[riot-notifications] [RIOT-OS/RIOT] cpu/stm32_common: fix issue while clearing EXTI->PR reg (#14100)

Abdulkerim Altuntas notifications at github.com
Tue May 19 13:10:26 CEST 2020

Since the **EXTI->PR** is an **rc_w1** type register, we need to be careful when clearing our interrupt flag in the register. When there are multiple interrupt flags set in the register, the `|=` operation will mistakenly clear all pending interrupts instead of just ours.

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### Contribution description
- The ISRs in **rtc.c** and **rtt.c** drivers are involved.
- If there are multiple bits set in the Pending Register in **EXTI**, clearing the bit by
`EXTI->PR |= EXTI_PR_BIT;` operation will mistakenly delete the other flags as well due to the fact that `EXTI->PR` is an `rc_w1` type register.
- Instead of this, we just need to do `EXTI->PR = EXTI_PR_BIT` in order to clear our flag and preserve the other possible flags like it is done so in `isr_exti` in **gpio.c**.

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- describe which part(s) of RIOT is (are) involved
- if it's a bug fix, describe the bug that it solves and how it is solved
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### Testing procedure
- We observed a strange issue in our FW where, _sometimes_, the `isr_exti` ISR is executed with `EXTI->PR = 0x0` which should ideally not happen. After some investigation, we detected this bug.
- It's really hard to describe a test procedure for this issue, since it completely depends on interrupt timings and ISR execution order.
- The expected behavior is that all ISRs only clear their corresponding interrupt flag.
Details steps to test your contribution:
- which test/example to compile for which board and is there a 'test' command
- how to know that it was not working/available in master
- the expected success test output

### Issues/PRs references
No known references.
Examples: Fixes #1234. See also #5678. Depends on PR #9876.

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-- Commit Summary --

  * cpu/stm32_common: fix issue while clearing EXTI->PR reg

-- File Changes --

    M cpu/stm32_common/periph/rtc.c (4)
    M cpu/stm32_common/periph/rtt.c (4)

-- Patch Links --


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