s390/udelay: make it work for the early code

Currently udelay relies on working EXT interrupts handler, which is not
the case during early startup. In such cases udelay_simple() has to be
used instead.

To avoid mistakes of calling udelay too early, which could happen from
the common code as well - make udelay work for the early code by
introducing static branch and redirecting all udelay calls to
udelay_simple until EXT interrupts handler is fully initialized and
async stack is allocated.
Reviewed-by: NHeiko Carstens <hca@linux.ibm.com>
Signed-off-by: NVasily Gorbik <gor@linux.ibm.com>
Signed-off-by: NHeiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/delay.h

@@ -13,6 +13,7 @@
 #ifndef _S390_DELAY_H
 #define _S390_DELAY_H
 
+void udelay_enable(void);
 void __ndelay(unsigned long long nsecs);
 void __udelay(unsigned long long usecs);
 void udelay_simple(unsigned long long usecs);

arch/s390/kernel/setup.c

@@ -336,6 +336,7 @@ int __init arch_early_irq_init(void)
 	if (!stack)
 		panic("Couldn't allocate async stack");
 	S390_lowcore.async_stack = stack + STACK_INIT_OFFSET;
+	udelay_enable();
 	return 0;
 }
 

arch/s390/lib/delay.c

@@ -13,11 +13,19 @@
 #include <linux/export.h>
 #include <linux/irqflags.h>
 #include <linux/interrupt.h>
+#include <linux/jump_label.h>
 #include <linux/irq.h>
 #include <asm/vtimer.h>
 #include <asm/div64.h>
 #include <asm/idle.h>
 
+static DEFINE_STATIC_KEY_FALSE(udelay_ready);
+
+void __init udelay_enable(void)
+{
+	static_branch_enable(&udelay_ready);
+}
+
 void __delay(unsigned long loops)
 {
         /*
@@ -77,6 +85,11 @@ void __udelay(unsigned long long usecs)
 {
 	unsigned long flags;
 
+	if (!static_branch_likely(&udelay_ready)) {
+		udelay_simple(usecs);
+		return;
+	}
+
 	preempt_disable();
 	local_irq_save(flags);
 	if (in_irq()) {