serial: stm32: rework RX dma initialization and release

The RX DMA channel is kept active forever (from the probe). That prevents
going to low power mode when it is used. This change moves the
DMA configuration and enabling procedures to startup routine to allow
transition to low power mode.
The DMA disabling procedure is implemented in stop_rx routine as this
ops has to stop characters reception, and DMA transation in shutdown.
Clean useless dma_async_tx_descriptor initialization to NULL value.
Signed-off-by: NValentin Caron <valentin.caron@foss.st.com>
Signed-off-by: NErwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20211025134229.8456-2-erwan.leray@foss.st.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -761,6 +761,10 @@ static void stm32_usart_stop_rx(struct uart_port *port)
 	struct stm32_port *stm32_port = to_stm32_port(port);
 	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
 
+	/* Disable DMA request line. */
+	if (stm32_port->rx_ch)
+		stm32_usart_clr_bits(port, ofs->cr3, USART_CR3_DMAR);
+
 	stm32_usart_clr_bits(port, ofs->cr1, stm32_port->cr1_irq);
 	if (stm32_port->cr3_irq)
 		stm32_usart_clr_bits(port, ofs->cr3, stm32_port->cr3_irq);
@@ -777,6 +781,7 @@ static int stm32_usart_startup(struct uart_port *port)
 	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
 	const struct stm32_usart_config *cfg = &stm32_port->info->cfg;
 	const char *name = to_platform_device(port->dev)->name;
+	struct dma_async_tx_descriptor *desc;
 	u32 val;
 	int ret;
 
@@ -797,6 +802,33 @@ static int stm32_usart_startup(struct uart_port *port)
 	if (ofs->rqr != UNDEF_REG)
 		writel_relaxed(USART_RQR_RXFRQ, port->membase + ofs->rqr);
 
+	if (stm32_port->rx_ch) {
+		stm32_port->last_res = RX_BUF_L;
+		/* Prepare a DMA cyclic transaction */
+		desc = dmaengine_prep_dma_cyclic(stm32_port->rx_ch,
+						 stm32_port->rx_dma_buf,
+						 RX_BUF_L, RX_BUF_P,
+						 DMA_DEV_TO_MEM,
+						 DMA_PREP_INTERRUPT);
+		if (!desc) {
+			dev_err(port->dev, "rx dma prep cyclic failed\n");
+			ret = -ENODEV;
+			goto err;
+		}
+
+		desc->callback = stm32_usart_rx_dma_complete;
+		desc->callback_param = port;
+
+		/* Push current DMA transaction in the pending queue */
+		ret = dma_submit_error(dmaengine_submit(desc));
+		if (ret) {
+			dmaengine_terminate_sync(stm32_port->rx_ch);
+			goto err;
+		}
+
+		/* Issue pending DMA requests */
+		dma_async_issue_pending(stm32_port->rx_ch);
+	}
 	/*
 	 * DMA request line not re-enabled at resume when port is throttled.
 	 * It will be re-enabled by unthrottle ops.
@@ -809,6 +841,11 @@ static int stm32_usart_startup(struct uart_port *port)
 	stm32_usart_set_bits(port, ofs->cr1, val);
 
 	return 0;
+
+err:
+	free_irq(port->irq, port);
+
+	return ret;
 }
 
 static void stm32_usart_shutdown(struct uart_port *port)
@@ -836,6 +873,10 @@ static void stm32_usart_shutdown(struct uart_port *port)
 	if (ret)
 		dev_err(port->dev, "Transmission is not complete\n");
 
+	/* Disable RX DMA. */
+	if (stm32_port->rx_ch)
+		dmaengine_terminate_async(stm32_port->rx_ch);
+
 	/* flush RX & TX FIFO */
 	if (ofs->rqr != UNDEF_REG)
 		writel_relaxed(USART_RQR_TXFRQ | USART_RQR_RXFRQ,
@@ -1304,7 +1345,6 @@ static int stm32_usart_of_dma_rx_probe(struct stm32_port *stm32port,
 	struct uart_port *port = &stm32port->port;
 	struct device *dev = &pdev->dev;
 	struct dma_slave_config config;
-	struct dma_async_tx_descriptor *desc = NULL;
 	int ret;
 
 	/*
@@ -1332,32 +1372,6 @@ static int stm32_usart_of_dma_rx_probe(struct stm32_port *stm32port,
 		return ret;
 	}
 
-	/* Prepare a DMA cyclic transaction */
-	desc = dmaengine_prep_dma_cyclic(stm32port->rx_ch,
-					 stm32port->rx_dma_buf,
-					 RX_BUF_L, RX_BUF_P, DMA_DEV_TO_MEM,
-					 DMA_PREP_INTERRUPT);
-	if (!desc) {
-		dev_err(dev, "rx dma prep cyclic failed\n");
-		stm32_usart_of_dma_rx_remove(stm32port, pdev);
-		return -ENODEV;
-	}
-
-	/* Set DMA callback */
-	desc->callback = stm32_usart_rx_dma_complete;
-	desc->callback_param = port;
-
-	/* Push current DMA transaction in the pending queue */
-	ret = dma_submit_error(dmaengine_submit(desc));
-	if (ret) {
-		dmaengine_terminate_sync(stm32port->rx_ch);
-		stm32_usart_of_dma_rx_remove(stm32port, pdev);
-		return ret;
-	}
-
-	/* Issue pending DMA requests */
-	dma_async_issue_pending(stm32port->rx_ch);
-
 	return 0;
 }
 
@@ -1535,7 +1549,6 @@ static int stm32_usart_serial_remove(struct platform_device *pdev)
 	}
 
 	if (stm32_port->rx_ch) {
-		dmaengine_terminate_async(stm32_port->rx_ch);
 		stm32_usart_of_dma_rx_remove(stm32_port, pdev);
 		dma_release_channel(stm32_port->rx_ch);
 	}