i2c: gpio: support write-only sda

index fc90293af..ab5a73e90 100644
@@ -184,8 +184,9 @@ static int i2c_outb(struct i2c_adapter *i2c_adap, unsigned char c)
 
 	/* read ack: SDA should be pulled down by slave, or it may
 	 * NAK (usually to report problems with the data we wrote).
+	 * Report ACK if SDA is write-only.
 	 */
-	ack = !getsda(adap);    /* ack: sda is pulled low -> success */
+	ack = !adap->getsda || !getsda(adap);    /* ack: sda is pulled low -> success */
 	bit_dbg(2, &i2c_adap->dev, "i2c_outb: 0x%02x %s\n", (int)c,
 		ack ? "A" : "NA");
 

@@ -203,6 +204,9 @@ static int i2c_inb(struct i2c_adapter *i2c_adap)
 	unsigned char indata = 0;
 	struct i2c_algo_bit_data *adap = i2c_adap->algo_data;
 
+	if (!adap->getsda)
+		return -EOPNOTSUPP;
+
 	/* assert: scl is low */
 	sdahi(adap);
 	for (i = 0; i < 8; i++) {

@@ -232,6 +236,10 @@ static int test_bus(struct i2c_adapter *i2c_adap)
 	const char *name = i2c_adap->name;
 	int scl, sda, ret;
 
+	/* Testing not possible if both pins are write-only. */
+	if (adap->getscl == NULL && adap->getsda == NULL)
+		return 0;
+
 	if (adap->pre_xfer) {
 		ret = adap->pre_xfer(i2c_adap);
 		if (ret < 0)

@@ -670,8 +678,10 @@ static int __i2c_bit_add_bus(struct i2c_adapter *adap,
 	if (ret < 0)
 		return ret;
 
-	/* Complain if SCL can't be read */
-	if (bit_adap->getscl == NULL) {
+	if (bit_adap->getscl == NULL && bit_adap->getsda == NULL)
+		dev_info(&adap->dev, "I2C-like interface, SDA and SCL are write-only\n");
+	else if (bit_adap->getscl == NULL) {
+		/* Complain if SCL can't be read */
 		dev_warn(&adap->dev, "Not I2C compliant: can't read SCL\n");
 		dev_warn(&adap->dev, "Bus may be unreliable\n");
 	}

index 7a048abbf..0a87af4a7 100644
@@ -316,6 +316,8 @@ static void of_i2c_gpio_get_props(struct device_node *np,
 		of_property_read_bool(np, "i2c-gpio,scl-open-drain");
 	pdata->scl_is_output_only =
 		of_property_read_bool(np, "i2c-gpio,scl-output-only");
+	pdata->sda_is_output_only =
+		of_property_read_bool(np, "i2c-gpio,sda-output-only");
 }
 
 static struct gpio_desc *i2c_gpio_get_desc(struct device *dev,

@@ -363,6 +365,7 @@ static int i2c_gpio_probe(struct platform_device *pdev)
 	struct device *dev = &pdev->dev;
 	struct device_node *np = dev->of_node;
 	enum gpiod_flags gflags;
+	bool sda_scl_output_only;
 	int ret;
 
 	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);

@@ -391,8 +394,12 @@ static int i2c_gpio_probe(struct platform_device *pdev)
 	 * marking these lines to be handled as open drain, and we should just
 	 * handle them as we handle any other output. Else we enforce open
 	 * drain as this is required for an I2C bus.
+	 * If SCL/SDA both are write-only, then this indicates I2C-like slaves
+	 * with read-only SCL/SDA. Such slaves don't need open-drain, and partially
+	 * don't even work with open-drain.
 	 */
-	if (pdata->sda_is_open_drain)
+	sda_scl_output_only = pdata->sda_is_output_only && pdata->scl_is_output_only;
+	if (pdata->sda_is_open_drain || sda_scl_output_only)
 		gflags = GPIOD_OUT_HIGH;
 	else
 		gflags = GPIOD_OUT_HIGH_OPEN_DRAIN;

@@ -400,7 +407,7 @@ static int i2c_gpio_probe(struct platform_device *pdev)
 	if (IS_ERR(priv->sda))
 		return PTR_ERR(priv->sda);
 
-	if (pdata->scl_is_open_drain)
+	if (pdata->scl_is_open_drain || sda_scl_output_only)
 		gflags = GPIOD_OUT_HIGH;
 	else
 		gflags = GPIOD_OUT_HIGH_OPEN_DRAIN;

@@ -418,7 +425,8 @@ static int i2c_gpio_probe(struct platform_device *pdev)
 
 	if (!pdata->scl_is_output_only)
 		bit_data->getscl = i2c_gpio_getscl;
-	bit_data->getsda = i2c_gpio_getsda;
+	if (!pdata->sda_is_output_only)
+		bit_data->getsda = i2c_gpio_getsda;
 
 	if (pdata->udelay)
 		bit_data->udelay = pdata->udelay;

index a907774fd..928c87b00 100644
@@ -16,6 +16,7 @@
  *	isn't actively driven high when setting the output value high.
  *	gpio_get_value() must return the actual pin state even if the
  *	pin is configured as an output.
+ * @sda_is_output_only: I2C-like interface, SDA is write-only.
  * @scl_is_open_drain: SCL is set up as open drain. Same requirements
  *	as for sda_is_open_drain apply.
  * @scl_is_output_only: SCL output drivers cannot be turned off.

@@ -24,6 +25,7 @@ struct i2c_gpio_platform_data {
 	int		udelay;
 	int		timeout;
 	unsigned int	sda_is_open_drain:1;
+	unsigned int	sda_is_output_only:1;
 	unsigned int	scl_is_open_drain:1;
 	unsigned int	scl_is_output_only:1;
 };