Implementing a driver that listen to a GPIO

[lroluk@gmail.com (Luca Ellero)]

Hi,

On 03/10/2014 02:33, Jinqiang Zeng wrote:
> 2014-10-02 10:01 GMT-07:00 Clemens Gruber <clemens.gruber@pqgruber.com>:
>> Hi,
>>
>> I am just trying to solve a similar problem as Luca described. I have a
>> TQMa28 board running Linux 3.17-rc7 though.
>> The capacitors keep the system up for about 1500 more milliseconds after
>> the power fails. A GPIO line is set to high as soon as the main power is
>> lost.
>>
>> So I have not much time to quit an important user space process and to
>> clean up the ext4 root filesystem before the power fails.
>>
>> At the moment I pass rootflags=data=journal,commit=1 in the kernel
>> commandline to lower the risk of data loss.
>> In the user space application I open important files using the O_SYNC flag.
>>
>> Is there something else I should do to avoid data loss?
>>
>> How would you implement the "emergency cleanup on power-failure" logic?
>> Dedicated driver (similar to the mentioned gpio-event driver?),
>> implement it with UIO, let a userspace application running as root
>> select /dev/uioX and cleaning up?
> how about  using netlink to notify the usersapce app on gpio interrupt .

thanks  to all for your replies.

Anyway, I solved the issue simply using the GPIO driver (no kernel 
modifications needed):

- export GPIO number (es 7):
$ echo 7 > /sys/class/gpio/export

- set direction:
$ echo "in" > /sys/class/gpio/gpio7/direction

- set event that triggers interrupt:
$ echo "falling" > /sys/class/gpio/gpio7/edge

Then I create a user space application that executes a poll system call 
on "/sys/class/gpio/gpio7/value".

Regards

---
Luca Ellero
http://http://www.brickedbrain.com