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>

diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c b/drivers/media/v4l2-core/v4l2-ctrls.c
index 76fa2db0e8fb24f02eb78448c455f4e72f4702dc..cd1ae016706f6f8f3a35e96272851180da27cae3 100644 (file)
--- a/drivers/media/v4l2-core/v4l2-ctrls.c
+++ b/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)
 {