iio: at91_adc: fix missing Sample and Hold time

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@jcrosoft.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

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:

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 */