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rp: Fix time driver hang #3763

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merged 2 commits into from
Feb 5, 2025
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robot-rover
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Fixes: #3758

This commit fixes a race condition in the embassy-rp time driver IRQ that causes the time driver to lock up when an alarm is scheduled that expires before the IRQ finishes.

I convinced myself that this fix worked by first changing the IRQ to be

fn check_alarm(&self) { 
     let n = 0; 
     critical_section::with(|cs| { 
         let alarm = &self.alarms.borrow(cs); 
         let timestamp = alarm.timestamp.get(); 
         if timestamp <= self.now() { 
             self.trigger_alarm(cs) 
         } else { 
             // Not elapsed, arm it again. 
             // This can happen if it was set more than 2^32 us in the future. 
             TIMER.alarm(n).write_value(timestamp as u32); 
         } 
     }); 

     // NEW LINES HERE
     for _ in 0..1000 {
         cortex_m::asm::nop()
     }
     // END NEW LINES
  
     // clear the irq 
     TIMER.intr().write(|w| w.set_alarm(n, true)); 
 }

This change made the corner case much easier to hit, and I was able to reproduce the issue in ~30 seconds rather than every 15 minutes or so. By moving the IRQ clear to the start of the ISR, I was able to stop the hang from happening altogether.

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@Dirbaio Dirbaio left a comment

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thank you!

@Dirbaio Dirbaio added this pull request to the merge queue Feb 4, 2025
Merged via the queue into embassy-rs:main with commit fdb9795 Feb 5, 2025
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embassy-rp time-driver-impl hangs
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