XON/XOFF are used for software flow-control on serial lines. XON and XOFF appear as characters within the stream but should be processed as soon as possible. The characters received by the UART drivers are in intermediate buffers until TTY receives them. In the case where the TTY is not read from, the characters may get stuck into those intermediate buffers until user-space reads from the TTY. Among the characters stuck in the buffers, can also be those XON/XOFF flow control characters. A stuck flow-control character is not very useful. This patch series addresses the issue by checking if TTY is slow to process characters, that is, eats less than the given amount. If TTY is slow, a lookahead is invoked for the characters that remain in the intermediate buffer(s). Then at a later time, receive_buf needs to ensure the flow-control actions are not retaken when those same characters get pushed to TTY. This patch series fixes an issue but I'm not able to pinpoint to a specific commit id to provide a Fixes tag. The last patch of the series is not needed for minimal fix (and has a small chance of regression too), thus that patch shouldn't be sent to stable. v1 -> v2: - Add flow ctrl char funcs in separate change & rework logic a bit. I believe it's now cleaner than it would have been with the suggestions during v1 review, please recheck. - Renamed n_tty_lookahead_special to n_tty_lookahead_flow_ctrl - Fixed logic for START_CHAR == STOP_CHAR case - Use unsigned int for lookahead_count in receive_buf call chain - Use consistent types in lookahead call chain - Improved indentation & line splits for function params - Corrected tty_ldisc.h comments documenting tty_ldisc_ops - Tweaked comment format v2 -> v3: - Split preparatory patch moving/rearranging code to two - Fix closing path giving change for ... || xx to execute instead of skipping the flow-control char - Use the same flow-control char function on closing path (just a cleanup, non-fix as last patch, a small change of regression exists) v3 -> v4: - Rework lookahead_count, it is now kept internal to n_tty ldisc rather than passed around through the whole callchain - Dropped Gilles' Tested-by due to major changes - Improve comments & changelogs v4 -> v5: - Rebased & removed the applied patch - Explain the algorithm a bit more in the commit message Ilpo Järvinen (2): tty: Implement lookahead to process XON/XOFF timely tty: Use flow-control char function on closing path drivers/tty/n_tty.c | 91 ++++++++++++++++++++++++++++++-------- drivers/tty/tty_buffer.c | 59 ++++++++++++++++++++---- drivers/tty/tty_port.c | 21 +++++++++ include/linux/tty_buffer.h | 1 + include/linux/tty_ldisc.h | 13 ++++++ include/linux/tty_port.h | 2 + 6 files changed, 159 insertions(+), 28 deletions(-) -- 2.30.2
