Role of close() in struct input_dev

[Johann D. <jo...@Do...>]

Hi !

I am trying to clean up my addition about multiple open-with-write-access.
My first idea was to use a flag, only_one_writer in struct evdev, and then
perform the necessay checks in evdev_open and evdev_release.

Vojtech suggested that we add a parameter to input_dev::open and pass
that parameter to device drivers and let them decide. This would indeed
allow for more flexibility.
I thought that input_dev::close should be modified the same way.

I tried to implement this, and while doing so, I noticed that it could be
that input_dev::open and input_dev::close are not called the same number
of times. This may or may not be a problem, I don't know.

Here is a sample scenario:
1. Application 1 opens /dev/input/event0
2. Application 2 opens /dev/input/event0 (nothing forbids that)
3. evdev_disconnect is called (usb device is unplugged, input_attach is
kiled...)

Steps 1 and 2 call input_dev::open, step 3 calls input_dev::close.

Now the question is: What is the role of input_dev::close/open ?
Are they meant as "init/deinit the device" or "establish/break connection
with processes" (or a mix of both) ?

I would vote for the second possibility.

By the way, if you add a step 4 to my scenario:
4. Application 1 or 2 writes to /dev/input/event0
then the application segfaults, and dmesg tells me someone tried to
dereference a NULL pointer.

-- 
Johann Deneux
CS student at DoCS (www.docs.uu.se/~johannd) and
ESIL (www.esil.univ-mrs.fr/~jdeneux)