Skip to contentSkip to conversation

arch/arm/kernel: use cpu_relax() in halt loops

PATCH In review Jonathan Neuschäfer 2 participants 3 messages Dec 14, 2010 Arm (32-bit)

lore
Share & tools
lore

Jonathan Neuschäfer

author
#0 lore raw

PATCH 1/1 arch/arm/kernel: use cpu_relax() in halt loops

Looking throught arch/arm/kernel/ I found some halt loops. I just thought we could save some power here using cpu_relax(), or am I missing something?

Signed-off-by Jonathan Neusch?fer <[email protected]>
---
 arch/arm/kernel/process.c |    6 ++++--
 arch/arm/kernel/setup.c   |    7 +++++--
 2 files changed, 9 insertions(+), 4 deletions(-)
arch/arm/kernel/process.c +4 -2
index e76fcaa..4a83fdf 100644
@@ -124,7 +124,8 @@ void arm_machine_restart(char mode, const char *cmd)
 	 */
 	mdelay(1000);
 	printk("Reboot failed -- System halted\n");
-	while (1);
+	while (1)
+		cpu_relax();
 }
 
 /*

@@ -235,7 +236,8 @@ void machine_shutdown(void)
 void machine_halt(void)
 {
 	machine_shutdown();
-	while (1);
+	while (1)
+		cpu_relax();
 }
 
 void machine_power_off(void)
arch/arm/kernel/setup.c +5 -2
index 336f14e..0d77329 100644
@@ -41,6 +41,7 @@
 #include <asm/cacheflush.h>
 #include <asm/cachetype.h>
 #include <asm/tlbflush.h>
+#include <asm/processor.h>
 
 #include <asm/mach/arch.h>
 #include <asm/mach/irq.h>

@@ -337,7 +338,8 @@ static void __init setup_processor(void)
 	if (!list) {
 		printk("CPU configuration botched (ID %08x), unable "
 		       "to continue.\n", read_cpuid_id());
-		while (1);
+		while (1)
+			cpu_relax();
 	}
 
 	cpu_name = list->cpu_name;

@@ -434,7 +436,8 @@ static struct machine_desc * __init setup_machine(unsigned int nr)
 	if (!list) {
 		printk("Machine configuration botched (nr %d), unable "
 		       "to continue.\n", nr);
-		while (1);
+		while (1)
+			cpu_relax();
 	}
 
 	printk("Machine: %s\n", list->name);
Signature · 1 line
1.7.2.3
#1 lore raw
Quoted 3 lines · Jonathan Neusch?fer

Looking throught arch/arm/kernel/ I found some halt loops. I just thought we could save some power here using cpu_relax(), or am I missing something?

#if __LINUX_ARM_ARCH__ == 6
#define cpu_relax() smp_mb()
#else
#define cpu_relax() barrier()
#endif

IOW, it's either a memory barrier or compiler barrier on ARM; it doesn't do any power saving.

author
#2 lore raw
Quoted 5 lines · Russell King - ARM Linux
Quoted 3 lines · Jonathan Neusch?fer

Looking throught arch/arm/kernel/ I found some halt loops. I just thought we could save some power here using cpu_relax(), or am I missing something?

Quoted 8 lines

#if __LINUX_ARM_ARCH__ == 6
#define cpu_relax() smp_mb()
#else
#define cpu_relax() barrier()
#endif

IOW, it's either a memory barrier or compiler barrier on ARM; it doesn't do any power saving.

Okay, thanks for that explanation.