[PATCH v2 0/5] minitty: a minimal TTY layer alternative for embedded systems

[Andy Shevchenko]

+Cc people, who have a key roles in all TTY stuff (btw, why you did
miss them? why you didn't include people who reacted on your v1
either?).
I'm pretty sure they are interested in what's going on here.

On Sun, Apr 2, 2017 at 1:21 AM, Nicolas Pitre <nicolas.pitre at linaro.org> wrote:
> Many embedded systems don't need the full TTY layer support. Most of the
> time, the TTY layer is only a conduit for outputting debugging messages
> over a serial port. The TTY layer also implements many features that are
> very unlikely to ever be used in such a setup. There is great potential
> for both code and dynamic memory size reduction on small systems. This is
> what this patch series is achieving.
>
> The existing TTY code is quite large and complex. Trying to shrink it
> is risky as the potential for breakage is non negligeable, and its
> interchangeable layers impose a lower limit on the code to implement it.
> Therefore, the approach used here consists in the creation of a parallel
> implementation with the very minimal amount of code collapsed together
> that interfaces with existing UART drivers directly and provides TTY-like
> character devices to user space. When the regular TTY layer is disabled,
> then this minitty alternative layer is proposed by Kconfig.
>
> For more details on the rationale and motivations driving this approach
> please see: https://lkml.org/lkml/2017/3/24/634
>
> Of course, making it "mini" means there are limitations to what it does:
>
> - This supports serial ports only. No VT's, no PTY's.
>
> - The default n_tty line discipline is hardcoded and no other line
>   discipline are supported.
>
> - The line discipline features are not all implemented. Notably, XON/XOFF
>   is currently not implemented (although this might not require a lot of
>   code to do it if someone were to need it).
>
> - Hung-up state is not implemented.
>
> - No error handling on RX bytes other than counting them.
>
> - Job control is currently not supported (this may change in the future and
>   be configurable).
>
> But again, most small embedded systems simply don't need those things.
>
> This can be used on any architecture of course, but here's some numbers
> using a minimal ARM config.
>
> When CONFIG_TTY=y, the following files are linked into the kernel:
>
>    text    data     bss     dec     hex filename
>    8796     128       0    8924    22dc drivers/tty/n_tty.o
>   11809     276       0   12085    2f35 drivers/tty/serial/fulltty_serial.o
>    1376       0       0    1376     560 drivers/tty/tty_buffer.o
>   13571     172     132   13875    3633 drivers/tty/tty_io.o
>    3072       0       0    3072     c00 drivers/tty/tty_ioctl.o
>    2457       2     120    2579     a13 drivers/tty/tty_ldisc.o
>    1328       0       0    1328     530 drivers/tty/tty_ldsem.o
>     316       0       0     316     13c drivers/tty/tty_mutex.o
>    2516       0       0    2516     9d4 drivers/tty/tty_port.o
>   5241     578     252   46071    b3f7 (TOTALS)
>
> When CONFIG_TTY=n and CONFIG_MINITTY_SERIAL=y, the above files are replaced
> by the following:
>
>    text    data     bss     dec     hex filename
>    8063       8      64    8135    1fc7 drivers/tty/serial/minitty_serial.o
>
> That's it!  And the runtime buffer usage is much less as well. Future plans
> such as removing runtime baudrate handling for those targets with a known
> fixed baudrate will shrink the code even more.
>
> Changes from v1:
>
> - Added an entry to the MAINTAINERS file.
> - Factored out more common core code into serial_lib.c.
> - Implemented a few more TTY callback functions to be compatible with
>   more UART drivers.
>
> Overall diffstat:
>
>  MAINTAINERS                                     |    8 +-
>  drivers/tty/Kconfig                             |   10 +-
>  drivers/tty/Makefile                            |    3 +-
>  drivers/tty/serial/Kconfig                      |   12 +-
>  drivers/tty/serial/Makefile                     |    7 +-
>  .../serial/{serial_core.c => fulltty_serial.c}  |  419 +---
>  drivers/tty/serial/minitty_serial.c             | 1793 +++++++++++++++++
>  drivers/tty/serial/serial_lib.c                 |  440 ++++
>  drivers/tty/tty_baudrate.c                      |  232 +++
>  drivers/tty/tty_io.c                            |   24 -
>  drivers/tty/tty_ioctl.c                         |  222 --
>  include/linux/console.h                         |    2 +
>  include/linux/serial_core.h                     |    1 +
>  include/linux/tty.h                             |    7 +-
>  include/linux/tty_flip.h                        |    9 +
>  init/main.c                                     |    2 +-
>  kernel/printk/printk.c                          |   24 +
>  17 files changed, 2538 insertions(+), 677 deletions(-)



-- 
With Best Regards,
Andy Shevchenko