For Synopsys DesignWare 8250 uart which version >= 4.00a, there's a
valid divisor latch fraction register. The fractional divisor width is
4bits ~ 6bits.

patch1 lets serial8250_get_divisor() get uart_port * as param
patch2 introduces necessary hooks to 8250 core.
patch3 implements the fractional divisor support for Synopsys DW 8250.

Since v1:
 - add an extra patch to let serial8250_get_divisor() get uart_port *
   as param
 - take Andy's suggestions to "integrates hooks in the same way like
   it's done for the rest of 8250 ones". Many thanks to Andy.

Jisheng Zhang (3):
  serial: 8250: let serial8250_get_divisor() get uart_port * as param
  serial: 8250: introduce get_divisor() and set_divisor() hook
  serial: 8250_dw: add fractional divisor support

 drivers/tty/serial/8250/8250_core.c |  4 +++
 drivers/tty/serial/8250/8250_dw.c   | 53 +++++++++++++++++++++++++++++
 drivers/tty/serial/8250/8250_port.c | 33 ++++++++++++++----
 include/linux/serial_core.h         |  7 ++++
 4 files changed, 90 insertions(+), 7 deletions(-)

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2.18.0