linux-kernel - [PATCH] arch/tile: add "nop" after "nap" to help GX idle power draw

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Message-Id: <201203302105.q2UL57lt006300@farm-0012.internal.tilera.com>
Date:	Thu, 29 Mar 2012 15:57:18 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	Chris Metcalf <cmetcalf@...era.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jesper Nilsson <jesper.nilsson@...s.com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] arch/tile: add "nop" after "nap" to help GX idle power draw

This avoids the hardware istream prefetcher doing unnecessary work.

Signed-off-by: Chris Metcalf <cmetcalf@...era.com>
---
 arch/tile/kernel/entry.S |    2 ++
 arch/tile/kernel/smp.c   |    2 +-
 2 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/arch/tile/kernel/entry.S b/arch/tile/kernel/entry.S
index f8d6155..133c4b5 100644
--- a/arch/tile/kernel/entry.S
+++ b/arch/tile/kernel/entry.S
@@ -85,6 +85,7 @@ STD_ENTRY(cpu_idle_on_new_stack)
 /* Loop forever on a nap during SMP boot. */
 STD_ENTRY(smp_nap)
 	nap
+	nop       /* avoid provoking the icache prefetch with a jump */
 	j smp_nap /* we are not architecturally guaranteed not to exit nap */
 	jrp lr    /* clue in the backtracer */
 	STD_ENDPROC(smp_nap)
@@ -106,5 +107,6 @@ STD_ENTRY(_cpu_idle)
 	.global _cpu_idle_nap
 _cpu_idle_nap:
 	nap
+	nop       /* avoid provoking the icache prefetch with a jump */
 	jrp lr
 	STD_ENDPROC(_cpu_idle)
diff --git a/arch/tile/kernel/smp.c b/arch/tile/kernel/smp.c
index 7579607..f86887a 100644
--- a/arch/tile/kernel/smp.c
+++ b/arch/tile/kernel/smp.c
@@ -122,7 +122,7 @@ static void smp_stop_cpu_interrupt(void)
 	set_cpu_online(smp_processor_id(), 0);
 	arch_local_irq_disable_all();
 	for (;;)
-		asm("nap");
+		asm("nap; nop");
 }
 
 /* This function calls the 'stop' function on all other CPUs in the system. */
-- 
1.6.5.2

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