[PATCH] ARM: imx6q: cpuidle: fix bug that CPU may not wake up

[Kohji Okuno <okuno.kohji@jp.panasonic.com>]

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Hi,
I found the bug in cpuidle-imx6q.c. Could you check my patch?

 In the current cpuidle implementation for i.MX6q, the CPU that sets
"WAIT_UNCLOCKED" and the CPU that returns to "WAIT_CLOCKED" are always
the same. While the CPU that sets "WAIT_UNCLOCKED" is in IDLE state of
"WAIT", if the other CPU wakes up and enters IDLE state of "WFI"
istead of "WAIT", this CPU can not wake up at expired time.
 Because, in the case of "WFI", the CPU must be waked up by the local
timer interrupt. But, while "WAIT_UNCLOCKED" is set, the local timer
is stopped, when all CPUs execute "wfi" instruction. As a result, the
local timer interrupt is not fired.
 In this situation, this CPU will wake up by IRQ different from local
timer. (e.g. broacast tiemr)

So, this fix changes CPU to return to "WAIT_CLOCKED".

Best regards,
 Kohji Okuno

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From 2cfeb5f68c2dd0de4854dbac091bf330e25f00d1 Mon Sep 17 00:00:00 2001
From: Kohji Okuno <okuno.kohji@jp.panasonic.com>
Date: Fri, 22 Feb 2019 17:45:26 +0900
Subject: [PATCH] ARM: imx6q: cpuidle: fix bug that CPU may not wake up at
 expected time

 In the current cpuidle implementation for i.MX6q, the CPU that sets
"WAIT_UNCLOCKED" and the CPU that returns to "WAIT_CLOCKED" are always
the same. While the CPU that sets "WAIT_UNCLOCKED" is in IDLE state of
"WAIT", if the other CPU wakes up and enters IDLE state of "WFI"
istead of "WAIT", this CPU can not wake up at expired time.
 Because, in the case of "WFI", the CPU must be waked up by the local
timer interrupt. But, while "WAIT_UNCLOCKED" is set, the local timer
is stopped, when all CPUs execute "wfi" instruction. As a result, the
local timer interrupt is not fired.
 In this situation, this CPU will wake up by IRQ different from local
timer. (e.g. broacast tiemr)

So, this fix changes CPU to return to "WAIT_CLOCKED".
---
 arch/arm/mach-imx/cpuidle-imx6q.c | 25 ++++++++++---------------
 1 file changed, 10 insertions(+), 15 deletions(-)

diff --git a/arch/arm/mach-imx/cpuidle-imx6q.c b/arch/arm/mach-imx/cpuidle-imx6q.c
index bfeb25aaf9a2..22d982d451d1 100644
--- a/arch/arm/mach-imx/cpuidle-imx6q.c
+++ b/arch/arm/mach-imx/cpuidle-imx6q.c
@@ -16,30 +16,25 @@
 #include "cpuidle.h"
 #include "hardware.h"
 
-static atomic_t master = ATOMIC_INIT(0);
+static int master = 0;
 static DEFINE_SPINLOCK(master_lock);
 
 static int imx6q_enter_wait(struct cpuidle_device *dev,
 			    struct cpuidle_driver *drv, int index)
 {
-	if (atomic_inc_return(&master) == num_online_cpus()) {
-		/*
-		 * With this lock, we prevent other cpu to exit and enter
-		 * this function again and become the master.
-		 */
-		if (!spin_trylock(&master_lock))
-			goto idle;
+	spin_lock(&master_lock);
+	if (++master == num_online_cpus()) {
 		imx6_set_lpm(WAIT_UNCLOCKED);
-		cpu_do_idle();
-		imx6_set_lpm(WAIT_CLOCKED);
-		spin_unlock(&master_lock);
-		goto done;
 	}
+	spin_unlock(&master_lock);
 
-idle:
 	cpu_do_idle();
-done:
-	atomic_dec(&master);
+
+	spin_lock(&master_lock);
+	if (master-- == num_online_cpus()) {
+		imx6_set_lpm(WAIT_CLOCKED);
+	}
+	spin_unlock(&master_lock);
 
 	return index;
 }
-- 
2.17.1


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