dmaengine: stm32-dma: add alternate REQ/ACK protocol management

STM32 USART/UART is not managing correctly the default DMA REQ/ACK protocol
leading to possibly lock the DMA stream.
Default protocol consists in maintaining ACK signal up to the removal of
REQuest and the transfer completion.
In case of alternative REQ/ACK protocol, ACK de-assertion does not wait the
removal of the REQuest, but only the transfer completion.

This patch retrieve the need of the alternative protocol through the device
tree, and sets the protocol accordingly.

Change-Id: I2c9fd6f33f3ad9a600449cbf02b9f29c88384618
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/linux-stm32/+/186668
Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Fabien DESSENNE <fabien.dessenne@st.com>

Author: ADESTM

drivers/dma/stm32-dma.c

@@ -62,6 +62,7 @@
 #define STM32_DMA_SCR_PSIZE_GET(n)	((n & STM32_DMA_SCR_PSIZE_MASK) >> 11)
 #define STM32_DMA_SCR_DIR_MASK		GENMASK(7, 6)
 #define STM32_DMA_SCR_DIR(n)		((n & 0x3) << 6)
+#define STM32_DMA_SCR_TRBUFF		BIT(20) /* Bufferable transfer for USART/UART */
 #define STM32_DMA_SCR_CT		BIT(19) /* Target in double buffer */
 #define STM32_DMA_SCR_DBM		BIT(18) /* Double Buffer Mode */
 #define STM32_DMA_SCR_PINCOS		BIT(15) /* Peripheral inc offset size */
@@ -143,6 +144,9 @@
 #define STM32_DMA_DIRECT_MODE_MASK	BIT(2)
 #define STM32_DMA_DIRECT_MODE_GET(n)	(((n) & STM32_DMA_DIRECT_MODE_MASK) \
 					 >> 2)
+#define STM32_DMA_ALT_ACK_MODE_MASK	BIT(4)
+#define STM32_DMA_ALT_ACK_MODE_GET(n)	(((n) & STM32_DMA_ALT_ACK_MODE_MASK) \
+					 >> 4)
 #define STM32_DMA_MDMA_CHAIN_FTR_MASK	BIT(31)
 #define STM32_DMA_MDMA_CHAIN_FTR_GET(n)	(((n) & STM32_DMA_MDMA_CHAIN_FTR_MASK) \
 					 >> 31)
@@ -1943,6 +1947,8 @@ static void stm32_dma_set_config(struct stm32_dma_chan *chan,
 		STM32_DMA_SRAM_GRANULARITY;
 	if (STM32_DMA_DIRECT_MODE_GET(cfg->features))
 		chan->threshold = STM32_DMA_FIFO_THRESHOLD_NONE;
+	if (STM32_DMA_ALT_ACK_MODE_GET(cfg->features))
+		chan->chan_reg.dma_scr |= STM32_DMA_SCR_TRBUFF;
 }
 
 static struct dma_chan *stm32_dma_of_xlate(struct of_phandle_args *dma_spec,