Why might `i2c_smbus_write_byte_data` be returning "Operation not permitted" on uClinux 2.4?

Question

I am writing a C program with the aim of configuring a peripheral device (the CS5368 ADC) via the I2C interface of a Dante Brooklyn II, a board based on a Microblaze soft-core processor running uClinux 2.4.

I have implemented the configuration following the Dante OEM docs for guidance, however when running my program I am encountering an "Operation not permitted" (EPERM) error when attempting to write data to I2C using i2c_smbus_write_byte_data.

Here is the section of code containing the culprit call to i2c_smbus_write_byte_data:

    // Set ADC I2S to "Slave mode all speeds".
    printf("Set the CS5368 I2S mode to slave\n");
    unsigned char adc_dif = 0x01;  // I2S mode
    unsigned char adc_mode = 0x03; // Slave mode all speeds
    unsigned char data = 0x90 | (adc_dif << 2) | adc_mode;
    result = i2c_smbus_write_byte_data(i2c_fd, CS5368_GCTL_MDE, data);
    if (result < 0) {
        perror("Failed to write to the 'Global Mode Control' register");
        return -1;
    }

Here is the code within context of the full source of the small program. The program begins by resetting the CS5368 via a GPIO pin before doing the configuration via I2C.

EPERM is returned whether or not I have the CS5368 wired up. I've been able to successfully configure the CS5368 using the I2C interface of an Arduino Uno, so the issue does not appear to be related to the CS5368.

To run the program I login to the board via telnet as root, so I doubt the error has anything to do with user permissions.

The OEM docs state:

The Brooklyn II module can operate as an I2C controller running at 100Khz and using 7 bit addressing mode. It supports multi-master operation. I2C devices can be accessed from user application running on the Brooklyn II module. The interface supports the SMBus (System Management Bus) protocol, which is a subset from the I2C protocol.

It goes on to list the supported i2c_smbus_* functions including i2c_smbus_write_byte_data, so the issue does not appear to be related to lack of support for SMBus or I2C.

I came across a related issue where a user was getting an EPERM error code when attempting to use the I2C write API, however the solution appears to have been to use the i2c_smbus_* API instead which I am already doing.

Any advice on what could be causing this error code to be returned or how to debug the issue further would be greatly appreciated!

---

Edit: In case it helps, here is the full output, starting from logging onto the board via telnet after having moved the exe to /tmp via ftp:

$ telnet 169.254.72.245                                                
Trying 169.254.72.245...                                                                                  
Connected to 169.254.72.245.                        
Escape character is '^]'.                            
login: root                                                                                               
Password:                                                                                                 
                                                                                                          
                                                     
BusyBox v1.23.2 (2018-05-31 11:33:18 AEST) hush - the humble shell                                        
                                                                                                          
/ # cd /tmp                                          
/var/tmp # ./cs5368-i2c-config                       
Open GPIO device                                                                                          
Set GPIO tristate outputs                            
Set GPIO pins low                                    
Sleep for 10 secs                                    
Set GPIO pins high
Close GPIO file descriptor                           
Searching for I2C device                                                                                  
  Opening /dev/i2c-0                                                                                      
  Setting I2C_SLAVE 4c...                                                                                 
  I2C Interface found: /dev/i2c-0                                                                         
Set the CS5368 as the slave                                                                               
Set the CS5368 I2S mode to slave                                                                          
Failed to write to the 'Global Mode Control' register: Operation not permitted