linux-kernel - [PATCH 0/1] Input: evdev - fix wrong timestamp after SYN

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Message-Id: <20241202123351.86957-1-guille.rodriguez@gmail.com>
Date: Mon,  2 Dec 2024 13:33:50 +0100
From: Guillermo Rodríguez <guille.rodriguez@...il.com>
To: linux-kernel@...r.kernel.org
Cc: Guillermo Rodríguez <guille.rodriguez@...il.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	linux-input@...r.kernel.org
Subject: [PATCH 0/1] Input: evdev - fix wrong timestamp after SYN_DROPPED

Hi all,

We are seeing an issue with gpio_keys where the first event after
a SYN_DROPPED has the same timestamp as the SYN_DROPPED event itself.
After some investigation it looks like this is an issue with evdev
and not specific to gpio_keys.

The issue was originally introduced in commit 3b51c44 ("Input: allow
drivers specify timestamp for input events").

This commit introduced input_set_timestamp and input_get_timestamp.
The latter (despite the name) actually generates and stores a timestamp
in dev->timestamp if the driver did not set one itself. This timestamp
needs to be reset when events are flushed; otherwise the next event
will use a stale timestamp. A partial fix was implemented in 4370b23
("Input: reset device timestamp on sync"), but this does not cover the
case of SYN_DROPPED.

If a SYN_DROPPED is generated (currently only done by evdev), the
following happens:

- evdev calls input_get_timestamp to generate a timestamp for the
  SYN_DROPPED event.
- input_get_timestamp generates a timestamp and stores it in
  dev->timestamp
- When the next real event is reported (in evdev_events), evdev
  calls input_get_timestamp, which uses the timestamp already
  stored in dev->timestamp, which corresponds to the SYN_DROPPED event

How to fix:

- When a SYN_DROPPED is generated, after calling input_get_timestamp,
  the timestamp stored in dev->timestamp should be reset (same as is
  currently done in input_handle_event). The attached patch does that.

(Perhaps the underlying problem is that it is not expected that a
function called input_get_timestamp will have side effects. The
commit history of input.c shows that this has actually caused a
few issues since 3b51c44.)

Guillermo Rodríguez (1):
  Input: evdev - fix wrong timestamp after SYN_DROPPED event

 drivers/input/evdev.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

base-commit: e70140ba0d2b1a30467d4af6bcfe761327b9ec95
2.25.1