* EXYNOS - USI(Universal Serial Interface) devicetree making guide

The USI can operate as UART, HSI2C and SPI.
So the configuration for one specific function
should be needed with SYSREG(System Registers).

To configure USI port as one specific function,
function string(ex, "spi" or "hsi2c0" or "hsi2c1" or "spi" or "uart" or
"hsi2c0_hsi2c1" or "uart_hsi2c1")
should be declared in "usi_mode" property in board devicetree file.

Becuase USI configuration can differ from board schematic.

* Required SoC Specific Properties:

 - compatible: should be one of the following.
   - samsung,exynos-usi: for exynos7, exynos8 platforms.

 - reg: physical base address of the SYSREG for specific USI port
	and length of memory mapped region.

* Required Board Specific Properties:
 - usi_mode: function string for one specific IP operation.
             hsi2c0: for HSI2C0
             hsi2c1: for HSI2C1
             spi: for SPI
             uart: for UART
             hsi2c0_hsi2c1: for HSI2C0 and HSI2C1
             uart_hsi2c1: for UART(without hardware flow control) and HSI2C1

   In case of HSI2C function of USI,
   hsi2c0 and hsi2c1 should be declared in according to board schemetic.
   If shemetic describes USI pin as "XUSI##_SDA0" and XUSI##_SCL0",
   we need to use "hsi2c0" for usi_mode.
   If shemetic describes USI pin as "XUSI##_SDA1" and XUSI##_SCL1",
   we need to use "hsi2c1" for usi_mode.
   If shemetic describes USI pin as "XUSI##_SDA0" and XUSI##_SCL0",
   and there is also "XUSI##_SDA1" and XUSI##_SCL1",
   we need to use "hsi2c0_hsi2c1" for usi_mode.

* Exsamples:

 - SoC Specific Portion

	/* USI_0 */
	usi_0: usi@10421000 {
		       compatible = "samsung,exynos-usi";
		       reg = <0x0 0x10421000 0x4>;
		       /* usi_mode = "hsi2c0" or "hsi2c1" or "spi" or "uart"
			  or "hsi2c0_hsi2c1" or "uart_hsi2c1"    */
		       status = "disabled";
       };

	/* USI_1 */
	usi_1: usi@10421004 {
		       compatible = "samsung,exynos-usi";
		       reg = <0x0 0x10421004 0x4>;
		       /* usi_mode = "hsi2c0" or "hsi2c1" or "spi" or "uart"
			  or "hsi2c0_hsi2c1" or "uart_hsi2c1"    */
		       status = "disabled";
       };

	/* USI_2 */
	usi_2: usi@10421008 {
		       compatible = "samsung,exynos-usi";
		       reg = <0x0 0x10421008 0x4>;
		       /* usi_mode = "hsi2c0" or "hsi2c1" or "spi" or "uart"
			  or "hsi2c0_hsi2c1" or "uart_hsi2c1"    */
		       status = "disabled";
       };

 - Board Specific Portion

	/* USI MODE SETTINGS

	   usi_mode = "hsi2c0" or "hsi2c1" or "spi" or "uart"
			or "hsi2c0_hsi2c1" or "uart_hsi2c1"
	*/

	usi_0: usi@10421000 {
		 usi_mode = "spi";
		 status = "okay";
	 };

	usi_1: usi@10421004 {
		       usi_mode = "spi";
		       status = "okay";
	};

	usi_2: usi@10421008 {
		       usi_mode = "hsi2c0";
		       status = "okay";
	};


If USI configuration was done successfully, the booting log will be shown as below.

[    0.700485]  [1:      swapper/0:    1] usi 10421000.usi: usi_probe() mode:4
[    0.702369]  [1:      swapper/0:    1] usi 10421004.usi: usi_probe() mode:4
[    0.704134]  [1:      swapper/0:    1] usi 10421008.usi: usi_probe() mode:1

This means the usi_0 was set as "spi" and usi_1 was set as "spi" and
usi_2 was set as "hsi2c0".

The mode values are as below.

/* USI mode */
#define USI_HSI2C0_SINGLE_MODE          0x1
#define USI_HSI2C1_SINGLE_MODE          0x2
#define USI_HSI2C0_HSI2C1_DUAL_MODE     0x3
#define USI_SPI_SINGLE_MODE             0x4
#define USI_UART_SINGLE_MODE            0x8
#define USI_UART_HSI2C1_DUAL_MODE       0xA