iio:adc:at91-sama5d2: cleanup mode register use

Do not erase previous configuration of the mode register when setting
the sampling frequency.

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

drivers/iio/adc/at91-sama5d2_adc.c

@@ -66,8 +66,10 @@
 #define	AT91_SAMA5D2_MR_PRESCAL(v)	((v) << AT91_SAMA5D2_MR_PRESCAL_OFFSET)
 #define	AT91_SAMA5D2_MR_PRESCAL_OFFSET	8
 #define	AT91_SAMA5D2_MR_PRESCAL_MAX	0xff
+#define AT91_SAMA5D2_MR_PRESCAL_MASK	GENMASK(15, 8)
 /* Startup Time */
 #define	AT91_SAMA5D2_MR_STARTUP(v)	((v) << 16)
+#define AT91_SAMA5D2_MR_STARTUP_MASK	GENMASK(19, 16)
 /* Analog Change */
 #define	AT91_SAMA5D2_MR_ANACH		BIT(23)
 /* Tracking Time */
@@ -226,7 +228,7 @@ static unsigned at91_adc_startup_time(unsigned startup_time_min,
 static void at91_adc_setup_samp_freq(struct at91_adc_state *st, unsigned freq)
 {
 	struct iio_dev *indio_dev = iio_priv_to_dev(st);
-	unsigned f_per, prescal, startup;
+	unsigned f_per, prescal, startup, mr;
 
 	f_per = clk_get_rate(st->per_clk);
 	prescal = (f_per / (2 * freq)) - 1;
@@ -234,10 +236,11 @@ static void at91_adc_setup_samp_freq(struct at91_adc_state *st, unsigned freq)
 	startup = at91_adc_startup_time(st->soc_info.startup_time,
 					freq / 1000);
 
-	at91_adc_writel(st, AT91_SAMA5D2_MR,
-			AT91_SAMA5D2_MR_TRANSFER(2)
-			| AT91_SAMA5D2_MR_STARTUP(startup)
-			| AT91_SAMA5D2_MR_PRESCAL(prescal));
+	mr = at91_adc_readl(st, AT91_SAMA5D2_MR);
+	mr &= ~(AT91_SAMA5D2_MR_STARTUP_MASK | AT91_SAMA5D2_MR_PRESCAL_MASK);
+	mr |= AT91_SAMA5D2_MR_STARTUP(startup);
+	mr |= AT91_SAMA5D2_MR_PRESCAL(prescal);
+	at91_adc_writel(st, AT91_SAMA5D2_MR, mr);
 
 	dev_dbg(&indio_dev->dev, "freq: %u, startup: %u, prescal: %u\n",
 		freq, startup, prescal);
@@ -444,6 +447,8 @@ static int at91_adc_probe(struct platform_device *pdev)
 
 	at91_adc_writel(st, AT91_SAMA5D2_CR, AT91_SAMA5D2_CR_SWRST);
 	at91_adc_writel(st, AT91_SAMA5D2_IDR, 0xffffffff);
+	/* Transfer field must be set to 2 according to the datasheet. */
+	at91_adc_writel(st, AT91_SAMA5D2_MR, AT91_SAMA5D2_MR_TRANSFER(2));
 
 	at91_adc_setup_samp_freq(st, st->soc_info.min_sample_rate);