GPIO and interrupt

[Ahmad Fatoum]

Hello Tony,

On 26.07.23 12:10, Tony He wrote:
> Hi,
> 
> I'm writing button driver. The button is based on GPIO.When the button
> is pressed or released, the interrupt is generated. According to the
> GPIO value(1 or 0) , I can know which action is taken(pressed or
> released).
> However, a question occurred to me. Do I need to consider when to get
> the GPIO value? I mean is it possible that GPIO value will be changed
> caused by another interrupt when I get the GPIO value in top or bottom
> half
> of first interrupt? I know the CPU is fast and pushing/releasing
> button is slow, so it should not happen in practice. But let's just
> consider this case in theory. You can assume the button is pushed or
> released very very fast.

This is not only a theoretical issue. The transition from button being
pressed to unpressed and back is not noise free, so a naive translation
of IRQ/GPIO state to input events may not be sufficient.

Dealing with this is called debouncing and involves taking multiple
samples before signalling the input event.

> Seems that GPIO interrupt is special comparing with other device with
> cache. There is no cache to store data(GPIO value) for GPIO. Right?
> Can someone help to clarify this question? Thanks a lot.

IRQ handlers have a second argument with a pointer to device specific data.
You can store information there and many drivers do so. You need to take
care that the data isn't accessed concurrently though, e.g. via a spinlock.

You can see how all this is implemented in the Linux gpio_keys driver.

Cheers,
Ahmad

> 
> Tony
> 
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