linux-kernel - [PATCH v2 12/12] Input: docs: add more details on the use of BTN

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Date:   Thu,  3 Feb 2022 15:32:26 +0100
From:   Benjamin Tissoires <benjamin.tissoires@...hat.com>
To:     Jiri Kosina <jikos@...nel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Ahelenia Ziemiańska 
        <nabijaczleweli@...ijaczleweli.xyz>,
        Ping Cheng <pinglinux@...il.com>,
        Aaron Armstrong Skomra <skomra@...il.com>,
        Jason Gerecke <killertofu@...il.com>,
        Peter Hutterer <peter.hutterer@...-t.net>
Cc:     linux-input@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>
Subject: [PATCH v2 12/12] Input: docs: add more details on the use of BTN_TOOL

The HID core stack used to be very relaxed considering the BTN_TOOL_*
usage. With the recent commits, we should now enforce to have only one
tool at a time, meaning that we can now express that requirement in the
docs.

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

---

changes in v2:
- changed to explain that switching tool in one EV_SYN report
  is not nice for userspace
---
 Documentation/input/event-codes.rst | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/Documentation/input/event-codes.rst b/Documentation/input/event-codes.rst
index b24ae7d292cc..8741d390b184 100644
--- a/Documentation/input/event-codes.rst
+++ b/Documentation/input/event-codes.rst
@@ -137,7 +137,11 @@ A few EV_KEY codes have special meanings:
     code should be set to a value of 1. When the tool is no longer interacting
     with the input device, the BTN_TOOL_<name> code should be reset to 0. All
     trackpads, tablets, and touchscreens should use at least one BTN_TOOL_<name>
-    code when events are generated.
+    code when events are generated. Likewise all trackpads, tablets, and
+    touchscreens should export only one BTN_TOOL_<name> at a time. To not break
+    existing userspace, it is recommended to not switch tool in one EV_SYN frame
+    but first emitting the old BTN_TOOL_<name> at 0, then emit one SYN_REPORT
+    and then set the new BTN_TOOL_<name> at 1.
 
 * BTN_TOUCH:
 
-- 
2.33.1