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

[Sam Protsenko]

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@xxxxxxxxxx>
---
 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.
+	 */
+	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)
-- 
2.30.2