[PATCH v3 0/3] serial: 8250_dw: add fractional divisor support

[PATCH v3 0/3] 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.

patch1 introduces necessary hooks to 8250 core.
patch2 exports serial8250_do_set_divisor()
patch3 implements the fractional divisor support for Synopsys DW 8250.

Since v2:
 - rebase to tty-next branch, since I need one patch from Andy which
   is in tty-next
 - drop the patch "serial: 8250: let serial8250_get_divisor() get
   uart_port * as param" since it's in tty-next now.
 - add a new patch to export serial8250_do_set_divisor(), and reuse it
   to complete dw8250_set_divisor().
 - remove DW 8250 version check, since the DLF register always exists
   and if fractional divisor isn't supported, the register read as 0
 - add comments to explain how dw8250_get_divisor() get quot and frac.
 - the frac calcuation is simplified with well implemented GENMASK
 - Add Andy's Reviewed-by tag to patch1.

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: introduce get_divisor() and set_divisor() hook
  serial: 8250: export serial8250_do_set_divisor()
  serial: 8250_dw: add fractional divisor support

 drivers/tty/serial/8250/8250_core.c |  4 +++
 drivers/tty/serial/8250/8250_dw.c   | 45 +++++++++++++++++++++++++++++
 drivers/tty/serial/8250/8250_port.c | 30 +++++++++++++++----
 include/linux/serial_8250.h         |  3 ++
 include/linux/serial_core.h         |  7 +++++
 5 files changed, 84 insertions(+), 5 deletions(-)

-- 
2.18.0

[PATCH v3 1/3] serial: 8250: introduce get_divisor() and set_divisor() hook

[Jisheng Zhang]

Add these two hooks so that they can be overridden with driver specific
implementations.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
 drivers/tty/serial/8250/8250_core.c |  4 ++++
 drivers/tty/serial/8250/8250_port.c | 27 +++++++++++++++++++++++----
 include/linux/serial_core.h         |  7 +++++++
 3 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 9342fc2ee7df..a0bb77290747 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -1023,6 +1023,10 @@ int serial8250_register_8250_port(struct uart_8250_port *up)
 			uart->port.get_mctrl = up->port.get_mctrl;
 		if (up->port.set_mctrl)
 			uart->port.set_mctrl = up->port.set_mctrl;
+		if (up->port.get_divisor)
+			uart->port.get_divisor = up->port.get_divisor;
+		if (up->port.set_divisor)
+			uart->port.set_divisor = up->port.set_divisor;
 		if (up->port.startup)
 			uart->port.startup = up->port.startup;
 		if (up->port.shutdown)
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 709fe6b4265c..ce0dc17f18ee 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2498,9 +2498,9 @@ static unsigned int npcm_get_divisor(struct uart_8250_port *up,
 	return DIV_ROUND_CLOSEST(port->uartclk, 16 * baud + 2) - 2;
 }
 
-static unsigned int serial8250_get_divisor(struct uart_port *port,
-					   unsigned int baud,
-					   unsigned int *frac)
+static unsigned int serial8250_do_get_divisor(struct uart_port *port,
+					      unsigned int baud,
+					      unsigned int *frac)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
 	unsigned int quot;
@@ -2532,6 +2532,16 @@ static unsigned int serial8250_get_divisor(struct uart_port *port,
 	return quot;
 }
 
+static unsigned int serial8250_get_divisor(struct uart_port *port,
+					   unsigned int baud,
+					   unsigned int *frac)
+{
+	if (port->get_divisor)
+		return port->get_divisor(port, baud, frac);
+
+	return serial8250_do_get_divisor(port, baud, frac);
+}
+
 static unsigned char serial8250_compute_lcr(struct uart_8250_port *up,
 					    tcflag_t c_cflag)
 {
@@ -2570,7 +2580,7 @@ static unsigned char serial8250_compute_lcr(struct uart_8250_port *up,
 	return cval;
 }
 
-static void serial8250_set_divisor(struct uart_port *port, unsigned int baud,
+static void serial8250_do_set_divisor(struct uart_port *port, unsigned int baud,
 			    unsigned int quot, unsigned int quot_frac)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
@@ -2603,6 +2613,15 @@ static void serial8250_set_divisor(struct uart_port *port, unsigned int baud,
 	}
 }
 
+static void serial8250_set_divisor(struct uart_port *port, unsigned int baud,
+				   unsigned int quot, unsigned int quot_frac)
+{
+	if (port->set_divisor)
+		port->set_divisor(port, baud, quot, quot_frac);
+	else
+		serial8250_do_set_divisor(port, baud, quot, quot_frac);
+}
+
 static unsigned int serial8250_get_baud_rate(struct uart_port *port,
 					     struct ktermios *termios,
 					     struct ktermios *old)
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 06ea4eeb09ab..406edae44ca3 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -127,6 +127,13 @@ struct uart_port {
 					     struct ktermios *);
 	unsigned int		(*get_mctrl)(struct uart_port *);
 	void			(*set_mctrl)(struct uart_port *, unsigned int);
+	unsigned int		(*get_divisor)(struct uart_port *,
+					       unsigned int baud,
+					       unsigned int *frac);
+	void			(*set_divisor)(struct uart_port *,
+					       unsigned int baud,
+					       unsigned int quot,
+					       unsigned int quot_frac);
 	int			(*startup)(struct uart_port *port);
 	void			(*shutdown)(struct uart_port *port);
 	void			(*throttle)(struct uart_port *port);
-- 
2.18.0

serial: 8250: introduce get_divisor() and set_divisor() hook

[Jisheng Zhang]

oops, sorry, please ignore this patch.

On Mon, 9 Jul 2018 16:19:22 +0800
Jisheng Zhang <Jisheng.Zhang@synaptics.com> wrote:

> Add these two hooks so that they can be overridden with driver specific
> implementations.
> 
> Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> ---
>  drivers/tty/serial/8250/8250_core.c |  4 ++++
>  drivers/tty/serial/8250/8250_port.c | 27 +++++++++++++++++++++++----
>  include/linux/serial_core.h         |  7 +++++++
>  3 files changed, 34 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> index 9342fc2ee7df..a0bb77290747 100644
> --- a/drivers/tty/serial/8250/8250_core.c
> +++ b/drivers/tty/serial/8250/8250_core.c
> @@ -1023,6 +1023,10 @@ int serial8250_register_8250_port(struct uart_8250_port *up)
>  			uart->port.get_mctrl = up->port.get_mctrl;
>  		if (up->port.set_mctrl)
>  			uart->port.set_mctrl = up->port.set_mctrl;
> +		if (up->port.get_divisor)
> +			uart->port.get_divisor = up->port.get_divisor;
> +		if (up->port.set_divisor)
> +			uart->port.set_divisor = up->port.set_divisor;
>  		if (up->port.startup)
>  			uart->port.startup = up->port.startup;
>  		if (up->port.shutdown)
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 709fe6b4265c..ce0dc17f18ee 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -2498,9 +2498,9 @@ static unsigned int npcm_get_divisor(struct uart_8250_port *up,
>  	return DIV_ROUND_CLOSEST(port->uartclk, 16 * baud + 2) - 2;
>  }
>  
> -static unsigned int serial8250_get_divisor(struct uart_port *port,
> -					   unsigned int baud,
> -					   unsigned int *frac)
> +static unsigned int serial8250_do_get_divisor(struct uart_port *port,
> +					      unsigned int baud,
> +					      unsigned int *frac)
>  {
>  	struct uart_8250_port *up = up_to_u8250p(port);
>  	unsigned int quot;
> @@ -2532,6 +2532,16 @@ static unsigned int serial8250_get_divisor(struct uart_port *port,
>  	return quot;
>  }
>  
> +static unsigned int serial8250_get_divisor(struct uart_port *port,
> +					   unsigned int baud,
> +					   unsigned int *frac)
> +{
> +	if (port->get_divisor)
> +		return port->get_divisor(port, baud, frac);
> +
> +	return serial8250_do_get_divisor(port, baud, frac);
> +}
> +
>  static unsigned char serial8250_compute_lcr(struct uart_8250_port *up,
>  					    tcflag_t c_cflag)
>  {
> @@ -2570,7 +2580,7 @@ static unsigned char serial8250_compute_lcr(struct uart_8250_port *up,
>  	return cval;
>  }
>  
> -static void serial8250_set_divisor(struct uart_port *port, unsigned int baud,
> +static void serial8250_do_set_divisor(struct uart_port *port, unsigned int baud,
>  			    unsigned int quot, unsigned int quot_frac)
>  {
>  	struct uart_8250_port *up = up_to_u8250p(port);
> @@ -2603,6 +2613,15 @@ static void serial8250_set_divisor(struct uart_port *port, unsigned int baud,
>  	}
>  }
>  
> +static void serial8250_set_divisor(struct uart_port *port, unsigned int baud,
> +				   unsigned int quot, unsigned int quot_frac)
> +{
> +	if (port->set_divisor)
> +		port->set_divisor(port, baud, quot, quot_frac);
> +	else
> +		serial8250_do_set_divisor(port, baud, quot, quot_frac);
> +}
> +
>  static unsigned int serial8250_get_baud_rate(struct uart_port *port,
>  					     struct ktermios *termios,
>  					     struct ktermios *old)
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index 06ea4eeb09ab..406edae44ca3 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -127,6 +127,13 @@ struct uart_port {
>  					     struct ktermios *);
>  	unsigned int		(*get_mctrl)(struct uart_port *);
>  	void			(*set_mctrl)(struct uart_port *, unsigned int);
> +	unsigned int		(*get_divisor)(struct uart_port *,
> +					       unsigned int baud,
> +					       unsigned int *frac);
> +	void			(*set_divisor)(struct uart_port *,
> +					       unsigned int baud,
> +					       unsigned int quot,
> +					       unsigned int quot_frac);
>  	int			(*startup)(struct uart_port *port);
>  	void			(*shutdown)(struct uart_port *port);
>  	void			(*throttle)(struct uart_port *port);

[PATCH v3 2/3] serial: 8250: export serial8250_do_set_divisor()

[Jisheng Zhang]

Some drivers could call serial8250_do_set_divisor() to complete its
own set_divisor routine. Export this symbol for code reusing.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
---
 drivers/tty/serial/8250/8250_port.c | 5 +++--
 include/linux/serial_8250.h         | 3 +++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index ce0dc17f18ee..945f8dc2d50f 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2580,8 +2580,8 @@ static unsigned char serial8250_compute_lcr(struct uart_8250_port *up,
 	return cval;
 }
 
-static void serial8250_do_set_divisor(struct uart_port *port, unsigned int baud,
-			    unsigned int quot, unsigned int quot_frac)
+void serial8250_do_set_divisor(struct uart_port *port, unsigned int baud,
+			       unsigned int quot, unsigned int quot_frac)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
 
@@ -2612,6 +2612,7 @@ static void serial8250_do_set_divisor(struct uart_port *port, unsigned int baud,
 		serial_port_out(port, 0x2, quot_frac);
 	}
 }
+EXPORT_SYMBOL_GPL(serial8250_do_set_divisor);
 
 static void serial8250_set_divisor(struct uart_port *port, unsigned int baud,
 				   unsigned int quot, unsigned int quot_frac)
diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index 76b9db71e489..18e21427bce4 100644
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -160,6 +160,9 @@ extern void serial8250_do_shutdown(struct uart_port *port);
 extern void serial8250_do_pm(struct uart_port *port, unsigned int state,
 			     unsigned int oldstate);
 extern void serial8250_do_set_mctrl(struct uart_port *port, unsigned int mctrl);
+extern void serial8250_do_set_divisor(struct uart_port *port, unsigned int baud,
+				      unsigned int quot,
+				      unsigned int quot_frac);
 extern int fsl8250_handle_irq(struct uart_port *port);
 int serial8250_handle_irq(struct uart_port *port, unsigned int iir);
 unsigned char serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr);
-- 
2.18.0

[PATCH v3 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.Zhang@synaptics.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..e90c3d229f00 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 = clk / (16 * baud) = div(I) + div(F)
+ * "I" means integer, "F" means fractional
+ *
+ * 2^dlf_size * clk / (16 * baud) = 2^dlf_size * (div(I) + div(F))
+ * so, (clk << (dlf_siz - 4)) / baud = (div(I) + div(F)) << dlf_size
+ *
+ * let quot = DIV_ROUND_CLOSEST(clk << (dlf_size - 4), baud), we get
+ * div(I) = quot >> dlf_size
+ * div(F) = quot & dlf_mask, where dlf_mask = GENMASK(dlf_size - 1, 0)
+ */
+static unsigned int dw8250_get_divisor(struct uart_port *p,
+				       unsigned int baud,
+				       unsigned int *frac)
+{
+	unsigned int quot;
+	struct dw8250_data *d = p->private_data;
+
+	quot = DIV_ROUND_CLOSEST(p->uartclk << (d->dlf_size - 4), baud);
+	*frac = quot & GENMASK(d->dlf_size - 1, 0);
+
+	return quot >> d->dlf_size;
+}
+
+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

[PATCH v3 3/3] serial: 8250_dw: add fractional divisor support

[Jisheng Zhang]

Hi Andy,

On Mon, 9 Jul 2018 16:23:15 +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.
> 
> Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.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..e90c3d229f00 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 = clk / (16 * baud) = div(I) + div(F)
> + * "I" means integer, "F" means fractional
> + *
> + * 2^dlf_size * clk / (16 * baud) = 2^dlf_size * (div(I) + div(F))
> + * so, (clk << (dlf_siz - 4)) / baud = (div(I) + div(F)) << dlf_size
> + *
> + * let quot = DIV_ROUND_CLOSEST(clk << (dlf_size - 4), baud), we get
> + * div(I) = quot >> dlf_size
> + * div(F) = quot & dlf_mask, where dlf_mask = GENMASK(dlf_size - 1, 0)
> + */
> +static unsigned int dw8250_get_divisor(struct uart_port *p,
> +				       unsigned int baud,
> +				       unsigned int *frac)
> +{
> +	unsigned int quot;
> +	struct dw8250_data *d = p->private_data;
> +
> +	quot = DIV_ROUND_CLOSEST(p->uartclk << (d->dlf_size - 4), baud);
> +	*frac = quot & GENMASK(d->dlf_size - 1, 0);
> +
> +	return quot >> d->dlf_size;

After more consideration, I sent out this version for the following two
points:

1. the max frac divisor width is 6bits now, but we dunno whether future IP
extends it or not. This patch version can still support > 6bits width except
the overflow concern. But fixing overflow(if there is) is as simple as using\
the DIV_ROUND_CLOSEST_ULL macro.

2. this version makes use of well implemented GENMASK to get the dlf_mask,
the micro is well understood, I think. so the code is simplified as well.

3. the magic "4" is explained in the comments, I hope it could help the
code.

what do you think?

Thanks in advance,
Jisheng

> +}
> +
> +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;

[PATCH v3 3/3] serial: 8250_dw: add fractional divisor support

[Jisheng Zhang]

Hi Andy,

On Mon, 9 Jul 2018 16:32:41 +0800 Jisheng Zhang wrote:

> Hi Andy,
> 
> On Mon, 9 Jul 2018 16:23:15 +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.
> > 
> > Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.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..e90c3d229f00 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 = clk / (16 * baud) = div(I) + div(F)
> > + * "I" means integer, "F" means fractional
> > + *
> > + * 2^dlf_size * clk / (16 * baud) = 2^dlf_size * (div(I) + div(F))
> > + * so, (clk << (dlf_siz - 4)) / baud = (div(I) + div(F)) << dlf_size
> > + *
> > + * let quot = DIV_ROUND_CLOSEST(clk << (dlf_size - 4), baud), we get
> > + * div(I) = quot >> dlf_size
> > + * div(F) = quot & dlf_mask, where dlf_mask = GENMASK(dlf_size - 1, 0)
> > + */
> > +static unsigned int dw8250_get_divisor(struct uart_port *p,
> > +				       unsigned int baud,
> > +				       unsigned int *frac)
> > +{
> > +	unsigned int quot;
> > +	struct dw8250_data *d = p->private_data;
> > +
> > +	quot = DIV_ROUND_CLOSEST(p->uartclk << (d->dlf_size - 4), baud);
> > +	*frac = quot & GENMASK(d->dlf_size - 1, 0);
> > +
> > +	return quot >> d->dlf_size;  
> 
> After more consideration, I sent out this version for the following two
> points:
> 
> 1. the max frac divisor width is 6bits now, but we dunno whether future IP
> extends it or not. This patch version can still support > 6bits width except
> the overflow concern. But fixing overflow(if there is) is as simple as using\
> the DIV_ROUND_CLOSEST_ULL macro.
> 
> 2. this version makes use of well implemented GENMASK to get the dlf_mask,
> the micro is well understood, I think. so the code is simplified as well.
> 
> 3. the magic "4" is explained in the comments, I hope it could help the
> code.

I agree with your "making the code simple" concern. So I have a consideration
again. I think I made the code complex unnecessarily, the simplest get_divisor code
could be:

get_divisor()
{
	unsigned int quot, rem;

	quot = clk / (16 * baud);
	rem = clk % (16 *baud);
	*frac = DIV_ROUND_CLOSEST(rem << dlf_size, 16*baud);

	return quot;
}

Compared with previous version, this one adds one more div instruction,
but remove several "shift, and" instructions, the performance isn't that bad.
>From another side, even this version gets a trivial performance regression,
the get_divisor() doesn't sit on the hot code path. Making the code simpler is
more important.

I will send a new version.

Thanks