diff mbox series

[04/12] tty: serial: samsung: Init USI to keep clocks running

Message ID 20210730144922.29111-5-semen.protsenko@linaro.org
State New
Headers show
Series Add minimal support for Exynos850 SoC | expand

Commit Message

Sam Protsenko July 30, 2021, 2:49 p.m. UTC 
UART block is a part of USI (Universal Serial Interface) IP-core in
Samsung SoCs since Exynos9810 (e.g. in Exynos850). USI allows one to
enable one of three types of serial interface: UART, SPI or I2C. That's
possible because USI shares almost all internal circuits within each
protocol. USI also provides some additional registers so it's possible
to configure it.

One USI register called USI_OPTION has reset value of 0x0. Because of
this the clock gating behavior is controlled by hardware (HWACG =
Hardware Auto Clock Gating), which simply means the serial won't work
after reset as is. In order to make it work, USI_OPTION[2:1] bits must
be set to 0b01, so that HWACG is controlled manually (by software).
Bits meaning:
  - CLKREQ_ON = 1: clock is continuously provided to IP
  - CLKSTOP_ON = 0: drive IP_CLKREQ to High (needs to be set along with
                    CLKREQ_ON = 1)

USI is not present on older chips, like s3c2410, s3c2412, s3c2440,
s3c6400, s5pv210, exynos5433, exynos4210. So the new boolean field
'.has_usi' was added to struct s3c24xx_uart_info. USI registers will be
only actually accessed when '.has_usi' field is set to "1".

This feature is needed for further serial enablement on Exynos850, but
some other new Exynos chips (like Exynos9810) may benefit from this
feature as well.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>

---
 drivers/tty/serial/samsung_tty.c | 33 +++++++++++++++++++++++++++++++-
 include/linux/serial_s3c.h       |  9 +++++++++
 2 files changed, 41 insertions(+), 1 deletion(-)

-- 
2.30.2

Comments

Krzysztof Kozlowski July 30, 2021, 4:31 p.m. UTC | #1 
On 30/07/2021 16:49, Sam Protsenko wrote:
> UART block is a part of USI (Universal Serial Interface) IP-core in
> Samsung SoCs since Exynos9810 (e.g. in Exynos850). USI allows one to
> enable one of three types of serial interface: UART, SPI or I2C. That's
> possible because USI shares almost all internal circuits within each
> protocol. USI also provides some additional registers so it's possible
> to configure it.
> 
> One USI register called USI_OPTION has reset value of 0x0. Because of
> this the clock gating behavior is controlled by hardware (HWACG =
> Hardware Auto Clock Gating), which simply means the serial won't work
> after reset as is. In order to make it work, USI_OPTION[2:1] bits must
> be set to 0b01, so that HWACG is controlled manually (by software).
> Bits meaning:
>   - CLKREQ_ON = 1: clock is continuously provided to IP
>   - CLKSTOP_ON = 0: drive IP_CLKREQ to High (needs to be set along with
>                     CLKREQ_ON = 1)
> 
> USI is not present on older chips, like s3c2410, s3c2412, s3c2440,
> s3c6400, s5pv210, exynos5433, exynos4210. So the new boolean field
> '.has_usi' was added to struct s3c24xx_uart_info. USI registers will be
> only actually accessed when '.has_usi' field is set to "1".
> 
> This feature is needed for further serial enablement on Exynos850, but
> some other new Exynos chips (like Exynos9810) may benefit from this
> feature as well.
> 
> Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
> ---
>  drivers/tty/serial/samsung_tty.c | 33 +++++++++++++++++++++++++++++++-
>  include/linux/serial_s3c.h       |  9 +++++++++
>  2 files changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
> index 9fbc61151c2e..0f3cbd0b37e3 100644
> --- a/drivers/tty/serial/samsung_tty.c
> +++ b/drivers/tty/serial/samsung_tty.c
> @@ -65,6 +65,7 @@ enum s3c24xx_port_type {
>  struct s3c24xx_uart_info {
>  	char			*name;
>  	enum s3c24xx_port_type	type;
> +	unsigned int		has_usi;
>  	unsigned int		port_type;
>  	unsigned int		fifosize;
>  	unsigned long		rx_fifomask;
> @@ -1352,6 +1353,29 @@ static int apple_s5l_serial_startup(struct uart_port *port)
>  	return ret;
>  }
>  
> +static void exynos_usi_init(struct uart_port *port)
> +{
> +	struct s3c24xx_uart_port *ourport = to_ourport(port);
> +	struct s3c24xx_uart_info *info = ourport->info;
> +
> +	if (!info->has_usi)
> +		return;
> +
> +	/*
> +	 * USI_RESET is an active High signal. Reset value of USI_RESET is 0x1
> +	 * to drive stable value to PAD. Due to this feature, the USI_RESET must
> +	 * be cleared (set as 0x0) before starting a transaction.

"before starting a transaction" suggests it is related with transaction
or something before starting it. Don't you need it simply after reset or
resume?

> +	 */
> +	wr_regl(port, USI_CON, USI_RESET);

You are clearing entire register, not only USI_RESET bitfield. Is it
really what you want?

> +	udelay(1);
> +
> +	/*
> +	 * Set the HWACG option bit in case of UART Rx mode.
> +	 * CLKREQ_ON = 1, CLKSTOP_ON = 0 (set USI_OPTION[2:1] = 0x1).
> +	 */
> +	wr_regl(port, USI_OPTION, USI_HWACG_CLKREQ_ON);
> +}
> +
>  /* power power management control */
>  
>  static void s3c24xx_serial_pm(struct uart_port *port, unsigned int level,
> @@ -1379,6 +1403,7 @@ static void s3c24xx_serial_pm(struct uart_port *port, unsigned int level,
>  		if (!IS_ERR(ourport->baudclk))
>  			clk_prepare_enable(ourport->baudclk);
>  
> +		exynos_usi_init(port);
>  		break;
>  	default:
>  		dev_err(port->dev, "s3c24xx_serial: unknown pm %d\n", level);
> @@ -2102,6 +2127,8 @@ static int s3c24xx_serial_init_port(struct s3c24xx_uart_port *ourport,
>  	if (ret)
>  		pr_warn("uart: failed to enable baudclk\n");
>  
> +	exynos_usi_init(port);
> +
>  	/* Keep all interrupts masked and cleared */
>  	switch (ourport->info->type) {
>  	case TYPE_S3C6400:
> @@ -2750,10 +2777,11 @@ static struct s3c24xx_serial_drv_data s5pv210_serial_drv_data = {
>  #endif
>  
>  #if defined(CONFIG_ARCH_EXYNOS)
> -#define EXYNOS_COMMON_SERIAL_DRV_DATA				\
> +#define EXYNOS_COMMON_SERIAL_DRV_DATA_USI(_has_usi)		\
>  	.info = &(struct s3c24xx_uart_info) {			\
>  		.name		= "Samsung Exynos UART",	\
>  		.type		= TYPE_S3C6400,			\
> +		.has_usi	= _has_usi,			\
>  		.port_type	= PORT_S3C6400,			\
>  		.has_divslot	= 1,				\
>  		.rx_fifomask	= S5PV210_UFSTAT_RXMASK,	\
> @@ -2773,6 +2801,9 @@ static struct s3c24xx_serial_drv_data s5pv210_serial_drv_data = {
>  		.has_fracval	= 1,				\
>  	}							\
>  
> +#define EXYNOS_COMMON_SERIAL_DRV_DATA				\
> +	EXYNOS_COMMON_SERIAL_DRV_DATA_USI(0)
> +
>  static struct s3c24xx_serial_drv_data exynos4210_serial_drv_data = {
>  	EXYNOS_COMMON_SERIAL_DRV_DATA,
>  	.fifosize = { 256, 64, 16, 16 },
> diff --git a/include/linux/serial_s3c.h b/include/linux/serial_s3c.h
> index f6c3323fc4c5..013c2646863e 100644
> --- a/include/linux/serial_s3c.h
> +++ b/include/linux/serial_s3c.h
> @@ -28,6 +28,15 @@
>  #define S3C2410_UFSTAT	  (0x18)
>  #define S3C2410_UMSTAT	  (0x1C)
>  
> +/* USI Control Register offset */
> +#define USI_CON			(0xC4)
> +/* USI Option Register offset */
> +#define USI_OPTION		(0xC8)
> +/* USI_CON[0] = 0b0: clear USI global software reset (Active High) */
> +#define USI_RESET		(0<<0)

Just 0x0. I understand you wanted to hint it is a bit field, but the
shift of 0 actually creates more questions.


Best regards,
Krzysztof
Sam Protsenko Aug. 2, 2021, 11:06 p.m. UTC | #2 
On Fri, 30 Jul 2021 at 19:32, Krzysztof Kozlowski
<krzysztof.kozlowski@canonical.com> wrote:
>
> On 30/07/2021 16:49, Sam Protsenko wrote:
> > UART block is a part of USI (Universal Serial Interface) IP-core in
> > Samsung SoCs since Exynos9810 (e.g. in Exynos850). USI allows one to
> > enable one of three types of serial interface: UART, SPI or I2C. That's
> > possible because USI shares almost all internal circuits within each
> > protocol. USI also provides some additional registers so it's possible
> > to configure it.
> >
> > One USI register called USI_OPTION has reset value of 0x0. Because of
> > this the clock gating behavior is controlled by hardware (HWACG =
> > Hardware Auto Clock Gating), which simply means the serial won't work
> > after reset as is. In order to make it work, USI_OPTION[2:1] bits must
> > be set to 0b01, so that HWACG is controlled manually (by software).
> > Bits meaning:
> >   - CLKREQ_ON = 1: clock is continuously provided to IP
> >   - CLKSTOP_ON = 0: drive IP_CLKREQ to High (needs to be set along with
> >                     CLKREQ_ON = 1)
> >
> > USI is not present on older chips, like s3c2410, s3c2412, s3c2440,
> > s3c6400, s5pv210, exynos5433, exynos4210. So the new boolean field
> > '.has_usi' was added to struct s3c24xx_uart_info. USI registers will be
> > only actually accessed when '.has_usi' field is set to "1".
> >
> > This feature is needed for further serial enablement on Exynos850, but
> > some other new Exynos chips (like Exynos9810) may benefit from this
> > feature as well.
> >
> > Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
> > ---
> >  drivers/tty/serial/samsung_tty.c | 33 +++++++++++++++++++++++++++++++-
> >  include/linux/serial_s3c.h       |  9 +++++++++
> >  2 files changed, 41 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
> > index 9fbc61151c2e..0f3cbd0b37e3 100644
> > --- a/drivers/tty/serial/samsung_tty.c
> > +++ b/drivers/tty/serial/samsung_tty.c
> > @@ -65,6 +65,7 @@ enum s3c24xx_port_type {
> >  struct s3c24xx_uart_info {
> >       char                    *name;
> >       enum s3c24xx_port_type  type;
> > +     unsigned int            has_usi;
> >       unsigned int            port_type;
> >       unsigned int            fifosize;
> >       unsigned long           rx_fifomask;
> > @@ -1352,6 +1353,29 @@ static int apple_s5l_serial_startup(struct uart_port *port)
> >       return ret;
> >  }
> >
> > +static void exynos_usi_init(struct uart_port *port)
> > +{
> > +     struct s3c24xx_uart_port *ourport = to_ourport(port);
> > +     struct s3c24xx_uart_info *info = ourport->info;
> > +
> > +     if (!info->has_usi)
> > +             return;
> > +
> > +     /*
> > +      * USI_RESET is an active High signal. Reset value of USI_RESET is 0x1
> > +      * to drive stable value to PAD. Due to this feature, the USI_RESET must
> > +      * be cleared (set as 0x0) before starting a transaction.
>
> "before starting a transaction" suggests it is related with transaction
> or something before starting it. Don't you need it simply after reset or
> resume?
>

Not sure what you are suggesting. USI_RESET is set to "1" at start up
(means USI block hangs in reset state), so we have to make it "0"
(that's what this code does); only then UART becomes functional and
UART transactions can be performed. And exynos_usi_init() is called
exactly where you hinted: at init and on resume.

Anyway, the whole comment is confusing, I'll simplify and rework it in
v2. Please let me know if I'm missing the point though.

> > +      */
> > +     wr_regl(port, USI_CON, USI_RESET);
>
> You are clearing entire register, not only USI_RESET bitfield. Is it
> really what you want?
>

Yeah, USI_CON[31:1] bits are reserved, and the reset value of this
register is 0x00000001. But anyway, I'm going to rework that code like
this, for clarity and consistence:

8<--------------------------------------------------------------------->8
    /* Clear the software reset of USI block (it's set at startup) */
    val = rd_regl(port, USI_CON);
    val &= ~(USI_RESET_MASK)
    wr_regl(port, USI_CON, val);
    udelay(1);

    /* Continuously provide the clock to USI IP w/o gating (for Rx mode) */
    val = rd_regl(port, USI_OPTION);
    val &= ~USI_HWACG_MASK;
    val |= USI_HWACG_CLKREQ_ON;
    wr_regl(port, USI_OPTION, val);
8<--------------------------------------------------------------------->8

> > +     udelay(1);
> > +
> > +     /*
> > +      * Set the HWACG option bit in case of UART Rx mode.
> > +      * CLKREQ_ON = 1, CLKSTOP_ON = 0 (set USI_OPTION[2:1] = 0x1).
> > +      */
> > +     wr_regl(port, USI_OPTION, USI_HWACG_CLKREQ_ON);
> > +}
> > +
> >  /* power power management control */
> >
> >  static void s3c24xx_serial_pm(struct uart_port *port, unsigned int level,
> > @@ -1379,6 +1403,7 @@ static void s3c24xx_serial_pm(struct uart_port *port, unsigned int level,
> >               if (!IS_ERR(ourport->baudclk))
> >                       clk_prepare_enable(ourport->baudclk);
> >
> > +             exynos_usi_init(port);
> >               break;
> >       default:
> >               dev_err(port->dev, "s3c24xx_serial: unknown pm %d\n", level);
> > @@ -2102,6 +2127,8 @@ static int s3c24xx_serial_init_port(struct s3c24xx_uart_port *ourport,
> >       if (ret)
> >               pr_warn("uart: failed to enable baudclk\n");
> >
> > +     exynos_usi_init(port);
> > +
> >       /* Keep all interrupts masked and cleared */
> >       switch (ourport->info->type) {
> >       case TYPE_S3C6400:
> > @@ -2750,10 +2777,11 @@ static struct s3c24xx_serial_drv_data s5pv210_serial_drv_data = {
> >  #endif
> >
> >  #if defined(CONFIG_ARCH_EXYNOS)
> > -#define EXYNOS_COMMON_SERIAL_DRV_DATA                                \
> > +#define EXYNOS_COMMON_SERIAL_DRV_DATA_USI(_has_usi)          \
> >       .info = &(struct s3c24xx_uart_info) {                   \
> >               .name           = "Samsung Exynos UART",        \
> >               .type           = TYPE_S3C6400,                 \
> > +             .has_usi        = _has_usi,                     \
> >               .port_type      = PORT_S3C6400,                 \
> >               .has_divslot    = 1,                            \
> >               .rx_fifomask    = S5PV210_UFSTAT_RXMASK,        \
> > @@ -2773,6 +2801,9 @@ static struct s3c24xx_serial_drv_data s5pv210_serial_drv_data = {
> >               .has_fracval    = 1,                            \
> >       }                                                       \
> >
> > +#define EXYNOS_COMMON_SERIAL_DRV_DATA                                \
> > +     EXYNOS_COMMON_SERIAL_DRV_DATA_USI(0)
> > +
> >  static struct s3c24xx_serial_drv_data exynos4210_serial_drv_data = {
> >       EXYNOS_COMMON_SERIAL_DRV_DATA,
> >       .fifosize = { 256, 64, 16, 16 },
> > diff --git a/include/linux/serial_s3c.h b/include/linux/serial_s3c.h
> > index f6c3323fc4c5..013c2646863e 100644
> > --- a/include/linux/serial_s3c.h
> > +++ b/include/linux/serial_s3c.h
> > @@ -28,6 +28,15 @@
> >  #define S3C2410_UFSTAT         (0x18)
> >  #define S3C2410_UMSTAT         (0x1C)
> >
> > +/* USI Control Register offset */
> > +#define USI_CON                      (0xC4)
> > +/* USI Option Register offset */
> > +#define USI_OPTION           (0xC8)
> > +/* USI_CON[0] = 0b0: clear USI global software reset (Active High) */
> > +#define USI_RESET            (0<<0)
>
> Just 0x0. I understand you wanted to hint it is a bit field, but the
> shift of 0 actually creates more questions.
>

After some consideration I decided to adhere to existing style and do
something like this (in v2):

8<--------------------------------------------------------------------->8
#define USI_CON          (0xC4)
#define USI_OPTION      (0xC8)

#define USI_CON_RESET_CLEAR        (0<<0)
#define USI_CON_RESET_SET        (1<<0)
#define USI_CON_RESET_MASK        (1<<0)

#define USI_OPTION_HWACG_CLKREQ_ON    (1<<1)
#define USI_OPTION_HWACG_CLKSTOP_ON    (1<<2)
#define USI_OPTION_HWACG_MASK        (3<<1)
8<--------------------------------------------------------------------->8

The whole reason for those comments was missing public TRM. But in the
end I decided it just looks ugly. Also, this way I can do RMW
operation (discussed above) in more logical way.

Please let me know if code snippets above look good to you.

>
> Best regards,
> Krzysztof
Krzysztof Kozlowski Aug. 3, 2021, 7:37 a.m. UTC | #3 
On 03/08/2021 01:06, Sam Protsenko wrote:

(...)

>>> diff --git a/include/linux/serial_s3c.h b/include/linux/serial_s3c.h
>>> index f6c3323fc4c5..013c2646863e 100644
>>> --- a/include/linux/serial_s3c.h
>>> +++ b/include/linux/serial_s3c.h
>>> @@ -28,6 +28,15 @@
>>>  #define S3C2410_UFSTAT         (0x18)
>>>  #define S3C2410_UMSTAT         (0x1C)
>>>
>>> +/* USI Control Register offset */
>>> +#define USI_CON                      (0xC4)
>>> +/* USI Option Register offset */
>>> +#define USI_OPTION           (0xC8)
>>> +/* USI_CON[0] = 0b0: clear USI global software reset (Active High) */
>>> +#define USI_RESET            (0<<0)
>>
>> Just 0x0. I understand you wanted to hint it is a bit field, but the
>> shift of 0 actually creates more questions.
>>
> 
> After some consideration I decided to adhere to existing style and do
> something like this (in v2):
> 
> 8<--------------------------------------------------------------------->8
> #define USI_CON          (0xC4)
> #define USI_OPTION      (0xC8)
> 
> #define USI_CON_RESET_CLEAR        (0<<0)
> #define USI_CON_RESET_SET        (1<<0)
> #define USI_CON_RESET_MASK        (1<<0)
> 
> #define USI_OPTION_HWACG_CLKREQ_ON    (1<<1)
> #define USI_OPTION_HWACG_CLKSTOP_ON    (1<<2)
> #define USI_OPTION_HWACG_MASK        (3<<1)
> 8<--------------------------------------------------------------------->8
> 
> The whole reason for those comments was missing public TRM. But in the
> end I decided it just looks ugly. Also, this way I can do RMW
> operation (discussed above) in more logical way.
> 
> Please let me know if code snippets above look good to you.

Please skip the USI_CON_RESET_CLEAR. There is no such pattern in the
code. Clearing bit is an obvious operation and such code is already
everywhere:
    val &= ~USI_CON_RESET

(or &= ~USI_RESET_MASK)

Therefore for USI_CON_RESET only:
    #define USI_CON_RESET             (1<<0)
    #define USI_CON_RESET_MASK        (1<<0)


Best regards,
Krzysztof
Sam Protsenko Aug. 3, 2021, 11:41 a.m. UTC | #4 
On Tue, 3 Aug 2021 at 10:37, Krzysztof Kozlowski
<krzysztof.kozlowski@canonical.com> wrote:
>
> On 03/08/2021 01:06, Sam Protsenko wrote:
>
> (...)
>
> >>> diff --git a/include/linux/serial_s3c.h b/include/linux/serial_s3c.h
> >>> index f6c3323fc4c5..013c2646863e 100644
> >>> --- a/include/linux/serial_s3c.h
> >>> +++ b/include/linux/serial_s3c.h
> >>> @@ -28,6 +28,15 @@
> >>>  #define S3C2410_UFSTAT         (0x18)
> >>>  #define S3C2410_UMSTAT         (0x1C)
> >>>
> >>> +/* USI Control Register offset */
> >>> +#define USI_CON                      (0xC4)
> >>> +/* USI Option Register offset */
> >>> +#define USI_OPTION           (0xC8)
> >>> +/* USI_CON[0] = 0b0: clear USI global software reset (Active High) */
> >>> +#define USI_RESET            (0<<0)
> >>
> >> Just 0x0. I understand you wanted to hint it is a bit field, but the
> >> shift of 0 actually creates more questions.
> >>
> >
> > After some consideration I decided to adhere to existing style and do
> > something like this (in v2):
> >
> > 8<--------------------------------------------------------------------->8
> > #define USI_CON          (0xC4)
> > #define USI_OPTION      (0xC8)
> >
> > #define USI_CON_RESET_CLEAR        (0<<0)
> > #define USI_CON_RESET_SET        (1<<0)
> > #define USI_CON_RESET_MASK        (1<<0)
> >
> > #define USI_OPTION_HWACG_CLKREQ_ON    (1<<1)
> > #define USI_OPTION_HWACG_CLKSTOP_ON    (1<<2)
> > #define USI_OPTION_HWACG_MASK        (3<<1)
> > 8<--------------------------------------------------------------------->8
> >
> > The whole reason for those comments was missing public TRM. But in the
> > end I decided it just looks ugly. Also, this way I can do RMW
> > operation (discussed above) in more logical way.
> >
> > Please let me know if code snippets above look good to you.
>
> Please skip the USI_CON_RESET_CLEAR. There is no such pattern in the
> code. Clearing bit is an obvious operation and such code is already
> everywhere:
>     val &= ~USI_CON_RESET
>
> (or &= ~USI_RESET_MASK)
>
> Therefore for USI_CON_RESET only:
>     #define USI_CON_RESET             (1<<0)
>     #define USI_CON_RESET_MASK        (1<<0)
>

Sure, I'll make it so.

>
> Best regards,
> Krzysztof
diff mbox series

Patch

diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
index 9fbc61151c2e..0f3cbd0b37e3 100644
--- a/drivers/tty/serial/samsung_tty.c
+++ b/drivers/tty/serial/samsung_tty.c
@@ -65,6 +65,7 @@  enum s3c24xx_port_type {
 struct s3c24xx_uart_info {
 	char			*name;
 	enum s3c24xx_port_type	type;
+	unsigned int		has_usi;
 	unsigned int		port_type;
 	unsigned int		fifosize;
 	unsigned long		rx_fifomask;
@@ -1352,6 +1353,29 @@  static int apple_s5l_serial_startup(struct uart_port *port)
 	return ret;
 }
 
+static void exynos_usi_init(struct uart_port *port)
+{
+	struct s3c24xx_uart_port *ourport = to_ourport(port);
+	struct s3c24xx_uart_info *info = ourport->info;
+
+	if (!info->has_usi)
+		return;
+
+	/*
+	 * USI_RESET is an active High signal. Reset value of USI_RESET is 0x1
+	 * to drive stable value to PAD. Due to this feature, the USI_RESET must
+	 * be cleared (set as 0x0) before starting a transaction.
+	 */
+	wr_regl(port, USI_CON, USI_RESET);
+	udelay(1);
+
+	/*
+	 * Set the HWACG option bit in case of UART Rx mode.
+	 * CLKREQ_ON = 1, CLKSTOP_ON = 0 (set USI_OPTION[2:1] = 0x1).
+	 */
+	wr_regl(port, USI_OPTION, USI_HWACG_CLKREQ_ON);
+}
+
 /* power power management control */
 
 static void s3c24xx_serial_pm(struct uart_port *port, unsigned int level,
@@ -1379,6 +1403,7 @@  static void s3c24xx_serial_pm(struct uart_port *port, unsigned int level,
 		if (!IS_ERR(ourport->baudclk))
 			clk_prepare_enable(ourport->baudclk);
 
+		exynos_usi_init(port);
 		break;
 	default:
 		dev_err(port->dev, "s3c24xx_serial: unknown pm %d\n", level);
@@ -2102,6 +2127,8 @@  static int s3c24xx_serial_init_port(struct s3c24xx_uart_port *ourport,
 	if (ret)
 		pr_warn("uart: failed to enable baudclk\n");
 
+	exynos_usi_init(port);
+
 	/* Keep all interrupts masked and cleared */
 	switch (ourport->info->type) {
 	case TYPE_S3C6400:
@@ -2750,10 +2777,11 @@  static struct s3c24xx_serial_drv_data s5pv210_serial_drv_data = {
 #endif
 
 #if defined(CONFIG_ARCH_EXYNOS)
-#define EXYNOS_COMMON_SERIAL_DRV_DATA				\
+#define EXYNOS_COMMON_SERIAL_DRV_DATA_USI(_has_usi)		\
 	.info = &(struct s3c24xx_uart_info) {			\
 		.name		= "Samsung Exynos UART",	\
 		.type		= TYPE_S3C6400,			\
+		.has_usi	= _has_usi,			\
 		.port_type	= PORT_S3C6400,			\
 		.has_divslot	= 1,				\
 		.rx_fifomask	= S5PV210_UFSTAT_RXMASK,	\
@@ -2773,6 +2801,9 @@  static struct s3c24xx_serial_drv_data s5pv210_serial_drv_data = {
 		.has_fracval	= 1,				\
 	}							\
 
+#define EXYNOS_COMMON_SERIAL_DRV_DATA				\
+	EXYNOS_COMMON_SERIAL_DRV_DATA_USI(0)
+
 static struct s3c24xx_serial_drv_data exynos4210_serial_drv_data = {
 	EXYNOS_COMMON_SERIAL_DRV_DATA,
 	.fifosize = { 256, 64, 16, 16 },
diff --git a/include/linux/serial_s3c.h b/include/linux/serial_s3c.h
index f6c3323fc4c5..013c2646863e 100644
--- a/include/linux/serial_s3c.h
+++ b/include/linux/serial_s3c.h
@@ -28,6 +28,15 @@ 
 #define S3C2410_UFSTAT	  (0x18)
 #define S3C2410_UMSTAT	  (0x1C)
 
+/* USI Control Register offset */
+#define USI_CON			(0xC4)
+/* USI Option Register offset */
+#define USI_OPTION		(0xC8)
+/* USI_CON[0] = 0b0: clear USI global software reset (Active High) */
+#define USI_RESET		(0<<0)
+/* USI_OPTION[2:1] = 0b01: continuously provide the clock to IP w/o gating */
+#define USI_HWACG_CLKREQ_ON	(1<<1)
+
 #define S3C2410_LCON_CFGMASK	  ((0xF<<3)|(0x3))
 
 #define S3C2410_LCON_CS5	  (0x0)