[PATCH 3/3] iio: at91_adc: fix missing Sample and Hold time

[ludovic.desroches at atmel.com]

From: Jean-Christophe PLAGNIOL-VILLARD <plagnioj at jcrosoft.com>

On the at91_adc a minimal Sample and Hold Time is necessary for the ADC to
guarantee the best converted final value between two channels selection.
This time has to be programmed through the bitfield SHTIM in the
Mode Register ADC_MR.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj at jcrosoft.com>
[ludovic.desroches at atmel.com: add unit for sample and hold time]
Signed-off-by: Ludovic Desroches <ludovic.desroches at atmel.com>
---
 Documentation/devicetree/bindings/arm/atmel-adc.txt |  1 +
 drivers/iio/adc/at91_adc.c                          | 15 ++++++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/Documentation/devicetree/bindings/arm/atmel-adc.txt b/Documentation/devicetree/bindings/arm/atmel-adc.txt
index 3a05492..16769d9 100644
--- a/Documentation/devicetree/bindings/arm/atmel-adc.txt
+++ b/Documentation/devicetree/bindings/arm/atmel-adc.txt
@@ -26,6 +26,7 @@ Optional properties:
 		       atmel,adc-res-names property. If not specified, the highest
 		       resolution will be used.
   - atmel,adc-sleep-mode: Boolean to enable sleep mode when no conversion
+  - atmel,adc-sample-hold-time: Sample and Hold Time in microseconds
  
 Optional trigger Nodes:
   - Required properties:
diff --git a/drivers/iio/adc/at91_adc.c b/drivers/iio/adc/at91_adc.c
index 8e38f1c..69fe8d4 100644
--- a/drivers/iio/adc/at91_adc.c
+++ b/drivers/iio/adc/at91_adc.c
@@ -52,6 +52,7 @@ struct at91_adc_state {
 	void __iomem		*reg_base;
 	struct at91_adc_reg_desc *registers;
 	u8			startup_time;
+	u8			sample_hold_time;
 	bool			sleep_mode;
 	struct iio_trigger	**trig;
 	struct at91_adc_trigger	*trigger_list;
@@ -465,6 +466,9 @@ static int at91_adc_probe_dt(struct at91_adc_state *st,
 	}
 	st->startup_time = prop;
 
+	prop = 0;
+	of_property_read_u32(node, "atmel,adc-sample-hold-time", &prop);
+	st->sample_hold_time = prop;
 
 	if (of_property_read_u32(node, "atmel,adc-vref", &prop)) {
 		dev_err(&idev->dev, "Missing adc-vref property in the DT.\n");
@@ -578,7 +582,7 @@ static const struct iio_info at91_adc_info = {
 
 static int at91_adc_probe(struct platform_device *pdev)
 {
-	unsigned int prsc, mstrclk, ticks, adc_clk;
+	unsigned int prsc, mstrclk, ticks, adc_clk, shtim;
 	int ret;
 	struct iio_dev *idev;
 	struct at91_adc_state *st;
@@ -691,12 +695,21 @@ static int at91_adc_probe(struct platform_device *pdev)
 	 */
 	ticks = round_up((st->startup_time * adc_clk /
 			  1000000) - 1, 8) / 8;
+	/*
+	 * a minimal Sample and Hold Time is necessary for the ADC to guarantee
+	 * the best converted final value between two channels selection
+	 * The formula thus is : Sample and Hold Time = (shtim + 1) / ADCClock
+	 */
+	shtim = round_up((st->sample_hold_time * adc_clk /
+			  1000000) - 1, 1);
+
 	reg = AT91_ADC_PRESCAL_(prsc) & AT91_ADC_PRESCAL;
 	reg |= AT91_ADC_STARTUP_(ticks) & AT91_ADC_STARTUP;
 	if (st->low_res)
 		reg |= AT91_ADC_LOWRES;
 	if (st->sleep_mode)
 		reg |= AT91_ADC_SLEEP;
+	reg |= AT91_ADC_SHTIM_(shtim) & AT91_ADC_SHTIM;
 	at91_adc_writel(st, AT91_ADC_MR, reg);
 
 	/* Setup the ADC channels available on the board */
-- 
1.7.11.3