1 files changed, 44 insertions, 4 deletions

diff --git a/arch/arc/kernel/mcip.c b/arch/arc/kernel/mcip.c
index e6ad6e64440a..35921c3ab394 100644
--- a/arch/arc/kernel/mcip.c
+++ b/arch/arc/kernel/mcip.c
@@ -33,27 +33,67 @@ void mcip_init_smp(unsigned int cpu)
 static void mcip_ipi_send(int cpu)
 {
 	unsigned long flags;
+	int ipi_was_pending;
+
+	/*
+	 * 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.
+	 */
+
+	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);
 
-	raw_spin_lock_irqsave(&mcip_lock, flags);
 	__mcip_cmd(CMD_INTRPT_GENERATE_IRQ, cpu);
 	raw_spin_unlock_irqrestore(&mcip_lock, flags);
+
+#ifdef CONFIG_ARC_IPI_DBG
+	if (ipi_was_pending)
+		pr_info("IPI ACK delayed from cpu %d\n", cpu);
+#endif
 }
 
 static void mcip_ipi_clear(int irq)
 {
-	unsigned int cpu;
+	unsigned int cpu, c;
 	unsigned long flags;
+	unsigned int __maybe_unused copy;
 
 	raw_spin_lock_irqsave(&mcip_lock, flags);
 
 	/* Who sent the IPI */
 	__mcip_cmd(CMD_INTRPT_CHECK_SOURCE, 0);
 
-	cpu = read_aux_reg(ARC_REG_MCIP_READBACK);	/* 1,2,4,8... */
+	copy = cpu = read_aux_reg(ARC_REG_MCIP_READBACK);	/* 1,2,4,8... */
 
-	__mcip_cmd(CMD_INTRPT_GENERATE_ACK, __ffs(cpu)); /* 0,1,2,3... */
+	/*
+	 * In rare case, multiple concurrent IPIs sent to same target can
+	 * possibly be coalesced by MCIP into 1 asserted IRQ, so @cpus can be
+	 * "vectored" (multiple bits sets) as opposed to typical single bit
+	 */
+	do {
+		c = __ffs(cpu);			/* 0,1,2,3 */
+		__mcip_cmd(CMD_INTRPT_GENERATE_ACK, c);
+		cpu &= ~(1U << c);
+	} while (cpu);
 
 	raw_spin_unlock_irqrestore(&mcip_lock, flags);
+
+#ifdef CONFIG_ARC_IPI_DBG
+	if (c != __ffs(copy))
+		pr_info("IPIs from %x coalesced to %x\n",
+			copy, raw_smp_processor_id());
+#endif
 }
 
 volatile int wake_flag;