sysfs: wait for GPIO interrupt

[Yegor Yefremov <yegorslists@xxxxxxxxxxxxxx>]

I've stumbled upon following issue. When waiting for interrupt with
poll() like described in this example [1], the program returns from
poll() immediately. This thread [2] describes a workaround for this
issue via making read() before invoking poll().

My system is am335x, kernel 3.15-rc4

I experience this behavior by both SoC's own GPIO and i2c expander tca6416.

Is this the proper way (invoke read() before poll()) to work with
GPIOs from user space, so that this approach should be incorporated
into such libraries as libsoc [3]

Yegor

[1] https://developer.ridgerun.com/wiki/index.php/Gpio-int-test.c
[2] http://comments.gmane.org/gmane.linux.distributions.gumstix.general/61286
[3] https://github.com/jackmitch/libsoc
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