ARC: [arcompact] entry.S: Document preemption games for L2 intr

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

diff --git a/arch/arc/kernel/entry-compact.S b/arch/arc/kernel/entry-compact.S
index 15d457b4403ae4a9ab55f455fd7e2ad1492e780c..d9087a1236eb691f425b80bb17ecb0a22ad014c6 100644 (file)
--- a/arch/arc/kernel/entry-compact.S
+++ b/arch/arc/kernel/entry-compact.S
@@ -175,12 +175,25 @@ ENTRY(handle_interrupt_level2)
 
        ;------------------------------------------------------
        ; if L2 IRQ interrupted a L1 ISR, disable preemption
+       ;
+       ; This is to avoid a potential L1-L2-L1 scenario
+       ;  -L1 IRQ taken
+       ;  -L2 interrupts L1 (before L1 ISR could run)
+       ;  -preemption off IRQ, user task in syscall picked to run
+       ;  -RTIE to userspace
+       ;       Returns from L2 context fine
+       ;       But both L1 and L2 re-enabled, so another L1 can be taken
+       ;       while prev L1 is still unserviced
+       ;
        ;------------------------------------------------------
 
+       ; L2 interrupting L1 implies both L2 and L1 active
+       ; However both A2 and A1 are NOT set in STATUS32, thus
+       ; need to check STATUS32_L2 to determine if L1 was active
+
        ld r9, [sp, PT_status32]        ; get statu32_l2 (saved in pt_regs)
        bbit0 r9, STATUS_A1_BIT, 1f     ; L1 not active when L2 IRQ, so normal
 
-       ; A1 is set in status32_l2
        ; bump thread_info->preempt_count (Disable preemption)
        GET_CURR_THR_INFO_FROM_SP   r10
        ld      r9, [r10, THREAD_INFO_PREEMPT_COUNT]