linux-kernel - Re: (EXT) RE: [PATCH] serial: Revert RS485 polarity change on UART open

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Date:   Tue, 29 Mar 2022 15:36:59 +0200
From:   Matthias Schiffer <matthias.schiffer@...tq-group.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Lino Sanfilippo <LinoSanfilippo@....de>,
        Lukas Wunner <lukas@...ner.de>
Subject: Re: (EXT) RE: [PATCH] serial: Revert RS485 polarity change on UART
 open

On Tue, 2022-03-29 at 13:19 +0000, David Laight wrote:
> From: Matthias Schiffer
> > Sent: 29 March 2022 14:03
> > 
> > On Tue, 2022-03-29 at 12:55 +0000, David Laight wrote:
> > > From: Matthias Schiffer
> > > > Sent: 29 March 2022 11:39
> > > ...
> > > > I guess that would work. The fact that even the different
> > > > variants of the 8250 are implemented inconsistently makes this
> > > > especially ugly... It certainly puts a damper on the efforts to
> > > > make
> > > > the handling of RS485 in serial drivers more generic.
> > > 
> > > One thing to remember is that RS232 (IIRC really V.38) line
> > > driver
> > > chips are typically inverting.
> > > 
> > > So the modem signals on a TTL level output will have the
> > > opposite polarity to that required on the actual connector.
> > > 
> > > Normally a UART will have an 'active high' register bit for
> > > a modem signal that drives and 'active low' pin so you get
> > > the correct polarity with an inverting line driver.
> > > 
> > > 	David
> > > 
> > 
> > Indeed. As far as I can tell, this property of UARTs is what got us
> > into this mess: Some people interpreted SER_RS485_RTS_ON_SEND as
> > "set
> > the RTS flag in the MCR register on send", while other thought it
> > should mean "set the RTS pin to high on send", leading to opposite
> > behaviours in different UART drivers (and even different UART
> > variants
> > in the same driver, in the case of the 8250 family).
> 
> Hmmm... A complete mess.
> The 'RTS pin' that needs to go high is the one on the (typically) 'D'
> connector after the inverting line driver.
> Not the pin on the uart package.
> I'd expect TTL level serial interfaces to require active low
> modem signals.
> 
> If RS485 is trying to do half duplex using RTS (request to send)
> and CTS (clear to send) you've typically got bigger problems
> than asserting RTS before a transmit.
> The real problem is removing RTS once the last transmit data bit
> (the stop bit) has left the UART pin.
> I've used local loopback (tx to rx) to detect that in the past.
> 
> Of course, if it is just doing flow control that should use RFS
> (ready for sending) to indicate space in the receive fifo but
> using the RTS pin instead that is a different matter.
> 
> 	David
> 


I'm aware of the difficulties of deasserting RTS after the transmission
is complete, but that's completely orthogonal to the issue I'm trying
to solve right now :)

Regards,
Matthias