Hi Adam, On Mon, Sep 10, 2018 at 1:54 PM, Adam Rudziński <adam.rudzinski@xxxxxxxxxx> wrote: > Hi, > > I'm preparing Linux for a device based on i.MX6UL by Freescale, now NXP. The > device uses RS485 interface, and I have noticed, that the current driver > SERIAL_IMX (driver/tty/serial/imx.c) doesn't do that correctly. Thus, I have > a question to you (four paragraphs below). Which kernel version are you using and what is the exact failure? > According to my understanding, at least for the i.MX family, the general > agreement is that CTS_B signal of the processor, which is an output, should > be used as "driver enable" (DE) signal of transmitter in the RS485 > transceiver chip. Typically this DE signal is "normal logic", ie. high > voltage == '1', while CTS_B is inverted logic signal, ie. low voltage == > '1'. UART peripheral of i.MX processor has bit field CTS in register UCR2, > and this bit field can be used for "manual" control of the CTS_B signal. > > That is what the driver SERIAL_IMX does for ports defined in device tree as > dedicated for RS485. Yes, this commit from Baruch helps to clarify about it: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v4.19-rc3&id=e51e597d9f692cd7d205e3b44bb35ea3b97e7329 > > However, Linux can, and does, disable serial port, here, by putting '0' into > UCR1 register of the UART peripheral. But then, CTS bit field is ignored and > CTS_B a.k.a. DE signal line is high. For RS485 this ends up in having > wrestling match of connected transmitters. > > The correct state should be that for unused or disabled UART assigned for > RS485 the DE signal line is low. That is something I want to implement. I > can see two ways of doing that in software (described below), and my > question is: which is preferred? > > 1) Prevent disabling UART peripheral. For that function imx_shutdown or > imx_uart_shutdown could check if RS485 is enabled (in rs485.flags) and if > yes, keep the enable bit in UCR1 asserted. This will not require engaging > other things, but will not allow to fully disable the port. I don't know why > Linux tries to do so, and I can't tell whether or not that is a potential > problem. > > 2) Use GPIO to define the state of the signal when UART peripheral is > disabled. That might be done by defining the GPIO muxed with CTS_B as output > in low state, and muxing the processor pin to CTS_B when enabling the UART, > and to GPIO when disabling it. This would allow to fully disable UART, but > will require undisclosed control of an additional feature (GPIO channel) > which somebody might want to use for another purpose. > > I would appreciate letting me know your opinion on that. Added a few folks who has worked with RS485 on i.MX in case they can help.
