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Commit 3dea30ca authored by Vineet Gupta's avatar Vineet Gupta
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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: default avatarVineet Gupta <vgupta@synopsys.com>
parent 96817879

arch/arc/kernel/mcip.c

+10 −17
Original line number Diff line number Diff line
@@ -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()

	 */



	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);



	if (!ipi_was_pending)

		__mcip_cmd(CMD_INTRPT_GENERATE_IRQ, cpu);



	raw_spin_unlock_irqrestore(&mcip_lock, flags);



#ifdef CONFIG_ARC_IPI_DBG