Merge #328

328: Prevent underflow when calling delay(n) with n<2 r=jonas-schievink a=ovidiusabou

Calling delay(1) causes a very long wait (freeze) otherwise.
86cd463 introduced this behaviour by
changing the cycle count from n / 4 + 1 to n / 2 which forces an
underflow when n<2.

Co-authored-by: Ovidiu Sabou <[email protected]>
Co-authored-by: Jonas Schievink <[email protected]>

Author: 3 people

asm/inline.rs

@@ -55,7 +55,8 @@ pub unsafe fn __delay(cyc: u32) {
     // The loop will normally take 3 to 4 CPU cycles per iteration, but superscalar cores
     // (eg. Cortex-M7) can potentially do it in 2, so we use that as the lower bound, since delaying
     // for more cycles is okay.
-    let real_cyc = cyc / 2;
+    // Add 1 to prevent an integer underflow which would cause a long freeze
+    let real_cyc = 1 + cyc / 2;
     asm!(
         // Use local labels to avoid R_ARM_THM_JUMP8 relocations which fail on thumbv6m.
         "1:",