Sometimes it is useful to observe (and maybe modify) data coming from
an input device, but only do that if there are other users of such input
device. An example is touchpad switching functionality on Lenovo IdeaPad
Z570 where it is desirable to suppress events coming from the touchpad
if user toggles touchpad on/off button (on this laptop the firmware does
not stop the device).

Introduce notion of passive observers for input handlers to solve this
issue. An input handler marked as passive observer behaves exactly like
any other input handler or filter, but with one exception: it does not
open/start underlying input device when attaching to it.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
---

 v2: fix incorrect handling in input_close_device() - we still need to
     decrement handle's open count and ensure that no more events will
     come through the handle even if the handler is an observer.
 v1: original posting:
     https://lore.kernel.org/all/ZteiClP9jabjHFkG@google.com

Maxim, I did not add "tested-by" by you because this version is
different from V1.

Thanks!

 drivers/input/input.c | 15 ++++++++++-----
 include/linux/input.h |  5 +++++
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/drivers/input/input.c b/drivers/input/input.c
index 3c321671793f..3b1e88ead97e 100644
--- a/drivers/input/input.c
+++ b/drivers/input/input.c
@@ -605,6 +605,9 @@ int input_open_device(struct input_handle *handle)
 
 	handle->open++;
 
+	if (handle->handler->passive_observer)
+		goto out;
+
 	if (dev->users++ || dev->inhibited) {
 		/*
 		 * Device is already opened and/or inhibited,
@@ -668,11 +671,13 @@ void input_close_device(struct input_handle *handle)
 
 	__input_release_device(handle);
 
-	if (!--dev->users && !dev->inhibited) {
-		if (dev->poller)
-			input_dev_poller_stop(dev->poller);
-		if (dev->close)
-			dev->close(dev);
+	if (!handle->handler->passive_observer) {
+		if (!--dev->users && !dev->inhibited) {
+			if (dev->poller)
+				input_dev_poller_stop(dev->poller);
+			if (dev->close)
+				dev->close(dev);
+		}
 	}
 
 	if (!--handle->open) {
diff --git a/include/linux/input.h b/include/linux/input.h
index 89a0be6ee0e2..6437c35f0796 100644
--- a/include/linux/input.h
+++ b/include/linux/input.h
@@ -286,6 +286,10 @@ struct input_handle;
  * @start: starts handler for given handle. This function is called by
  *	input core right after connect() method and also when a process
  *	that "grabbed" a device releases it
+ * @passive_observer: set to %true by drivers only interested in observing
+ *	data stream from devices if there are other users present. Such
+ *	drivers will not result in starting underlying hardware device
+ *	when input_open_device() is called for their handles
  * @legacy_minors: set to %true by drivers using legacy minor ranges
  * @minor: beginning of range of 32 legacy minors for devices this driver
  *	can provide
@@ -321,6 +325,7 @@ struct input_handler {
 	void (*disconnect)(struct input_handle *handle);
 	void (*start)(struct input_handle *handle);
 
+	bool passive_observer;
 	bool legacy_minors;
 	int minor;
 	const char *name;
-- 
2.47.0.105.g07ac214952-goog


-- 
Dmitry