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

[Nicolas Pitre]

On Sun, 2 Apr 2017, Andy Shevchenko wrote:

> +Cc people, who have a key roles in all TTY stuff (btw, why you did
> miss them?

I used what MAINTAINERS and get_maintainer.pl gave me.

> 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.

The only one I missed is Ard.

Thanks for pulling more people in.


> 
> 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
>