Writes from I2C bus often fail when testing the i2c-designware-slave.c
with the slave-eeprom backend. The same writes work correctly when
testing with a real 24c02 EEPROM chip.

In the tests below an i2c-designware-slave.c instance with the
slave-eeprom backend is configured to act as a simulated 24c02 at
address 0x65 on an I2C host bus 6.

1. i2cset -y 6 0x65 0x00 0x55

Single byte 0x55 write into address 0x00. No data goes into simulated
EEPROM. Debug prints from the i2c_dw_irq_handler_slave():

0x1 STATUS SLAVE_ACTIVITY=0x0 : RAW_INTR_STAT=0x714 : INTR_STAT=0x204
0x1 STATUS SLAVE_ACTIVITY=0x0 : RAW_INTR_STAT=0x514 : INTR_STAT=0x4

2. i2ctransfer -y 6 w9@0x65 0x00 0xff-

Write 8 bytes with decrementing value starting from 0xff at address 0x00
and forward. Only some of the data goes into arbitrary addresses.
Content is something like below but varies:

00000000  f9 f8 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000050  00 00 00 00 00 00 ff fe  00 00 00 00 00 00 00 00  |................|
000000f0  00 00 00 00 00 00 00 00  00 00 00 00 00 fc fb fa  |................|

In this case debug prints were:

0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x514 : INTR_STAT=0x4
0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x514 : INTR_STAT=0x4
0x1 STATUS SLAVE_ACTIVITY=0x0 : RAW_INTR_STAT=0x714 : INTR_STAT=0x204
0x1 STATUS SLAVE_ACTIVITY=0x0 : RAW_INTR_STAT=0x514 : INTR_STAT=0x4
0x1 STATUS SLAVE_ACTIVITY=0x0 : RAW_INTR_STAT=0x514 : INTR_STAT=0x4
0x1 STATUS SLAVE_ACTIVITY=0x0 : RAW_INTR_STAT=0x514 : INTR_STAT=0x4
0x1 STATUS SLAVE_ACTIVITY=0x0 : RAW_INTR_STAT=0x514 : INTR_STAT=0x4
0x1 STATUS SLAVE_ACTIVITY=0x0 : RAW_INTR_STAT=0x514 : INTR_STAT=0x4
0x1 STATUS SLAVE_ACTIVITY=0x0 : RAW_INTR_STAT=0x514 : INTR_STAT=0x4
0x1 STATUS SLAVE_ACTIVITY=0x0 : RAW_INTR_STAT=0x510 : INTR_STAT=0x0

Both cases show there is more data coming from the receive FIFO still
after detecting the STOP condition. This can be seen from interrupt
status bits DW_IC_INTR_STOP_DET (0x200) and DW_IC_INTR_RX_FULL (0x4).

Perhaps due interrupt latencies the receive FIFO is not read fast
enough, STOP detection happens synchronously when it occurs on the I2C
bus and the DW_IC_INTR_RX_FULL keeps coming as long as there are more
bytes in the receive FIFO.

Fix this by reading the receive FIFO completely empty whenever
DW_IC_INTR_RX_FULL occurs. Use RFNE, Receive FIFO Not Empty bit in the
DW_IC_STATUS register to loop through bytes in the FIFO.

While at it do not test the return code from i2c_slave_event() for the
I2C_SLAVE_WRITE_RECEIVED since to my understanding this hardware cannot
generate NACK to incoming bytes and debug print itself does not have
much value.
Reported-by: Tian Ye <tianye@sugon.com>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>