[PATCH v2 3/4] iio: at91: ADC start-up time calculation changed since at91sam9x5

Since in at91sam9x5, sama5d3x chip. the start up time calucation is
changed.  This patch can choose different start up time calculation
formula for different chips.

Signed-off-by: Josh Wu <redacted>
---
v1 --> v2:
  add a new variable for different start_time calculation.

 drivers/iio/adc/at91_adc.c |   42 +++++++++++++++++++++++++++++++++++-------
 1 file changed, 35 insertions(+), 7 deletions(-)

diff --git a/drivers/iio/adc/at91_adc.c b/drivers/iio/adc/at91_adc.c
index b41eb60..9b0ffff 100644
--- a/drivers/iio/adc/at91_adc.c
+++ b/drivers/iio/adc/at91_adc.c

@@ -40,6 +40,7 @@
 	(writel_relaxed(val, st->reg_base + reg))
 
 struct at91_adc_caps {
+	bool	start_time_use_lookup_tab;
 	u32	mr_prescal_mask;
 	u32	mr_startup_mask;
 };

@@ -700,13 +701,39 @@ static int at91_adc_probe(struct platform_device *pdev)
 		goto error_disable_adc_clk;
 	}
 
-	/*
-	 * Number of ticks needed to cover the startup time of the ADC as
-	 * defined in the electrical characteristics of the board, divided by 8.
-	 * The formula thus is : Startup Time = (ticks + 1) * 8 / ADC Clock
-	 */
-	ticks = round_up((st->startup_time * adc_clk_khz /
-			  1000) - 1, 8) / 8;
+	if (!st->caps->start_time_use_lookup_tab) {
+		/*
+		 * Number of ticks needed to cover the startup time of the ADC
+		 * as defined in the electrical characteristics of the board,
+		 * divided by 8. The formula thus is :
+		 *   Startup Time = (ticks + 1) * 8 / ADC Clock
+		 */
+		ticks = round_up((st->startup_time * adc_clk_khz /
+				  1000) - 1, 8) / 8;
+	} else {
+		/*
+		 * For sama5d3x and at91sam9x5, the formula changes to:
+		 * Startup Time = <lookup_table_value> / ADC Clock
+		 */
+		const int startup_lookup[] = {
+			0  , 8  , 16 , 24 ,
+			64 , 80 , 96 , 112,
+			512, 576, 640, 704,
+			768, 832, 896, 960
+			};
+
+		int i, size = ARRAY_SIZE(startup_lookup);
+		ticks = st->startup_time * adc_clk_khz / 1000;
+		for (i = 0; i < size; i++)
+			if (ticks < startup_lookup[i])
+				break;
+
+		ticks = i;
+		if (ticks == size)
+			/* Reach the end of lookup table */
+			ticks = size - 1;
+	}
+