On Fri, Sep 26, 2014 at 02:59:18PM +0200, Janusz Użycki wrote:

But serial8250_get_mctrl() in 8250_core.c calls serial8250_modem_status()
which calls eg. uart_handle_cts_change() even if enable_ms() wasn't called.
This is the difference.
The serial8250_modem_status() is also called in the interrupt
and, what I don't understand, in serial8250_console_write().

Reading the MSR register clears the interrupts.  So, whenever MSR is read,
you have to deal with any state changes which _would_ have been passed
to the interrupt function.

Plus, it's not quite as you make out above.

If enable_ms() is not called, then UART_IER_MSI will not be set in up->ier.
Hence, uart_handle_cts_change() will not be called.

The reason for the call in the console function is to account for the
state changes during console write with CTS flow control - this again
needs the MSR register to be read, and we have to account for MSR state
changes after the console write has completed.

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