Getting help in writing tty driver

[Ayush Singh]

Hello everyone, I am working on a Google Summer of Code 2023 project [1]
under BeagleBoard.or. In this project, I have to write a Linux driver that
exposes `dev/ttyMCU0` (or something else) which can be accessed as a Serial
tty by MCUmgr [2]. This then sends the data and gets the response from the
CC1352 processor in BeagelPlay [3], which runs Zephyr over HDLC UART (the
data for this tty port has a particular HDLC address).

However, all the resources I could find seem to use deprecated or old APIs
(like `alloc_tty_driver` instead of `tty_alloc_driver`). After some trial
and error, I was able to transfer the data written to `ttyMCU0` to CC1352
using `write` in `tty_operations`. However, I am stumped on how to send
data to tty from the driver so that the userspace application can read it
(like using `cat`). Supposedly it should be done using
`tty_insert_flip_char` but the signature for this function in the tutorials
and the one present at `tty_flip.h` is quite different and does not seem to
work for me. Also, there seems to be something about the Echo flag but not
sure what all that is.

Finally, I am not sure I understand the relationship between tty port, tty
device, and tty driver either so it would be great if someone can guide me
to resources related to tty. I think a simple tty echo driver should allow
me to get the hang of what I am supposed to do.

Ayush

[1]: https://elinux.org/BeagleBoard/GSoC/2023_Proposal/AyushSingh
[2]: https://github.com/apache/mynewt-mcumgr
[3]: https://beagleboard.org/play
[4]:
https://git.beagleboard.org/gsoc/greybus/beagleplay-greybus-driver/-/tree/develop
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