[PATCH v2 12/12] Input: docs: add more details on the use of BTN_TOOL

[Benjamin Tissoires]

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@xxxxxxxxxx>

---

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