[PATCH v3 3/3] i2c: gpio: support write-only sda

[Heiner Kallweit <hkallweit1@gmail.com>]

There are slave devices that understand I2C but have read-only
SDA and SCL. Examples are FD650 7-segment LED controller and
its derivatives. Typical board designs don't even have a
pull-up for both pins. Therefore don't enforce open-drain
if SDA and SCL both are unidirectional. This patch makes
i2c-gpio usable with such devices, based on new DT property
i2c-gpio,sda-output-only.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
---
v3:
- improve description of attribute sda_is_output_only
---
 drivers/i2c/busses/i2c-gpio.c          | 14 +++++++++++---
 include/linux/platform_data/i2c-gpio.h |  3 +++
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/i2c/busses/i2c-gpio.c b/drivers/i2c/busses/i2c-gpio.c
index 0e4385a9b..ea108d7e4 100644
--- a/drivers/i2c/busses/i2c-gpio.c
+++ b/drivers/i2c/busses/i2c-gpio.c
@@ -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;
diff --git a/include/linux/platform_data/i2c-gpio.h b/include/linux/platform_data/i2c-gpio.h
index a907774fd..e9536c078 100644
--- a/include/linux/platform_data/i2c-gpio.h
+++ b/include/linux/platform_data/i2c-gpio.h
@@ -16,6 +16,8 @@
  *	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: SDA output drivers can't be turned off.
+ *	This is for clients that can only read SDA/SCL.
  * @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 +26,7 @@ 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;
 };
-- 
2.39.0