Re: Need help with libgpiod gpiod_line_request_both_edges_events()

[Bartosz Gołaszewski <bartekgola@xxxxxxxxx>]

2018-03-06 21:49 GMT+01:00 Timur Tabi <timur@xxxxxxxxxxxxxx>:
> On 03/06/2018 01:30 PM, Timur Tabi wrote:
>>
>>
>> Never mind, I figured it out.  I was passing the wrong parameter to
>> pthread_create().  Everything works now.
>>
>> Thanks for all your help.  libgpiod makes a lot more sense to me now.
>
>
> So I found one small oddity.
>
> When I start the interrupt test, the output pin is set to 1, and therefore
> the input pin is also set to 1 since they're wired together.
>
> It appears that gpiod_line_event_wait() is receiving a rising-edge event
> when I call gpiod_line_set_value(..., 1).  That is, the input is reading 1,
> and it is still set to 1, but I get a rising event anyway.
>

This is normal, you're generating an interrupt anyway.

> At one point, I told it to write a 0 to output, and I still got a rising
> event:
>

This isn't normal - do you have an strace output for that? We need to
figure out if it's something in libgpiod or in the kernel.

> Oscillate thread started
> Initial input value: 1
> Setting output to 0
> Received rising edge event, value=0, elapsed=-0.000204s
>
> And it apparently went backwards in time.
>
> What happens in this scenario:
>
> 1. gpiod_line_request_both_edges_events() is called
> 2. GPIO line transitions from 0 to 1
> 3. gpiod_line_event_wait() is called
>

Is it possible that this event was queued in the kernel buffer and
that you actually read one of the previous events?

> Will gpiod_line_event_wait() return immediately, or will it wait until the
> next transition?
>

It will return immediately if there's an unread event pending.

>
> --
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> Technologies, Inc.  Qualcomm Technologies, Inc. is a member of the
> Code Aurora Forum, a Linux Foundation Collaborative Project.



-- 
Best regards,
Bartosz Golaszewski
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