[PATCH 4.16 29/47] iio: adc: stm32-dfsdm: fix successive oversampling settings

[Greg Kroah-Hartman]

4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Fabrice Gasnier <fabrice.gasnier@xxxxxx>

commit 7531cf59bfa082769887ec70c2029838ea139f11 upstream.

When doing successive oversampling settings, it may fail to update filter
parameters silently:
- First time oversampling is being set, it will be successful, as fl->res
is 0 initially.
- Next attempts with various oversamp value may return 0 (success), but
keep previous filter parameters, due to 'res' never reaches above or
equal current 'fl->res'.

This is particularly true when setting sampling frequency (that relies on
oversamp). Typical failure without error:
- run 1st test @16kHz samp freq will succeed
- run new test @8kHz will succeed as well
- run new test @16kHz (again): sample rate will remain 8kHz without error

Fixes: e2e6771c6462 ("IIO: ADC: add STM32 DFSDM sigma delta ADC support")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@xxxxxx>
Acked-by: Arnaud Pouliquen <arnaud.pouliquen@xxxxxx>
Cc: <Stable@xxxxxxxxxxxxxxx>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/iio/adc/stm32-dfsdm-adc.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/iio/adc/stm32-dfsdm-adc.c
+++ b/drivers/iio/adc/stm32-dfsdm-adc.c
@@ -144,6 +144,7 @@ static int stm32_dfsdm_set_osrs(struct s
 	 * Leave as soon as if exact resolution if reached.
 	 * Otherwise the higher resolution below 32 bits is kept.
 	 */
+	fl->res = 0;
 	for (fosr = 1; fosr <= DFSDM_MAX_FL_OVERSAMPLING; fosr++) {
 		for (iosr = 1; iosr <= DFSDM_MAX_INT_OVERSAMPLING; iosr++) {
 			if (fast)
@@ -193,7 +194,7 @@ static int stm32_dfsdm_set_osrs(struct s
 		}
 	}
 
-	if (!fl->fosr)
+	if (!fl->res)
 		return -EINVAL;
 
 	return 0;