linux-kernel - Move RS485 implementation from drivers to serial core (v4)

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Date:   Mon, 28 Feb 2022 03:39:19 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     gregkh@...uxfoundation.org, jirislaby@...nel.org,
        u.kleine-koenig@...gutronix.de
Cc:     linux@...linux.org.uk, richard.genoud@...il.com,
        nicolas.ferre@...rochip.com, alexandre.belloni@...tlin.com,
        ludovic.desroches@...rochip.com, shawnguo@...nel.org,
        s.hauer@...gutronix.de, kernel@...gutronix.de, festevam@...il.com,
        linux-imx@....com, mcoquelin.stm32@...il.com,
        alexandre.torgue@...s.st.com, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-stm32@...md-mailman.stormreply.com, lukas@...ner.de,
        p.rosenberger@...bus.com
Subject: Move RS485 implementation from drivers to serial core (v4)

This patch series is an attempt to simplify rs485 implementation in drivers
by moving the following tasks out of the drivers into the serial core:

- ensure sane RTS settings: in case of an invalid configuration (both RTS
  after send and RTS on send set or both unset) enable RTS on send and
  disable RTS after send

- nullify the padding field of the serial_rs485 struct before it is
  returned to userspace

- copy the configuration stored in the serial_rs485 struct to the port
  configuration if setting the configuration in the driver was successfull

- limit the RTS delay to 100ms


Redundant code has been removed from the following drivers for now:

- atmel
- fsl_lpuart
- amba
- imx
- max310x
- omap-serial
- sc16is7xx
- stm32-usart

The code has been tested with the amba pl011 driver. This series applies
against Gregs tty-testing branch.

Changes in v2:
- use a makro for max RTS delays and comment it (as requested by Jiri)
- add a comment concerning the memset of a structures padding field - correct
  typos in the commit message (found by Uwe) 
- rephrase all commit messages to make more clear that function
  uart_set_rs485_config() has been extended by checks and other functionalities
  (as requested by Uwe)

Changes in v3:
- add warning messages if the serial core corrects RS485 values (as requested by
  Lukas Wunner)
- dont expose the macro for max RTS delays to userspace (as requested by Greg)

Changes in v4:
- use ratelimit warning messages and also print device, uart port name and line
  (as requested by Jiri)