serial: 8250_dw: add fractional divisor support
Subsystems (4)
v3 cover 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.
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(-)
Signature · 1 line
2.18.0
serial: 8250: introduce get_divisor() and set_divisor() hook
Add these two hooks so that they can be overridden with driver specific implementations.
--- 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(-)
drivers/tty/serial/8250/8250_core.c +4 -0
@@ -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)
drivers/tty/serial/8250/8250_port.c +23 -4
@@ -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)
include/linux/serial_core.h +7 -0
@@ -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);
Signature · 1 line
2.18.0
Re: serial: 8250: introduce get_divisor() and set_divisor() hook
oops, sorry, please ignore this patch.
Quoted 103 lines · On Mon, 9 Jul 2018 16:19:22 +0800 Jisheng Zhang <[email protected]>…
Add these two hooks so that they can be overridden with driver specific implementations.
Signed-off-by Jisheng Zhang <[email protected]>Reviewed-by Andy Shevchenko <[email protected]>--- 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(-)drivers/tty/serial/8250/8250_core.c +4
-0@@ -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)drivers/tty/serial/8250/8250_port.c +23
-4@@ -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)include/linux/serial_core.h +7
-0@@ -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 1/3 serial: 8250: introduce get_divisor() and set_divisor() hook
In reply to Jisheng ZhangAdd these two hooks so that they can be overridden with driver specific implementations.
--- 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(-)
drivers/tty/serial/8250/8250_core.c +4 -0
@@ -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)
drivers/tty/serial/8250/8250_port.c +23 -4
@@ -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)
include/linux/serial_core.h +7 -0
@@ -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);
Signature · 1 line
2.18.0
PATCH v3 2/3 serial: 8250: export serial8250_do_set_divisor()
In reply to Jisheng ZhangSome drivers could call serial8250_do_set_divisor() to complete its own set_divisor routine. Export this symbol for code reusing.
--- drivers/tty/serial/8250/8250_port.c | 5 +++-- include/linux/serial_8250.h | 3 +++ 2 files changed, 6 insertions(+), 2 deletions(-)
drivers/tty/serial/8250/8250_port.c +3 -2
@@ -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)
include/linux/serial_8250.h +3 -0
@@ -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);
Signature · 1 line
2.18.0
PATCH v3 3/3 serial: 8250_dw: add fractional divisor support
In reply to Jisheng ZhangFor 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.
--- drivers/tty/serial/8250/8250_dw.c | 45 +++++++++++++++++++++++++++++++ 1 file changed, 45 insertions(+)
drivers/tty/serial/8250/8250_dw.c +45 -0
@@ -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 ;
Signature · 1 line
2.18.0
Hi Andy,
Quoted 49 lines · 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()andset_divisor()hook.Signed-off-by Jisheng Zhang <[email protected]>--- drivers/tty/serial/8250/8250_dw.c | 45 +++++++++++++++++++++++++++++++ 1 file changed, 45 insertions(+)drivers/tty/serial/8250/8250_dw.c +14
-0@@ -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,
drivers/tty/serial/8250/8250_dw.c static bool dw8250_idma_filter(struct dma_chan *chan, void *param) · line 392
Quoted 10 lines
drivers/tty/serial/8250/8250_dw.c +10
-0+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
Quoted 31 lines
drivers/tty/serial/8250/8250_dw.c +21
-0+} + +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 ;
Hi Andy,
Quoted 72 lines · On Mon, 9 Jul 2018 16:32:41 +0800 Jisheng Zhang wrote:
Hi Andy,
Quoted 49 lines · 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()andset_divisor()hook.Signed-off-by Jisheng Zhang <[email protected]>--- drivers/tty/serial/8250/8250_dw.c | 45 +++++++++++++++++++++++++++++++ 1 file changed, 45 insertions(+)drivers/tty/serial/8250/8250_dw.c +14
-0@@ -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,drivers/tty/serial/8250/8250_dw.c static bool dw8250_idma_filter(struct dma_chan *chan, void *param) · line 392
Quoted 10 lines
drivers/tty/serial/8250/8250_dw.c +10
-0+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\
theDIV_ROUND_CLOSEST_ULLmacro.
Quoted 5 lines
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.
Quoted 1 line
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