Re: [PATCH] Input: evdev: drain queued events before reporting device removal

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

Hi,

On Wed, Jul 29, 2026 at 05:27:53PM +0800, zhangliuyang.zly wrote:
> When an input device is unregistered, the input core calls
> input_dev_release_keys() and sends a SYN_REPORT before closing the input
> handles. This is intended to deliver synthetic key-up events for keys that
> are still pressed when the device disappears.
> 
> However, evdev_read() currently checks evdev->exist before draining the
> per-client event queue. If evdev_disconnect() marks the evdev node dead
> before userspace reads the queued synthetic release packet, read() returns
> -ENODEV immediately and the queued key-up events are lost from userspace's
> point of view.
> 
> This can happen on Android with USB OTG 2.4G keyboard receivers. The input
> core generates the release event during disconnect, but Android EventHub 
> may observe the evdev hangup/removal first and then fail to read the 
> pending EV_KEY value=0 event. The framework then falls back to device 
> reset/cancel semantics instead of dispatching a normal ACTION_UP.
> 
> Allow evdev_read() to drain already queued events even after evdev->exist 
> is cleared. Return -ENODEV only when the client queue is empty, or when 
> the client has been revoked. Keep zero-length reads compatible with the 
> previous error-checking behavior.

I'd rather Android handle devices going away properly. Input core tries
to clear the key state for its own internal purposes (sysrq handling,
etc); userspace should take care of this on its own.

Thanks.

-- 
Dmitry