Re: [PATCH] i2c: designware: fix idx_write_cnt in read loop

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Hi

On 5/18/23 21:06, David Zheng wrote:
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@xxxxxxxxx>
---
  drivers/i2c/busses/i2c-designware-core.h  | 2 ++
  drivers/i2c/busses/i2c-designware-slave.c | 6 ++++--
  2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/i2c/busses/i2c-designware-core.h b/drivers/i2c/busses/i2c-designware-core.h
index c5d87aae39c6..8b85147bd518 100644
--- a/drivers/i2c/busses/i2c-designware-core.h
+++ b/drivers/i2c/busses/i2c-designware-core.h
@@ -123,6 +123,8 @@
  #define DW_IC_COMP_PARAM_1_SPEED_MODE_HIGH	(BIT(2) | BIT(3))
  #define DW_IC_COMP_PARAM_1_SPEED_MODE_MASK	GENMASK(3, 2)
+#define DW_IC_DATA_CMD_FIRST_DATA_BYTE BIT(11)
+
  /*
   * Sofware status flags
   */
diff --git a/drivers/i2c/busses/i2c-designware-slave.c b/drivers/i2c/busses/i2c-designware-slave.c
index cec25054bb24..9549cbcf50aa 100644
--- a/drivers/i2c/busses/i2c-designware-slave.c
+++ b/drivers/i2c/busses/i2c-designware-slave.c
@@ -170,12 +170,14 @@ static irqreturn_t i2c_dw_isr_slave(int this_irq, void *dev_id)
  		if (!(dev->status & STATUS_WRITE_IN_PROGRESS)) {
  			dev->status |= STATUS_WRITE_IN_PROGRESS;
  			dev->status &= ~STATUS_READ_IN_PROGRESS;
-			i2c_slave_event(dev->slave, I2C_SLAVE_WRITE_REQUESTED,
-					&val);
  		}
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);
I fear this might cause regression on some use case on HW that doesn't have the FIRST_DATA_BYTE bit in IC_DATA_CMD. That is available on newer Synopsys I2C IPs only. For example my test HW doesn't have it.

This means the I2C_SLAVE_WRITE_REQUESTED is never delivered on these HWs that don't implement the FIRST_DATA_BYTE.

My quick tests using i2c-slave-eeprom didn't show regression but I'm sure there is a case that will regress because of that.



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