Hi,

there is a problem in samsung soc serial driver with the report of
TIOCM_CTS in get_mctrl.

The uart layer check the status of the TIOCM_CTS in the open or
set_termios function and cache the result in the tty->hw_stopped flag.

Then when sending data, in the function uart_start, it check
tty->hw_stopped flag before calling the start_tx uart driver callback.


For the samsung uart driver it means :
- if the other side is not connected or in a reset state, then CTS
signal is not activated
- we open and configure the uart. get_mctrl don't report TIOCM_CTS flag.
uart layer set tty->hw_stopped flag.
- we connect or start the other side. CTS signal become activated.
- we send some characters on the uart. This call uart_write which fill
uart tx queue and call uart_start. uart_start doesn't call driver
start_tx because tty->hw_stopped flag is set.
- because driver start_tx is never call, the data is never send on the uart.

>From what I see, working driver managing TIOCM_CTS flag (amba-pl011,
8250), call uart_handle_cts_change when the uart driver detect a CTS
signal change (via a interrupt).

Because the samsung controller doesn't generate an interrupt when CTS
signal change, we can't call uart_handle_cts_change.

And easy fix could be to always report TIOCM_CTS.

I believe this bug is tricky and can affect other drivers.
Can this be documented in Documentation/serial/driver like the attached
patch ?


Matthieu