[PATCH v2 1/8] iio: adc: stm32-dfsdm: make spi_master_freq more accurate

Fabrice Gasnier
2019-03-21T16:48:12Z

Current ckout divider may be set to a value that makes ckout to exceed
spi-max-frequency. Rather use lower value (e.g. round up divider when
ckout isn't accurate).

Also when the SPI clock isn't accurate, 'spi_master_freq' is filled in
with expected frequency. Use computed value instead to be more accurate:
- e.g. source clock / (CKOUTDIV + 1)

Enforce checks on the divider: ckoutdiv range can be from 1-255 to provide
divider of 2-256.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
---
Changes in v2:
- rework the way to determine ckoutdiv bitfield: deal with divider directly
to explicitly handle the rounding, the range constraints of 2-256 and the
divider = ckoutdiv + 1.
---
drivers/iio/adc/stm32-dfsdm-core.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/adc/stm32-dfsdm-core.c b/drivers/iio/adc/stm32-dfsdm-core.c
index bf089f5..472b809 100644
--- a/drivers/iio/adc/stm32-dfsdm-core.c
+++ b/drivers/iio/adc/stm32-dfsdm-core.c
@@ -199,7 +199,7 @@ static int stm32_dfsdm_parse_of(struct platform_device *pdev,
{
struct device_node *node = pdev->dev.of_node;
struct resource *res;
- unsigned long clk_freq;
+ unsigned long clk_freq, divider;
unsigned int spi_freq, rem;
int ret;

@@ -243,13 +243,20 @@ 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;
- if (!priv->spi_clk_out_div) {
- /* spi_clk_out_div == 0 means ckout is OFF */
+ divider = div_u64_rem(clk_freq, spi_freq, &rem);
+ /* Round up divider when ckout isn't precise, not to exceed spi_freq */
+ if (rem)
+ divider++;
+
+ /* programmable divider is in range of [2:256] */
+ if (divider < 2 || divider > 256) {
dev_err(&pdev->dev, "spi-max-frequency not achievable\n");
return -EINVAL;
}
- priv->dfsdm.spi_master_freq = spi_freq;
+
+ /* SPI clock output divider is: divider = CKOUTDIV + 1 */
+ priv->spi_clk_out_div = divider - 1;
+ 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