dt-bindings: dma: add alternative REQ/ACK protocol selection in stm32-dma

Default REQ/ACK 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.
Due to a possible DMA stream lock when transferring data to/from STM32
USART/UART, this new bindings allow to select this alternative protocol in
device tree, especially for STM32 USART/UART nodes.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210624093959.142265-2-amelie.delaunay@foss.st.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/dma/st,stm32-dma.yaml

@@ -40,6 +40,13 @@ description: |
          0x0: FIFO mode with threshold selectable with bit 0-1
          0x1: Direct mode: each DMA request immediately initiates a transfer
               from/to the memory, FIFO is bypassed.
+       -bit 4: alternative DMA request/acknowledge protocol
+         0x0: Use standard DMA ACK management, where ACK signal is maintained
+              up to the removal of request and transfer completion
+         0x1: Use alternative DMA ACK management, where ACK de-assertion does
+              not wait for the de-assertion of the REQuest, ACK is only managed
+              by transfer completion. This must only be used on channels
+              managing transfers for STM32 USART/UART.
 
 
 maintainers: