ARCv2: Elide sending new cross core intr if receiver didn't ack prev

ARConnect/MCIP IPI sending has a retry-wait loop in case caller had
not seen a previous such interrupt. Turns out that it is not needed at
all. Linux cross core calling allows coalescing multiple IPIs to same
receiver - it is fine as long as there is one.

This logic is built into upper layer already, at a higher level of
abstraction. ipi_send_msg_one() sets the actual msg payload, but it only
calls MCIP IPI sending if msg holder was empty (using
atomic-set-new-and-get-old construct). Thus it is unlikely that the
retry-wait looping was ever getting exercised at all.

Cc: Chuck Jordan <cjordan@synopsys.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

diff --git a/arch/arc/kernel/mcip.c b/arch/arc/kernel/mcip.c
index e30d5d4283307bfb0331dfaa9acb756a7f8b9573..7afc3c703ed102d037e86d738fc7eb282d235b14 100644 (file)
--- a/arch/arc/kernel/mcip.c
+++ b/arch/arc/kernel/mcip.c
@@ -40,26 +40,19 @@ static void mcip_ipi_send(int cpu)
                return;
        }
 
+       raw_spin_lock_irqsave(&mcip_lock, flags);
+
        /*
-        * NOTE: We must spin here if the other cpu hasn't yet
-        * serviced a previous message. This can burn lots
-        * of time, but we MUST follows this protocol or
-        * ipi messages can be lost!!!
-        * Also, we must release the lock in this loop because
-        * the other side may get to this same loop and not
-        * be able to ack -- thus causing deadlock.
+        * If receiver already has a pending interrupt, elide sending this one.
+        * Linux cross core calling works well with concurrent IPIs
+        * coalesced into one
+        * see arch/arc/kernel/smp.c: ipi_send_msg_one()
         */
+       __mcip_cmd(CMD_INTRPT_READ_STATUS, cpu);
+       ipi_was_pending = read_aux_reg(ARC_REG_MCIP_READBACK);
+       if (!ipi_was_pending)
+               __mcip_cmd(CMD_INTRPT_GENERATE_IRQ, cpu);
 
-       do {
-               raw_spin_lock_irqsave(&mcip_lock, flags);
-               __mcip_cmd(CMD_INTRPT_READ_STATUS, cpu);
-               ipi_was_pending = read_aux_reg(ARC_REG_MCIP_READBACK);
-               if (ipi_was_pending == 0)
-                       break; /* break out but keep lock */
-               raw_spin_unlock_irqrestore(&mcip_lock, flags);
-       } while (1);
-
-       __mcip_cmd(CMD_INTRPT_GENERATE_IRQ, cpu);
        raw_spin_unlock_irqrestore(&mcip_lock, flags);
 
 #ifdef CONFIG_ARC_IPI_DBG