On Wed, Nov 08, 2017 at 11:32:34AM +0100, Lukas Wunner wrote: > rs485 allows for robust half-duplex serial communication. It is often > implemented by attaching an rs485 transceiver to a UART. The UART's > RTS line is wired to the transceiver's Transmit Enable pin and > determines whether the transceiver is sending or receiving. > > Examples for such transceivers are Maxim MAX13451E and TI SN65HVD1781A: > https://datasheets.maximintegrated.com/en/ds/MAX13450E-MAX13451E.pdf > http://www.ti.com/lit/ds/symlink/sn65hvd1781a-q1.pdf > > In the devicetree, the transceiver itself is not represented, only the > UART is. A few rs485-specific dt-bindings already exist and these go > into the UART's device node. That often ends up being a mistake. We don't describe things, then add a few properties and then finally realize we need a full representation. Connectors are a good example. > This commit adds a binding to set the RTS polarity. Most (if not all) > transceivers require the Transmit Enable pin be driven high for sending, > but in some cases boards may negate the pin and RTS must then be driven > low. Consequently the polarity defaults to active high but can be > inverted with the newly added "rs485-rts-active-low" binding. > > Document this binding in rs485.txt and in the two drivers fsl-imx-uart > and fsl-lpuart that are about to be amended with support for it. > > Curiously, the omap_serial driver defaults to active low and already > supports an "rs485-rts-active-high" binding to invert the polarity. > This is left unchanged to retain compatibility, but the binding is > herewith documented. > > Cc: Mark Jackson <mpfj@xxxxxxxxxxxxx> > Cc: Michał Oleszczyk <oleszczyk.m@xxxxxxxxx> > Cc: Sascha Hauer <s.hauer@xxxxxxxxxxxxxx> > Cc: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> > Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx> > --- > Documentation/devicetree/bindings/serial/fsl-imx-uart.txt | 3 ++- > Documentation/devicetree/bindings/serial/fsl-lpuart.txt | 3 ++- > Documentation/devicetree/bindings/serial/omap_serial.txt | 1 + > Documentation/devicetree/bindings/serial/rs485.txt | 1 + > 4 files changed, 6 insertions(+), 2 deletions(-) Acked-by: Rob Herring <robh@xxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
