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commitd3b3404df3upstream. Commita6845e1e1b("serial: core: Consider rs485 settings to drive RTS") sought to deassert RTS when opening an rs485-enabled uart port. That way, the transceiver does not occupy the bus until it transmits data. Unfortunately, the commit mixed up the logic and *asserted* RTS instead of *deasserting* it: The commit amended uart_port_dtr_rts(), which raises DTR and RTS when opening an rs232 port. "Raising" actually means lowering the signal that's coming out of the uart, because an rs232 transceiver not only changes a signal's voltage level, it also *inverts* the signal. See the simplified schematic in the MAX232 datasheet for an example: https://www.ti.com/lit/ds/symlink/max232.pdf So, to raise RTS on an rs232 port, TIOCM_RTS is *set* in port->mctrl and that results in the signal being driven low. In contrast to rs232, the signal level for rs485 Transmit Enable is the identity, not the inversion: If the transceiver expects a "high" RTS signal for Transmit Enable, the signal coming out of the uart must also be high, so TIOCM_RTS must be *cleared* in port->mctrl. The commit did the exact opposite, but it's easy to see why given the confusing semantics of rs232 and rs485. Fix it. Fixes:a6845e1e1b("serial: core: Consider rs485 settings to drive RTS") Cc: stable@vger.kernel.org # v4.14+ Cc: Rafael Gago Castano <rgc@hms.se> Cc: Jan Kiszka <jan.kiszka@siemens.com> Cc: Su Bao Cheng <baocheng.su@siemens.com> Signed-off-by: Lukas Wunner <lukas@wunner.de> Link: https://lore.kernel.org/r/9395767847833f2f3193c49cde38501eeb3b5669.1639821059.git.lukas@wunner.de Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.