From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bich HEMON Subject: [PATCH 17/20] dt-bindings: serial: stm32: add dma using note Date: Mon, 26 Jun 2017 12:49:16 +0000 Message-ID: <1498481318-1894-18-git-send-email-bich.hemon@st.com> References: <1498481318-1894-1-git-send-email-bich.hemon@st.com> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 8BIT Return-path: In-Reply-To: <1498481318-1894-1-git-send-email-bich.hemon-qxv4g6HH51o@public.gmane.org> Content-Language: en-US Sender: devicetree-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Greg Kroah-Hartman , Rob Herring , Mark Rutland , Maxime Coquelin , Alexandre TORGUE , Jiri Slaby , "linux-serial-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org" , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" Cc: Bich HEMON List-Id: devicetree@vger.kernel.org From: Bich Hemon Signed-off-by: Gerald Baeza --- .../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 devicetree" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html