How linux driver can delegate functions to another driver (shared hardware) - driver

Question

I'm writing a custom IR transmitter driver for lirc on an embedded board. The board has a i2c-to-gpio extender (FXL6408).
The problem is that only one GPIO pin is needed for my driver (and hence LIRC) and the other pins are needed for use by other applications. Something like this:
I've read LWM, LDD3, and tons of sites about i2c-slave, i2c adaptors, buses, pinctrl, gpio, stacking etc but its not clear how to do what I want:
my-driver needs to control a single pin on the GPIO extender while still allowing other applications to control the other 7 pins via /dev/i2c-0.
Following this SO suggestion was promising but i2c_new_dummy failed, returning NULL:
i2cAdaptor = i2c_get_adapter(ECP_I2CBUS); // 1 means i2c-1 bus
if (i2cAdaptor == NULL)
{
printk("ecp_gpio: Could not acquire i2c adaptor\n");
return -EBUSY;
}
i2cClient = i2c_new_dummy(i2cAdaptor, ECP_I2CDEV); // 0x43 - slave address on i2c bus
if (i2cClient == NULL)
{
printk("ecp_gpio: Could not acquire i2c client\n");
return -EBUSY;
}
if ( (rc = i2c_smbus_write_byte(i2cClient, 0xF0)) < 0)
{
printk("ecp_gpio: Error writing byte - error %d", rc);
return -EIO;
}
What is the correct way to hook up the plumbing to achieve what I want?
OS Info:
# uname -a
Linux ecp 4.4.127-1.el6.elrepo.i686 #1 SMP Sun Apr 8 09:44:43 EDT 2018 i686 i686 i386 GNU/Linux

Answer 1

After trying many different things, I found one that works. I don't know if it is "the right way" to do it, but it does work.
Instead of trying to create a dummy client, just call the i2c_xx functions directly. So the code looks like:
i2cAdaptor = i2c_get_adapter(ECP_I2CBUS); // 1 means i2c-1 bus etc
if (i2cAdaptor == NULL)
{
printk("ecp_gpio: Could not acquire i2c adaptor\n");
return -EBUSY;
}
union i2c_smbus_data data;
data.byte = 0xF0;
if ((rc = i2c_smbus_xfer(ecpHardware.i2cAdaptor, ECP_I2CDEV, 0, I2C_SMBUS_WRITE, 0x05, I2C_SMBUS_BYTE_DATA, &data)) < 0)
{
printk("ecp_gpio: i2c_smbus_xfer failed: %d\n", rc);
return -EIO;
}
data.byte = 0xE0;
if ((rc = i2c_smbus_xfer(ecpHardware.i2cAdaptor, ECP_I2CDEV, 0, I2C_SMBUS_WRITE, 0x05, I2C_SMBUS_BYTE_DATA, &data)) < 0)
{
printk("ecp_gpio: i2c_smbus_xfer failed: %d\n", rc);
return -EIO;
}