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Subject[PATCH 17/20] dt-bindings: serial: stm32: add dma using note
Date
From: Bich Hemon <bich.hemon@st.com>

Signed-off-by: Gerald Baeza <gerald.baeza@st.com>
---
.../devicetree/bindings/serial/st,stm32-usart.txt | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)

diff --git a/Documentation/devicetree/bindings/serial/st,stm32-usart.txt b/Documentation/devicetree/bindings/serial/st,stm32-usart.txt
index 3b42138..6c1cfc8 100644
--- a/Documentation/devicetree/bindings/serial/st,stm32-usart.txt
+++ b/Documentation/devicetree/bindings/serial/st,stm32-usart.txt
@@ -18,6 +18,28 @@ Optional properties:
- dmas: phandle(s) to DMA controller node(s). Refer to stm32-dma.txt
- dma-names: "rx" and/or "tx"

+Note for dma using:
+- "tx" dma can be used without any constraint since it uses single
+dma transfers.
+- "rx" dma using requires some attention:
+ 1) if you cannot anticipate the length of your received packets
+ and if your usart device embeds an internal fifo, then DON'T use
+ dma mode.
+ 2) if you enable dma mode WITHOUT mdma intermediate copy (cf.
+ stm32-dma.txt), then the availability of the received data will
+ depend on the dma driver policy and it may be delayed until dma
+ internal fifo is full. The usart driver will see this checking
+ the dma residue when rx interrupt (RXNE or RTO) occurs.
+ 3) if you enable dma mode WITH mdma intermediate copy (cf.
+ stm32-dma.txt) then the usart driver will never see the dma
+ residue becoming smaller than RX_BUF_P but it will get its
+ rx dma complete callback called when the cyclic transfer period
+ (RX_BUF_P) is reached.
+The three possibilities above are ordered from the most cpu time
+consuming one to the least one. The counterpart of this optimisation
+is the reception granularity achievable by the usart driver, from
+one byte up to RX_BUF_P.
+
Note: Each usart controller should have an alias correctly numbered
in "aliases" node.

--
1.9.1
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