[PATCH 1/8] iio: adc: stm32-dfsdm: make spi_master_freq more accurate

[Fabrice Gasnier <fabrice.gasnier@st.com>]

When SPI clock isn't accurate, 'spi_master_freq' is filled in with
expected frequency. Use computed value instead:
- e.g. source clock / (CKOUTDIV + 1)
Also, current divider may be set to value that makes CKOUT to exceed
spi-max-frequency. Rather use lower value (e.g. round up divider when
ckout isn't accurate).

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
---
 drivers/iio/adc/stm32-dfsdm-core.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/adc/stm32-dfsdm-core.c b/drivers/iio/adc/stm32-dfsdm-core.c
index bf089f5..65b7556 100644
--- a/drivers/iio/adc/stm32-dfsdm-core.c
+++ b/drivers/iio/adc/stm32-dfsdm-core.c
@@ -243,13 +243,18 @@ static int stm32_dfsdm_parse_of(struct platform_device *pdev,
 		return 0;
 	}
 
-	priv->spi_clk_out_div = div_u64_rem(clk_freq, spi_freq, &rem) - 1;
+	priv->spi_clk_out_div = div_u64_rem(clk_freq, spi_freq, &rem);
+
+	/* round up divider when clkout isn't accurate (e.g. !rem) */
+	if (priv->spi_clk_out_div && !rem)
+		priv->spi_clk_out_div--;
+
 	if (!priv->spi_clk_out_div) {
 		/* spi_clk_out_div == 0 means ckout is OFF */
 		dev_err(&pdev->dev, "spi-max-frequency not achievable\n");
 		return -EINVAL;
 	}
-	priv->dfsdm.spi_master_freq = spi_freq;
+	priv->dfsdm.spi_master_freq = clk_freq / (priv->spi_clk_out_div + 1);
 
 	if (rem) {
 		dev_warn(&pdev->dev, "SPI clock not accurate\n");
-- 
2.7.4