 
+/*
+ * Compound controls validation requires setting unused fields/flags to zero
+ * in order to properly detect unchanged controls with std_equal's memcmp.
+ */
 static int std_validate_compound(const struct v4l2_ctrl *ctrl, u32 idx,
 				 union v4l2_ctrl_ptr ptr)
 {