i2c: testunit: on errors, repeat NACK until STOP

This backend requests a NACK from the controller driver when it detects
an error. If that request gets ignored from some reason, subsequent
accesses will wrongly be handled OK. To fix this, an error now changes
the state machine, so the backend will report NACK until a STOP
condition has been detected. This make the driver more robust against
controllers which will sadly apply the NACK not to the current byte but
the next one.

Fixes: a8335c64c5f0 ("i2c: add slave testunit driver")
Signed-off-by: Wolfram Sang <[e-mail]>

drivers/i2c/i2c-slave-testunit.c

···
 
 enum testunit_flags {
 	TU_FLAG_IN_PROCESS,
+	TU_FLAG_NACK,
 };
 
 struct testunit_data {
···
 
 	switch (event) {
 	case I2C_SLAVE_WRITE_REQUESTED:
-		if (test_bit(TU_FLAG_IN_PROCESS, &tu->flags))
-			return -EBUSY;
+		if (test_bit(TU_FLAG_IN_PROCESS | TU_FLAG_NACK, &tu->flags)) {
+			ret = -EBUSY;
+			break;
+		}
 
 		memset(tu->regs, 0, TU_NUM_REGS);
 		tu->reg_idx = 0;
···
 		break;
 
 	case I2C_SLAVE_WRITE_RECEIVED:
-		if (test_bit(TU_FLAG_IN_PROCESS, &tu->flags))
-			return -EBUSY;
+		if (test_bit(TU_FLAG_IN_PROCESS | TU_FLAG_NACK, &tu->flags)) {
+			ret = -EBUSY;
+			break;
+		}
 
 		if (tu->reg_idx < TU_NUM_REGS)
 			tu->regs[tu->reg_idx] = *val;
···
 		 * here because we still need them in the workqueue!
 		 */
 		tu->reg_idx = 0;
+
+		clear_bit(TU_FLAG_NACK, &tu->flags);
 		break;
 
 	case I2C_SLAVE_READ_PROCESSED:
···
 					tu->regs[TU_REG_CMD] : 0;
 		break;
 	}
+
+	/* If an error occurred somewhen, we NACK everything until next STOP */
+	if (ret)
+		set_bit(TU_FLAG_NACK, &tu->flags);
 
 	return ret;
 }