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 45241 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(-)
