i2c: omap: fix bus recovery setup

At least on the AM335x, enabling OMAP_I2C_SYSTEST_ST_EN is not enough to
allow direct access to the SCL and SDA pins. In addition to ST_EN, we
need to set the TMODE to 0b11 (Loop back & SDA/SCL IO mode select).
Also, as the reset values of SCL_O and SDA_O are 0 (which means "drive
low level"), we need to set them to 1 (which means "high-impedance") to
avoid unwanted changes on the pins.

As a precaution, reset all these bits to their default values after
recovery is complete.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Tested-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/busses/i2c-omap.c

@@ -1247,7 +1247,14 @@ static void omap_i2c_prepare_recovery(struct i2c_adapter *adap)
 	u32 reg;
 
 	reg = omap_i2c_read_reg(dev, OMAP_I2C_SYSTEST_REG);
+	/* enable test mode */
 	reg |= OMAP_I2C_SYSTEST_ST_EN;
+	/* select SDA/SCL IO mode */
+	reg |= 3 << OMAP_I2C_SYSTEST_TMODE_SHIFT;
+	/* set SCL to high-impedance state (reset value is 0) */
+	reg |= OMAP_I2C_SYSTEST_SCL_O;
+	/* set SDA to high-impedance state (reset value is 0) */
+	reg |= OMAP_I2C_SYSTEST_SDA_O;
 	omap_i2c_write_reg(dev, OMAP_I2C_SYSTEST_REG, reg);
 }
 
@@ -1257,7 +1264,11 @@ static void omap_i2c_unprepare_recovery(struct i2c_adapter *adap)
 	u32 reg;
 
 	reg = omap_i2c_read_reg(dev, OMAP_I2C_SYSTEST_REG);
+	/* restore reset values */
 	reg &= ~OMAP_I2C_SYSTEST_ST_EN;
+	reg &= ~OMAP_I2C_SYSTEST_TMODE_MASK;
+	reg &= ~OMAP_I2C_SYSTEST_SCL_O;
+	reg &= ~OMAP_I2C_SYSTEST_SDA_O;
 	omap_i2c_write_reg(dev, OMAP_I2C_SYSTEST_REG, reg);
 }