linux-kernel - [PATCH v7 1/4] media: v4l2-ctrl: Add a comment on why we zero out compound controls fields

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Date:   Thu, 25 Jul 2019 20:55:59 +0200
From:   Paul Kocialkowski <paul.kocialkowski@...tlin.com>
To:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        devel@...verdev.osuosl.org, linux-arm-kernel@...ts.infradead.org,
        linux-sunxi@...glegroups.com
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Hans Verkuil <hverkuil@...all.nl>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Alexandre Courbot <acourbot@...omium.org>,
        Nicolas Dufresne <nicolas@...fresne.ca>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Jonas Karlman <jonas@...boo.se>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>
Subject: [PATCH v7 1/4] 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@...tlin.com>
---
 drivers/media/v4l2-core/v4l2-ctrls.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c b/drivers/media/v4l2-core/v4l2-ctrls.c
index 76fa2db0e8fb..cd1ae016706f 100644
--- 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)
 {
-- 
2.22.0