xtensa: clean up udelay

Replace division with shift, use ccount_freq instead of loops_per_jiffy.
Introduce __MAX_UDELAY and (undefined) __bad_udelay, break build for too
big udelay constants.
Remove irrelevant comment, clean up code style.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/delay.h

@@ -27,18 +27,27 @@ static inline void __delay(unsigned long loops)
 				: "+r" (loops));
 }
 
-/* For SMP/NUMA systems, change boot_cpu_data to something like
- * local_cpu_data->... where local_cpu_data points to the current
- * cpu. */
+/* Undefined function to get compile-time error */
+void __bad_udelay(void);
 
-static __inline__ void udelay (unsigned long usecs)
+#define __MAX_UDELAY 30000
+
+static inline void __udelay(unsigned long usecs)
 {
 	unsigned long start = get_ccount();
-	unsigned long cycles = usecs * (loops_per_jiffy / (1000000UL / HZ));
+	unsigned long cycles = (usecs * (ccount_freq >> 15)) >> 5;
 
 	/* Note: all variables are unsigned (can wrap around)! */
 	while (((unsigned long)get_ccount()) - start < cycles)
-		;
+		cpu_relax();
+}
+
+static inline void udelay(unsigned long usec)
+{
+	if (__builtin_constant_p(usec) && usec >= __MAX_UDELAY)
+		__bad_udelay();
+	else
+		__udelay(usec);
 }
 
 #endif