driver: uart: stm32: Disable UART DMA before shutdown

Use LL_USART_DisableDMAReq_RX to disable UART RX DMA requests.
This should be called before entering shutdown mode.

Force to use UART_STM32U5_ERRATA_DMAT_LOWPOWER
when POWEROFF is selected. This ensure the system to shut down
properly instead of hanging due to DMA staying active.

Signed-off-by: Khaoula Bidani <[email protected]>
Signed-off-by: Julien Racki <[email protected]>

Author: kbidani

drivers/serial/Kconfig.stm32

@@ -39,8 +39,8 @@ if UART_STM32U5_ERRATA_DMAT_AFFECTED
 
 choice UART_STM32U5_ERRATA_DMAT
 	prompt "Workaround for DMAT errata on selected devices"
-	default UART_STM32U5_ERRATA_DMAT_LOWPOWER if PM
 	default UART_STM32U5_ERRATA_DMAT_NOCLEAR if !PM
+	default UART_STM32U5_ERRATA_DMAT_LOWPOWER
 	help
 	  Handles erratum "USART does not generate DMA requests after
 	  setting/clearing DMAT bit" as described in the errata sheets:
@@ -59,6 +59,7 @@ config UART_STM32U5_ERRATA_DMAT_LOWPOWER
 
 config UART_STM32U5_ERRATA_DMAT_NOCLEAR
 	bool "Do not clear DMAT"
+	depends on !POWEROFF # DMAT must be clear to enter LL_PWR_SHUTDOWN_MODE
 	help
 	  This option keeps DMAT bit set. This may cause additional power
 	  consumption in STOP low-power modes.

drivers/serial/uart_stm32.c

@@ -1508,8 +1508,11 @@ static inline void uart_stm32_dma_rx_enable(const struct device *dev)
 
 static inline void uart_stm32_dma_rx_disable(const struct device *dev)
 {
+	const struct uart_stm32_config *config = dev->config;
 	struct uart_stm32_data *data = dev->data;
 
+	LL_USART_DisableDMAReq_RX(config->usart);
+
 	data->dma_rx.enabled = false;
 }