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

Heiner Kallweit
2023-01-15T10:18:52Z

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