i2c: designware: fix idx_write_cnt in read loop

With IC_INTR_RX_FULL slave interrupt handler reads data in a loop until
RX FIFO is empty. When testing with the slave-eeprom, each transaction
has 2 bytes for address/index and 1 byte for value, the address byte
can be written as data byte due to dropping STOP condition.

In the test below, the master continuously writes to the slave, first 2
bytes are index, 3rd byte is value and follow by a STOP condition.

 i2c_write: i2c-3 #0 a=04b f=0000 l=3 [00-D1-D1]
 i2c_write: i2c-3 #0 a=04b f=0000 l=3 [00-D2-D2]
 i2c_write: i2c-3 #0 a=04b f=0000 l=3 [00-D3-D3]

Upon receiving STOP condition slave eeprom would reset `idx_write_cnt` so
next 2 bytes can be treated as buffer index for upcoming transaction.
Supposedly the slave eeprom buffer would be written as

 EEPROM[0x00D1] = 0xD1
 EEPROM[0x00D2] = 0xD2
 EEPROM[0x00D3] = 0xD3

When CPU load is high the slave irq handler may not read fast enough,
the interrupt status can be seen as 0x204 with both DW_IC_INTR_STOP_DET
(0x200) and DW_IC_INTR_RX_FULL (0x4) bits. The slave device may see
the transactions below.

 0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1594 : INTR_STAT=0x4
 0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1594 : INTR_STAT=0x4
 0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1594 : INTR_STAT=0x4
 0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1794 : INTR_STAT=0x204
 0x1 STATUS SLAVE_ACTIVITY=0x0 : RAW_INTR_STAT=0x1790 : INTR_STAT=0x200
 0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1594 : INTR_STAT=0x4
 0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1594 : INTR_STAT=0x4
 0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1594 : INTR_STAT=0x4

After `D1` is received, read loop continues to read `00` which is the
first bype of next index. Since STOP condition is ignored by the loop,
eeprom buffer index increased to `D2` and `00` is written as value.

So the slave eeprom buffer becomes

 EEPROM[0x00D1] = 0xD1
 EEPROM[0x00D2] = 0x00
 EEPROM[0x00D3] = 0xD3

The fix is to use `FIRST_DATA_BYTE` (bit 11) in `IC_DATA_CMD` to split
the transactions. The first index byte in this case would have bit 11
set. Check this indication to inject I2C_SLAVE_WRITE_REQUESTED event
which will reset `idx_write_cnt` in slave eeprom.
Signed-off-by: David Zheng <david.zheng@intel.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/busses/i2c-designware-core.h

@@ -40,6 +40,7 @@
 #define DW_IC_CON_BUS_CLEAR_CTRL		BIT(11)
 
 #define DW_IC_DATA_CMD_DAT			GENMASK(7, 0)
+#define DW_IC_DATA_CMD_FIRST_DATA_BYTE		BIT(11)
 
 /*
  * Registers offset

drivers/i2c/busses/i2c-designware-slave.c

@@ -176,6 +176,10 @@ static irqreturn_t i2c_dw_isr_slave(int this_irq, void *dev_id)
 
 		do {
 			regmap_read(dev->map, DW_IC_DATA_CMD, &tmp);
+			if (tmp & DW_IC_DATA_CMD_FIRST_DATA_BYTE)
+				i2c_slave_event(dev->slave,
+						I2C_SLAVE_WRITE_REQUESTED,
+						&val);
 			val = tmp;
 			i2c_slave_event(dev->slave, I2C_SLAVE_WRITE_RECEIVED,
 					&val);