serial: 8250_dw: add fractional divisor support
For Synopsys DesignWare 8250 uart which version >= 4.00a, there's a valid divisor latch fraction register. The fractional divisor width is 4bits ~ 6bits. Now the preparation is done, it's easy to add the feature support. This patch firstly tries to get the fractional divisor width during probe, then setups dw specific get_divisor() and set_divisor() hook. Signed-off-by: NJisheng Zhang <Jisheng.Zhang@synaptics.com> Reviewed-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com> Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
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