The formula for the ADC sampling period in ast2400/ast2500 is:
ADC clock period = PCLK * 2 * (ADC0C[31:17] + 1) * (ADC0C[9:0])
When ADC0C[9:0] is set to 0 the sampling voltage will be lower than
expected, because the hardware may not have enough time to
charge/discharge to a stable voltage. This patch use the flag
CLK_DIVIDER_ONE_BASED which will use the raw value read from the
register, with the value of zero considered invalid to conform to the
corrected formula.
Fixes: 573803234e72 ("iio: Aspeed ADC")
Reported-by: Konstantin Klubnichkin <kitsok@xxxxxxxxxxxxxx>
Signed-off-by: Billy Tsai <billy_tsai@xxxxxxxxxxxxxx>
Reviewed-by: Joel Stanley <joel@xxxxxxxxx>
Link: https://lore.kernel.org/r/20220221012705.22008-1-billy_tsai@xxxxxxxxxxxxxx
Cc: <Stable@xxxxxxxxxxxxxxx>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
diff --git a/drivers/iio/adc/aspeed_adc.c b/drivers/iio/adc/aspeed_adc.c
index e939b84cbb56..0793d2474cdc 100644
--- a/drivers/iio/adc/aspeed_adc.c
+++ b/drivers/iio/adc/aspeed_adc.c
@@ -539,7 +539,9 @@ static int aspeed_adc_probe(struct platform_device *pdev)
data->clk_scaler = devm_clk_hw_register_divider(
&pdev->dev, clk_name, clk_parent_name, scaler_flags,
data->base + ASPEED_REG_CLOCK_CONTROL, 0,
- data->model_data->scaler_bit_width, 0, &data->clk_lock);
+ data->model_data->scaler_bit_width,
+ data->model_data->need_prescaler ? CLK_DIVIDER_ONE_BASED : 0,
+ &data->clk_lock);
if (IS_ERR(data->clk_scaler))
return PTR_ERR(data->clk_scaler);
