On Tue, 2018-07-10 at 11:15 +0800, Jisheng Zhang wrote:

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.

Thanks for an update, my comments below.

+/*
+ * divisor = div(I) + div(F)
+ * "I" means integer, "F" means fractional
+ * quot = div(I) = clk / (16 * baud)
+ * frac = div(F) * 2^dlf_size
+ *
+ * let rem = clk % (16 * baud)
+ * we have: div(F) * (16 * baud) = rem
+ * so frac = 2^dlf_size * rem / (16 * baud) = (rem << dlf_size) / (16
* baud)
+ */
+static unsigned int dw8250_get_divisor(struct uart_port *p,
+				       unsigned int baud,
+				       unsigned int *frac)
+{

unsigned int base_baud = baud * 16;

+	unsigned int quot, rem;
+	struct dw8250_data *d = p->private_data;
+
+	quot = p->uartclk / (16 * baud);
+	rem = p->uartclk % (16 * baud);
+	*frac = DIV_ROUND_CLOSEST(rem << d->dlf_size, 16 * baud);
+

While it looks indeed better, I would rather like to have a confirmation
it's working as designed.

For example, when I did some calculus, I cooked a preliminary check in
Python (easy and fast to prototype), for example:
https://gist.github.com/andy-shev/06b084488b3629898121 in Python, or
commit 9df461eca18f ("spi: pxa2xx: replace ugly table by approximation")
in the kernel.

Or another one here https://gist.github.com/andy-shev/8b2a73aeca2874f4cc
89 and commits c1a67b48f6a5 ("serial: 8250_pci: replace switch-case by
formula for Intel MID"), 21947ba654a6 ("serial: 8250_pci: replace
switch-case by formula")

P.S. The code itself looks good to me, thanks!

-- 
Andy Shevchenko [off-list ref]
Intel Finland Oy