On Mon, Dec 20, 2021 at 07:30:52AM +0100, Jiri Slaby wrote:

On 20. 12. 21, 7:28, Jiri Slaby wrote:

quoted

On 18. 12. 21, 10:58, Lukas Wunner wrote:

quoted

Commit a6845e1e1b78 ("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: a6845e1e1b78 ("serial: core: Consider rs485 settings to drive
RTS")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: stable@vger.kernel.org # v4.14+
Cc: Rafael Gago Castano <redacted>

Rafael, can you ack/test this, please?

Definitely on that e-mail:
 550 5.4.1 Recipient address rejected: Access denied. AS(201806281)
[DB5EUR03FT039.eop-EUR03.prod.protection.outlook.com]

Trying rafael.gago@gmail.com from the Author field.

A bit of GitHub sleuthing turned up the following alternative addresses:

rafael_gago_81@hotmail.com
rafael.gago@zenuity.com
rafael.gago@zenseact.com

Unfortunately none of them is responsive.  I was hoping that the Siemens
folks might be willing to attest correctness of the patch.

Thanks,

Lukas