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From:   Benjamin Tissoires <benjamin.tissoires@gmail.com>
To:     Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Jiri Kosina <jikos@kernel.org>,
        Harry Cutts <hcutts@chromium.org>,
        Peter Hutterer <peter.hutterer@who-t.net>
Cc:     linux-input@vger.kernel.org, linux-kernel@vger.kernel.org,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>
Subject: [PATCH] Input: reserve 2 events code because of HID
Date:   Fri,  7 Sep 2018 10:51:15 +0200
Message-Id: <20180907085115.30242-1-benjamin.tissoires@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: Benjamin Tissoires <benjamin.tissoires@redhat.com>

Prior to commit 190d7f02ce8e ("HID: input: do not increment usages when
a duplicate is found") from the v4.18 kernel, HID used to shift the
event codes if a duplicate usage was found. This ended up in a situation
where a device would export a ton of ABS_MISC+n event codes, or a ton
of REL_MISC+n event codes.

This is now fixed, however userspace needs to detect those situation.
Fortunately, ABS_MISC+1 was never assigned a code, and so libinput
can detect fake multitouch devices from genuine ones by checking is
ABS_MISC+1 is set.

Now that we have REL_WHEEL_HI_RES, libinput won't be able to differentiate
true high res mice from some other device in a pre-v4.18 kernel.

Set in stone that the ABS_MISC+1 and REL_MISC+1 are reserved and should not
be used so userspace can properly work around those old kernels.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
---

Hi,

while reviewing my local tree, I realize that we might want to be able
to differentiate older kernels from new ones that export REL_WHEEL_HI_RES.

I know Dmitry was against adding several REL_MISC, so I hope just moving
REL_WHEEL_HI_RES by one and reserving the faulty event codes would be good
this time.

This patch applies on top of the branch for-4.20/logitech-highres from
Jiri's tree. It should go through Jiri's tree as well.

Cheers,
Benjamin

 include/uapi/linux/input-event-codes.h | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/input-event-codes.h b/include/uapi/linux/input-event-codes.h
index 29fb891ea337..30149939249a 100644
--- a/include/uapi/linux/input-event-codes.h
+++ b/include/uapi/linux/input-event-codes.h
@@ -708,7 +708,12 @@
 #define REL_DIAL		0x07
 #define REL_WHEEL		0x08
 #define REL_MISC		0x09
-#define REL_WHEEL_HI_RES	0x0a
+/*
+ * 0x0a is reserved and should not be used.
+ * It was used by HID as REL_MISC+1 and usersapce needs to detect if
+ * the next REL_* event is correct or is just REL_MISC + n.
+ */
+#define REL_WHEEL_HI_RES	0x0b
 #define REL_MAX			0x0f
 #define REL_CNT			(REL_MAX+1)
 
@@ -745,6 +750,12 @@
 
 #define ABS_MISC		0x28
 
+/*
+ * 0x29 is reserved and should not be used.
+ * It was used by HID as ABS_MISC+1 and usersapce needs to detect if
+ * the next ABS_* event is correct or is just ABS_MISC + n.
+ */
+
 #define ABS_MT_SLOT		0x2f	/* MT slot being modified */
 #define ABS_MT_TOUCH_MAJOR	0x30	/* Major axis of touching ellipse */
 #define ABS_MT_TOUCH_MINOR	0x31	/* Minor axis (omit if circular) */
-- 
2.14.3