[PATCH v6 3/3] serial: 8250_dw: add fractional divisor support

[Jisheng Zhang]

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: Jisheng Zhang <jisheng.zh...@synaptics.com>
Reviewed-by: Andy Shevchenko <andriy.shevche...@linux.intel.com>
---
 drivers/tty/serial/8250/8250_dw.c | 45 +++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_dw.c 
b/drivers/tty/serial/8250/8250_dw.c
index fa8a00e8c9c6..5a60c4814d62 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -31,6 +31,7 @@
 
 /* Offsets for the DesignWare specific registers */
 #define DW_UART_USR    0x1f /* UART Status Register */
+#define DW_UART_DLF    0xc0 /* Divisor Latch Fraction Register */
 #define DW_UART_CPR    0xf4 /* Component Parameter Register */
 #define DW_UART_UCV    0xf8 /* UART Component Version */
 
@@ -55,6 +56,7 @@
 
 struct dw8250_data {
        u8                      usr_reg;
+       u8                      dlf_size;
        int                     line;
        int                     msr_mask_on;
        int                     msr_mask_off;
@@ -366,6 +368,37 @@ static bool dw8250_idma_filter(struct dma_chan *chan, void 
*param)
        return param == chan->device->dev->parent;
 }
 
+/*
+ * 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 quot, rem, base_baud = baud * 16;
+       struct dw8250_data *d = p->private_data;
+
+       quot = p->uartclk / base_baud;
+       rem = p->uartclk % base_baud;
+       *frac = DIV_ROUND_CLOSEST(rem << d->dlf_size, base_baud);
+
+       return quot;
+}
+
+static void dw8250_set_divisor(struct uart_port *p, unsigned int baud,
+                              unsigned int quot, unsigned int quot_frac)
+{
+       dw8250_writel_ext(p, DW_UART_DLF, quot_frac);
+       serial8250_do_set_divisor(p, baud, quot, quot_frac);
+}
+
 static void dw8250_quirks(struct uart_port *p, struct dw8250_data *data)
 {
        if (p->dev->of_node) {
@@ -426,6 +459,18 @@ static void dw8250_setup_port(struct uart_port *p)
        dev_dbg(p->dev, "Designware UART version %c.%c%c\n",
                (reg >> 24) & 0xff, (reg >> 16) & 0xff, (reg >> 8) & 0xff);
 
+       dw8250_writel_ext(p, DW_UART_DLF, ~0U);
+       reg = dw8250_readl_ext(p, DW_UART_DLF);
+       dw8250_writel_ext(p, DW_UART_DLF, 0);
+
+       if (reg) {
+               struct dw8250_data *d = p->private_data;
+
+               d->dlf_size = fls(reg);
+               p->get_divisor = dw8250_get_divisor;
+               p->set_divisor = dw8250_set_divisor;
+       }
+
        reg = dw8250_readl_ext(p, DW_UART_CPR);
        if (!reg)
                return;
-- 
2.18.0