Implement generic delay timer

Add delay timer implementation based on the system generic counter.
This either uses the platform's implementation of
`plat_get_syscnt_freq()` or explicit clock multiplier/divider values
provided by the platform.

The current implementation of udelay has been modified to avoid
unnecessary calculations while waiting on the loop and to make it
easier to check for overflows.

Change-Id: I9062e1d506dc2f68367fd9289250b93444721732

1 files changed, 13 insertions, 10 deletions

diff --git a/drivers/delay_timer/delay_timer.c b/drivers/delay_timer/delay_timer.c
index 0bee876f..ed7ed52e 100644
--- a/drivers/delay_timer/delay_timer.c
+++ b/drivers/delay_timer/delay_timer.c
@@ -48,19 +48,22 @@ void udelay(uint32_t usec)
 		(ops->clk_div != 0) &&
 		(ops->get_timer_value != 0));
 
-	uint32_t start, cnt, delta, delta_us;
+	uint32_t start, delta, total_delta;
+
+	assert(usec < UINT32_MAX / ops->clk_div);
 
-	/* counter is decreasing */
 	start = ops->get_timer_value();
+
+	total_delta = (usec * ops->clk_div) / ops->clk_mult;
+
 	do {
-		cnt = ops->get_timer_value();
-		if (cnt > start) {
-			delta = UINT32_MAX - cnt;
-			delta += start;
-		} else
-			delta = start - cnt;
-		delta_us = (delta * ops->clk_mult) / ops->clk_div;
-	} while (delta_us < usec);
+		/*
+		 * If the timer value wraps around, the subtraction will
+		 * overflow and it will still give the correct result.
+		 */
+		delta = start - ops->get_timer_value(); /* Decreasing counter */
+
+	} while (delta < total_delta);
 }
 
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