[PATCHv4 2/4] ARM: Allow machines to override __delay()

[Stephen Boyd <sboyd@codeaurora.org>]

Some machines want to implement their own __delay() routine based
on fixed rate timers. Expose functionality to set the __delay()
routine at runtime. This should allow two machines with different
__delay() routines to happily co-exist within the same kernel
with minimal overhead.

Russell expressed concern that using a timer based __delay()
would cause problems when an iomapped device isn't mapped in
prior to a delay call being made (see
http://article.gmane.org/gmane.linux.ports.arm.kernel/78543 for
more info). We can sidestep that issue with this approach since
the __delay() routine _should_ only be pointed to a timer based
delay once the timer has been properly mapped. Up until that
point __delay() and udelay() will use delay_loop() which is
always safe to call.

This patch is inspired by x86's delay.c

Reviewed-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
---

Changed to inline set_delay_fn()

 arch/arm/include/asm/delay.h |    7 +++++++
 arch/arm/lib/delay.c         |   14 +++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/arch/arm/include/asm/delay.h b/arch/arm/include/asm/delay.h
index ccc5ed5..82ef82a 100644
--- a/arch/arm/include/asm/delay.h
+++ b/arch/arm/include/asm/delay.h
@@ -40,5 +40,12 @@ extern void __const_udelay(unsigned long);
 			__const_udelay((n) * ((2199023U*HZ)>>11))) :	\
 	  __udelay(n))
 
+extern void (*delay_fn)(unsigned long);
+
+static inline void set_delay_fn(void (*fn)(unsigned long))
+{
+	delay_fn = fn;
+}
+
 #endif /* defined(_ARM_DELAY_H) */
 
diff --git a/arch/arm/lib/delay.c b/arch/arm/lib/delay.c
index f92aca0..7468bc6 100644
--- a/arch/arm/lib/delay.c
+++ b/arch/arm/lib/delay.c
@@ -11,11 +11,9 @@
 #include <linux/delay.h>
 
 /*
- * loops = usecs * HZ * loops_per_jiffy / 1000000
- *
  * Oh, if only we had a cycle counter...
  */
-void __delay(unsigned long loops)
+static void delay_loop(unsigned long loops)
 {
 	asm volatile(
 	"1:	subs %0, %0, #1 \n"
@@ -24,6 +22,16 @@ void __delay(unsigned long loops)
 	: "r" (loops)
 	);
 }
+
+void (*delay_fn)(unsigned long) = delay_loop;
+
+/*
+ * loops = usecs * HZ * loops_per_jiffy / 1000000
+ */
+void __delay(unsigned long loops)
+{
+	delay_fn(loops);
+}
 EXPORT_SYMBOL(__delay);
 
 /*
-- 
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