Both I2C_DESIGNWARE_PLATFORM and I2C_DESIGNWARE_PCI will add `i2c-designware-core.o' to the list of object files to be linked against the respective module. When these features are compiled built-in in the kernel, both modules are part of a `built-in.o' object. The linker fails because both `i2c-designware-platform.o' and `i2c-designware-pci.o' contain duplicate definitions coming from `i2c-designware-core'.

Bug fixed by commit e68bb91baa0

#ifdef CONFIG_I2C_DESIGNWARE_PLATFORM
int i2c_dw_init()
{
	return 0;
}
#endif

#ifdef CONFIG_I2C_DESIGNWARE_PCI
int i2c_dw_init()	// ERROR
{
	return 0;
}

#endif

int main(void)
{
#if defined(CONFIG_I2C_DESIGNWARE_PLATFORM) || defined(CONFIG_I2C_DESIGNWARE_PCI)
	i2c_dw_init();
#endif
	return 0;
}