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>

1 files changed, 14 insertions, 1 deletions

diff --git a/drivers/iio/adc/at91_adc.c b/drivers/iio/adc/at91_adc.c
index 7295bc5280bd..e5b88d5d3b59 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 */