Nested interrupts on ARM

[Andrew Parlane <andrewp@carallon.com>]

I found a bug with our product which boils down to a high priority 
interrupt not being called fast enough sometimes because we are in a 
different ISR (lower priority).  We have 
CYGSEM_HAL_COMMON_INTERRUPTS_ALLOW_NESTING on, but looking through the 
code this does very little on ARM chips (just sets the min stack size).

Looking at packages/hal/arm/arch/current/src/vectors.S it looks like the 
framework for nested interrupts is in place, but during an interrupt the 
IRQ bit in the CPSR register is never cleared. I wrote the following 
patch, which enables IRQs runs the ISR and disables IRQs again. This 
works, and I can now see nested interrupts working, however I'm now 
seeing it take 30us to get into my high priority ISR when I'm already in 
an ISR, rather than the 7us when I'm not already in an ISR.

So my questions:
1) Does my patch make sense?
2) Any idea why this wasn't already in place? It seems like something so 
simple must have been left out for a reason.
3) Any idea on the 30us to get into my ISR? This may be the first ISR 
disabling IRQs for a bit, I need to look more into this area.

Thanks,
Andrew Parlane
Carallon Ltd.


--- a/packages/hal/arm/arch/current/src/vectors.S
+++ b/packages/hal/arm/arch/current/src/vectors.S
@@ -914,7 +914,14 @@ IRQ:
  #endif // CYGIMP_HAL_COMMON_INTERRUPTS_IGNORE_SPURIOUS
          b       spurious_IRQ

-10:     ldr     r1,.hal_interrupt_data
+10:
+        // re-enable IRQs so we can have nested interrutps
+        ARM_MODE(r1,10)
+        mrs     r1,cpsr
+        bic     r1,r1,#CPSR_IRQ_DISABLE
+        msr     cpsr, r1
+
+        ldr     r1,.hal_interrupt_data
          ldr     r1,[r1,v1,lsl #2]       // handler data
          ldr     r2,.hal_interrupt_handlers
          ldr     v3,[r2,v1,lsl #2]       // handler (indexed by vector #)
@@ -926,6 +933,11 @@ IRQ:
          mov     lr,pc                   // invoke handler (call indirect
          bx      v3                      // thru v3)

+        // disable IRQs
+        mrs     r1,cpsr
+        orr     r1,r1,#CPSR_IRQ_DISABLE
+        msr     cpsr, r1
+
  spurious_IRQ: