From: Bich Hemon <bich.hemon@xxxxxx> Signed-off-by: Gerald Baeza <gerald.baeza@xxxxxx> --- .../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 -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
