iio: adc: stm32: add optional dma support

Add DMA optional support to STM32 ADC, as there is a limited number DMA
channels (request lines) that can be assigned to ADC. This way, driver
may fall back using interrupts when all DMA channels are in use for
other IPs.
Use dma cyclic mode with two periods. Allow to tune period length by
using watermark. Coherent memory is used for dma (max buffer size is
fixed to PAGE_SIZE).

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/Kconfig

@@ -497,6 +497,7 @@ config ROCKCHIP_SARADC
 config STM32_ADC_CORE
 	tristate "STMicroelectronics STM32 adc core"
 	depends on ARCH_STM32 || COMPILE_TEST
+	depends on HAS_DMA
 	depends on OF
 	depends on REGULATOR
 	select IIO_BUFFER

drivers/iio/adc/stm32-adc-core.c

@@ -201,6 +201,7 @@ static int stm32_adc_probe(struct platform_device *pdev)
 	priv->common.base = devm_ioremap_resource(&pdev->dev, res);
 	if (IS_ERR(priv->common.base))
 		return PTR_ERR(priv->common.base);
+	priv->common.phys_base = res->start;
 
 	priv->vref = devm_regulator_get(&pdev->dev, "vref");
 	if (IS_ERR(priv->vref)) {

drivers/iio/adc/stm32-adc-core.h

@@ -42,10 +42,12 @@
 /**
  * struct stm32_adc_common - stm32 ADC driver common data (for all instances)
  * @base:		control registers base cpu addr
+ * @phys_base:		control registers base physical addr
  * @vref_mv:		vref voltage (mv)
  */
 struct stm32_adc_common {
 	void __iomem			*base;
+	phys_addr_t			phys_base;
 	int				vref_mv;
 };