w1: add UART w1 bus driver

Hello!

Krzysztof, thank your very much for your feedback!

This patch contains a driver for a 1-Wire bus over UART. The driver
utilizes the UART interface via the Serial Device Bus to create the
1-Wire timing patterns.

Version 1

- In v1, the driver requests a baud-rate (9600 for reset and 115200 for
write/read) and tries to adapt the transmitted byte according to the
actual baud-rate returned from serdev. Is this the correct direction or
should the baud-rate be specified in the device-tree? Alternatively,
it could make sense to specify the minimum and maximum times for the
1-Wire operations in the device-tree, instead of using hard-coded ones
similar as in "Figure 11. Configuration tab" of the linked document
"Using UART to Implement a 1-Wire Bus Master".

- In addition, the received byte is now protected with a mutex - instead
of the atomic, which I used before due to the concurrent store and load.

- Receiving more than one byte results in an error, since the w1-uart
driver is the only writer, it writes a single-byte and should receive
a single byte.

Changes:
- support different baud-rates
- fix variable names, errno-returns, wrong define CONFIG_OF
- fix log flooding
- fix locking problem for serdev-receive and w1-master reset/touch
- fix driver remove (error-path for rxtx-function)
- add documentation for dt-binding


It was tested on a "Raspberry Pi 3 Model B+" with a DS18B20 and on a
"Variscite DART-6UL" with a DS18S20 temperature sensor.

Content:
- Patch 1: device tree binding
- Patch 2: driver and documentation

The patch was created against the w1 subsytem tree (branch w1-next):
  Link: https://git.kernel.org/pub/scm/linux/kernel/git/krzk/linu...

The checkpatch.pl script reported the following error - which I am not
sure how to fix:
  WARNING: added, moved or deleted file(s), does MAINTAINERS need
  updating?

The technical details for 1-Wire over UART are in the document:
  Link: https://www.analog.com/en/technical-articles/using-a-uart...

  In short, the UART peripheral must support full-duplex and operate in
open-drain mode. The timing patterns are generated by a specific
combination of baud-rate and transmitted byte, which corresponds to a
1-Wire read bit, write bit or reset pulse.

For instance the timing pattern for a 1-Wire reset and presence detect
uses the baud-rate 9600, i.e. 104.2 us per bit. The transmitted byte
0xf0 over UART (least significant bit first, start-bit low) sets the
reset low time for 1-Wire to 521 us. A present 1-Wire device changes the
received byte by pulling the line low, which is used by the driver to
evaluate the result of the 1-Wire operation.

Similar for a 1-Wire read bit or write bit, which uses the baud-rate
115200, i.e. 8.7 us per bit. The transmitted byte 0x00 is used for a
Write-0 operation and the byte 0xff for Read-0, Read-1 and Write-1.

Hope the driver is helpful.

Thanks,
Christoph

Christoph Winklhofer (2):
  dt-bindings: w1: UART 1-wire bus
  w1: add UART w1 bus driver

 .../devicetree/bindings/w1/w1-uart.yaml       |  44 +++
 Documentation/w1/masters/index.rst            |   1 +
 Documentation/w1/masters/w1-uart.rst          |  53 +++
 drivers/w1/masters/Kconfig                    |  10 +
 drivers/w1/masters/Makefile                   |   1 +
 drivers/w1/masters/w1-uart.c                  | 307 ++++++++++++++++++
 6 files changed, 416 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/w1/w1-uart.yaml
 create mode 100644 Documentation/w1/masters/w1-uart.rst
 create mode 100644 drivers/w1/masters/w1-uart.c


base-commit: efc19c44aa442197ddcbb157c6ca54a56eba8c4e
-- 
2.43.0