drm/amd/display: get refclk from MICROSECOND_TIME_BASE_DIV HW register

[why]
recent VBIOS dce_infotable reference clock change caused a I2c regression.
instead of relying on vbios, let's get it from HW directly.

Signed-off-by: Charlene Liu <[e-mail]>
Reviewed-by: Chris Park <[e-mail]>
Reviewed-by: Nicholas Kazlauskas <[e-mail]>
Acked-by: Bindu Ramamurthy <[e-mail]>
Tested-by: Daniel Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/dce/dce_i2c_hw.c

···
 	struct dce_i2c_hw *dce_i2c_hw,
 	uint32_t speed)
 {
-	uint32_t xtal_ref_div = 0;
+	uint32_t xtal_ref_div = 0, ref_base_div = 0;
 	uint32_t prescale = 0;
+	uint32_t i2c_ref_clock = 0;
 
 	if (speed == 0)
 		return;
 
-	REG_GET(MICROSECOND_TIME_BASE_DIV, XTAL_REF_DIV, &xtal_ref_div);
+	REG_GET_2(MICROSECOND_TIME_BASE_DIV, MICROSECOND_TIME_BASE_DIV, &ref_base_div,
+		XTAL_REF_DIV, &xtal_ref_div);
 
 	if (xtal_ref_div == 0)
 		xtal_ref_div = 2;
 
-	prescale = ((dce_i2c_hw->reference_frequency * 2) / xtal_ref_div) / speed;
+	if (ref_base_div == 0)
+		i2c_ref_clock = (dce_i2c_hw->reference_frequency * 2);
+	else
+		i2c_ref_clock = ref_base_div * 1000;
+
+	prescale = (i2c_ref_clock / xtal_ref_div) / speed;
 
 	if (dce_i2c_hw->masks->DC_I2C_DDC1_START_STOP_TIMING_CNTL)
 		REG_UPDATE_N(SPEED, 3,

drivers/gpu/drm/amd/display/dc/dce/dce_i2c_hw.h

···
 	I2C_SF(DC_I2C_DATA, DC_I2C_INDEX, mask_sh),\
 	I2C_SF(DC_I2C_DATA, DC_I2C_INDEX_WRITE, mask_sh),\
 	I2C_SF(MICROSECOND_TIME_BASE_DIV, XTAL_REF_DIV, mask_sh),\
+	I2C_SF(MICROSECOND_TIME_BASE_DIV, MICROSECOND_TIME_BASE_DIV, mask_sh),\
 	I2C_SF(DC_I2C_ARBITRATION, DC_I2C_REG_RW_CNTL_STATUS, mask_sh)
 
 #define I2C_COMMON_MASK_SH_LIST_DCE110(mask_sh)\
···
 	uint8_t DC_I2C_INDEX;
 	uint8_t DC_I2C_INDEX_WRITE;
 	uint8_t XTAL_REF_DIV;
+	uint8_t MICROSECOND_TIME_BASE_DIV;
 	uint8_t DC_I2C_DDC1_SEND_RESET_LENGTH;
 	uint8_t DC_I2C_REG_RW_CNTL_STATUS;
 	uint8_t I2C_LIGHT_SLEEP_FORCE;
···
 	uint32_t DC_I2C_INDEX;
 	uint32_t DC_I2C_INDEX_WRITE;
 	uint32_t XTAL_REF_DIV;
+	uint32_t MICROSECOND_TIME_BASE_DIV;
 	uint32_t DC_I2C_DDC1_SEND_RESET_LENGTH;
 	uint32_t DC_I2C_REG_RW_CNTL_STATUS;
 	uint32_t I2C_LIGHT_SLEEP_FORCE;