Fix two races related TCSADRAIN/FLUSH. One in tty core and the other
that can occur when 8250 UART is using DMA for Tx.

These might be slightly controversial as it requires userspace to race
write() with tcsetattr() to trigger these races. But since the races
still seem fixable on kernel side, I made these patches.

v2:
- Switch DMA Tx running / LSR Tx empty checks order due to fast/slowness
- Calculate the result in serial8250_tx_empty() directly

Ilpo Järvinen (2):
  tty: Prevent writing chars during tcsetattr TCSADRAIN/FLUSH
  serial: 8250: Fix serial8250_tx_empty() race with DMA Tx

 drivers/tty/serial/8250/8250.h      | 12 ++++++++
 drivers/tty/serial/8250/8250_port.c |  7 +++--
 drivers/tty/tty.h                   |  2 ++
 drivers/tty/tty_io.c                |  4 +--
 drivers/tty/tty_ioctl.c             | 45 +++++++++++++++++++++--------
 5 files changed, 53 insertions(+), 17 deletions(-)

-- 
2.30.2