[PATCH 0/5] serial: fix TX kfifo underflow on flush in five drivers

[Samarth Narang <samarth.colleges@gmail.com>]

This series addresses the TX kfifo underflow reported in the parent
thread, where uart_flush_buffer() resets the transmit kfifo but a
completion callback from an earlier DMA transfer may still call
uart_xmit_advance() using the byte count from before the flush.

The fixes take two approaches depending on how each driver manages its
TX DMA mappings. In samsung and tegra, which map the TX buffer once,
samsung's new flush_buffer() terminates the outstanding transfer, as
tegra's already does, following the approach introduced in 8250 by
9e512eaaf8f4. A completion already in progress skips the kfifo update if
the transfer has since been retired by flush_buffer() or stop_tx().

In sprd, msm, and pch_uart, which map each transfer separately,
terminating DMA there would leave the flush hook to unmap a buffer the
controller may still be reading. These drivers therefore leave the
transfer running and mark it as flushed, allowing the completion
callback to unmap the buffer without advancing the kfifo. This means
that the data already queued for DMA before the flush may still be
transmitted.

All five patches have only been compile-tested with arm64 defconfig and
the relevant drivers enabled, including pch_uart through COMPILE_TEST,
using W=1 with no new warnings. I do not have access to the affected
hardware, so testing by someone with suitable devices would be
appreciated.

One race remains, which is also present in the 8250 fix: a completion
may already be running when the flush occurs but acquire the port lock
only after a new transfer has started, potentially mistaking the new
transfer's state for its own. Preventing this would require tracking
individual transfers, for example with a generation counter, which I
have left out of this series to keep the changes focused.

Samarth Narang (5):
  tty: serial: samsung: fix TX kfifo underflow on flush
  serial: tegra: don't advance the TX kfifo for a flushed transfer
  serial: sprd: fix TX kfifo underflow on flush
  tty: serial: msm: fix TX kfifo underflow on flush
  serial: pch_uart: fix TX kfifo underflow on flush

 drivers/tty/serial/msm_serial.c   | 19 ++++++++++++++++++-
 drivers/tty/serial/pch_uart.c     | 27 +++++++++++++++++++++++++--
 drivers/tty/serial/samsung_tty.c  | 22 ++++++++++++++++++++++
 drivers/tty/serial/serial-tegra.c |  8 ++++++++
 drivers/tty/serial/sprd_serial.c  | 24 ++++++++++++++++++++++--
 5 files changed, 95 insertions(+), 5 deletions(-)


base-commit: 36aad21d50d1af3f6954bb0c988257a908202477
-- 
2.34.1