misc: ioc4: simplify wave period measurement in clock_calibrate

The loop for measuring the square wave periods over some cycles is
refactored to be more easily readable. This includes avoiding a
"by-hand-implemented" for loop with a "real" one and adding some
comments.

Furthermore the following compiler warning is avoided by this patch:
drivers/misc/ioc4.c: In function ‘ioc4_probe’:
drivers/misc/ioc4.c:194:16: warning: ‘start’ may be used uninitialized
in this function [-Wmaybe-uninitialized]
  period = (end - start) /
                ^
drivers/misc/ioc4.c:148:11: note: ‘start’ was declared here
  uint64_t start, end, period;
           ^

Signed-off-by: Richard Leitner <dev@g0hl1n.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/ioc4.c

@@ -144,9 +144,9 @@ ioc4_clock_calibrate(struct ioc4_driver_data *idd)
 {
 	union ioc4_int_out int_out;
 	union ioc4_gpcr gpcr;
-	unsigned int state, last_state = 1;
+	unsigned int state, last_state;
 	uint64_t start, end, period;
-	unsigned int count = 0;
+	unsigned int count;
 
 	/* Enable output */
 	gpcr.raw = 0;
@@ -167,19 +167,20 @@ ioc4_clock_calibrate(struct ioc4_driver_data *idd)
 	mmiowb();
 
 	/* Check square wave period averaged over some number of cycles */
-	do {
-		int_out.raw = readl(&idd->idd_misc_regs->int_out.raw);
-		state = int_out.fields.int_out;
-		if (!last_state && state) {
-			count++;
-			if (count == IOC4_CALIBRATE_END) {
-				end = ktime_get_ns();
-				break;
-			} else if (count == IOC4_CALIBRATE_DISCARD)
-				start = ktime_get_ns();
-		}
-		last_state = state;
-	} while (1);
+	start = ktime_get_ns();
+	state = 1; /* make sure the first read isn't a rising edge */
+	for (count = 0; count <= IOC4_CALIBRATE_END; count++) {
+		do { /* wait for a rising edge */
+			last_state = state;
+			int_out.raw = readl(&idd->idd_misc_regs->int_out.raw);
+			state = int_out.fields.int_out;
+		} while (last_state || !state);
+
+		/* discard the first few cycles */
+		if (count == IOC4_CALIBRATE_DISCARD)
+			start = ktime_get_ns();
+	}
+	end = ktime_get_ns();
 
 	/* Calculation rearranged to preserve intermediate precision.
 	 * Logically: