[PATCH v2] Input: document inhibiting

Andrzej Pietrasiewicz
2020-06-17T10:18:41Z

Document inhibiting input devices and its relation to being
a wakeup source.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
---
v1..v2:

- Addressed editorial comments from Randy
- Added a paragraph by Hans

Documentation/input/input-programming.rst | 40 +++++++++++++++++++++++
1 file changed, 40 insertions(+)

diff --git a/Documentation/input/input-programming.rst b/Documentation/input/input-programming.rst
index 45a4c6e05e39..7432315cc829 100644
--- a/Documentation/input/input-programming.rst
+++ b/Documentation/input/input-programming.rst
@@ -164,6 +164,46 @@ disconnects. Calls to both callbacks are serialized.
The open() callback should return a 0 in case of success or any nonzero value
in case of failure. The close() callback (which is void) must always succeed.

+Inhibiting input devices
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Inhibiting a device means ignoring input events from it. As such it is about maintaining
+relationships with input handlers - either already existing relationships, or relationships
+to be established while the device is in inhibited state.
+
+If a device is inhibited, no input handler will receive events from it.
+
+The fact that nobody wants events from the device is exploited further, by calling device's
+close() (if there are users) and open() (if there are users) on inhibit and uninhibit
+operations, respectively. Indeed, the meaning of close() is to stop providing events
+to the input core and that of open() is to start providing events to the input core.
+
+Calling the device's close() method on inhibit (if there are users) allows the driver
+to save power. Either by directly powering down the device or by releasing the
+runtime-pm reference it got in open() when the driver is using runtime-pm.
+
+Inhibiting and uninhibiting are orthogonal to opening and closing the device by input
+handlers. Userspace might want to inhibit a device in anticipation before any handler is
+positively matched against it.
+
+Inhibiting and uninhibiting are orthogonal to device's being a wakeup source, too. Being a
+wakeup source plays a role when the system is sleeping, not when the system is operating.
+How drivers should program their interaction between inhibiting, sleeping and being a wakeup
+source is driver-specific.
+
+Taking the analogy with the network devices - bringing a network interface down doesn't mean
+that it should be impossible be wake the system up on LAN through this interface. So, there
+may be input drivers which should be considered wakeup sources even when inhibited. Actually,
+in many I2C input devices their interrupt is declared a wakeup interrupt and its handling
+happens in driver's core, which is not aware of input-specific inhibit (nor should it be).
+Composite devices containing several interfaces can be inhibited on a per-interface basis and
+e.g. inhibiting one interface shouldn't affect the device's capability of being a wakeup source.
+
+If a device is to be considered a wakeup source while inhibited, special care must be taken when
+programming its suspend(), as it might need to call device's open(). Depending on what close()
+means for the device in question, not opening() it before going to sleep might make it
+impossible to provide any wakeup events. The device is going to sleep anyway.
+
Basic event types
~~~~~~~~~~~~~~~~~

--
2.17.1