serial: 8250: Fix reporting real baudrate value in c_ospeed field

serial: 8250: Fix reporting real baudrate value in c_ospeed field

commit 32262e2e429cdb31f9e957e997d53458762931b7 upstream.

In most cases it is not possible to set exact baudrate value to hardware.

So fix reporting real baudrate value which was set to hardware via c_ospeed
termios field. It can be retrieved by ioctl(TCGETS2) from userspace.

Real baudrate value is calculated from chosen hardware divisor and base
clock. It is implemented in a new function serial8250_compute_baud_rate()
which is inverse of serial8250_get_divisor() function.

With this change is fixed also UART timeout value (it is updated via
uart_update_timeout() function), which is calculated from the now fixed
baudrate value too.

Cc: stable@vger.kernel.org
Signed-off-by: Pali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20210927093704.19768-1-pali@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>