Android Serial Port is being used for no permission to open
Android Serial Port Access: Troubleshooting “Permission Denied” Errors and Achieving Seamless Communication
We understand the frustration of encountering “permission denied” errors when attempting to access serial ports on an Android device, particularly when working with embedded systems like your Wandboard i.MX6. This comprehensive guide delves deep into the complexities of Android serial port communication, providing a detailed roadmap to diagnose, troubleshoot, and ultimately resolve the issues preventing your application from successfully reading RS232 data. Drawing upon our collective experience in embedded systems and Android development, we aim to equip you with the knowledge and strategies necessary to overcome these hurdles. We’ll cover everything from fundamental concepts to advanced techniques, ensuring you possess the tools to achieve reliable serial port communication.
Understanding the Android Serial Port Ecosystem
The Android operating system, built upon a Linux kernel, manages hardware resources, including serial ports, through a layered architecture. To effectively address permission issues, it’s crucial to understand the key components and processes involved.
The Role of Serial Port Drivers
The foundation of serial port communication lies within the kernel-level drivers. These drivers are responsible for the low-level interactions with the UART (Universal Asynchronous Receiver/Transmitter) hardware, enabling the transmission and reception of data. On the Wandboard i.MX6, the drivers, likely related to ttyS* or ttymxc* devices, manage the specific UART interfaces. These drivers expose the serial ports as character devices, accessible via the /dev/ directory.
Permission Management: The Android Security Model
Android’s security model employs a robust permission system to protect sensitive hardware resources. Access to serial ports is governed by permissions, which regulate which applications are authorized to interact with specific devices. Without the appropriate permissions, your application will inevitably encounter “permission denied” errors. These permissions are managed at various levels: the device tree, the Linux kernel, and the Android operating system itself.
Identifying Serial Port Devices
Android presents serial ports as device nodes within the /dev/ directory. The names often reflect the UART interfaces of the hardware. The ttymxc0 and ttymxc2 entries you observe in your dropdown menu are typical examples of these device nodes. These names will vary based on the device’s specific hardware configuration.
Diagnosing “Permission Denied” Errors
The core of resolving your problem lies in pinpointing the specific cause of the permission errors. This involves a systematic approach, using several different techniques.
Verify Serial Port Availability and Configuration
Before diving into permissions, confirm that the serial ports are correctly configured and accessible from the Linux console.
Checking Serial Port Existence
Use the ls -l /dev/ command in the Android terminal or via an ADB shell to list the contents of the /dev/ directory. This command will confirm the presence of your desired serial port devices, such as ttymxc0 or ttymxc2. If the device nodes are missing, there may be a hardware or kernel driver issue.
Examining Serial Port Settings
Use the stty -F /dev/ttymxc0 -a command (replace /dev/ttymxc0 with the actual serial port you are attempting to use) to view the serial port configuration. This will display settings such as baud rate, parity, data bits, and stop bits. Compare these settings with the requirements of your RS232 device. Mismatched configurations can lead to communication failures, although not directly “permission denied” errors.
Analyzing Kernel Messages with dmesg
The kernel log, accessible through the dmesg command, is a critical resource for debugging hardware-related issues.
Filtering Kernel Logs for Serial Port Activity
Run dmesg | grep tty or dmesg | grep mxc to filter the kernel log and focus on entries related to serial ports. This can reveal errors during driver initialization, hardware conflicts, or other relevant information.
Interpreting Kernel Error Messages
Carefully examine the output of dmesg. Look for error messages associated with your serial port devices. These messages may offer clues about the root cause of the “permission denied” error, such as device ownership issues.
Investigating Application-Level Errors
Your Qt Creator application can provide valuable insights into the problem.
Error Handling in Your C++ Code
Ensure your C++ code includes robust error handling. Specifically, check the return values of system calls like open(), read(), write(), and close(). Implement error logging using qDebug() or equivalent, to capture details about failures.
Logging Open() and Other System Call Results
When attempting to open the serial port, meticulously log the return value of the open() system call. A return value of -1 indicates an error. Use errno to determine the specific error that occurred. You should log both the value of errno and the corresponding textual error message using strerror(errno).
Examining Qt Creator Output
Review the output from Qt Creator’s debugger. This will often provide crucial details about the state of your application, including any error messages or unexpected behaviors.
Resolving “Permission Denied” Errors: A Step-by-Step Guide
Now, let’s move on to the key steps to resolve the “permission denied” error.
Granting Serial Port Permissions in the Manifest
The Android Manifest file (AndroidManifest.xml) is the central configuration file for your application. It’s where you declare required permissions.
Adding the android.permission.SERIAL_PORT Permission
Include the <uses-permission> tag within the <manifest> tag of your AndroidManifest.xml file:
<uses-permission android:name="android.permission.SERIAL_PORT" />
Important Consideration: Some Android devices and versions don’t use android.permission.SERIAL_PORT. In these cases, you must use alternatives, like the user-defined permissions, and modify the hardware configuration.
Understanding Permission Scopes: Android 6.0 (API Level 23) and Beyond
With Android 6.0 (Marshmallow, API level 23) and later, runtime permissions were introduced. However, serial port access might work differently, depending on the device manufacturer. If android.permission.SERIAL_PORT alone doesn’t resolve the issue, you’ll need to explore other methods.
Alternative Permission Strategies
If android.permission.SERIAL_PORT is insufficient, consider these advanced techniques.
Using Custom Permissions (Requires Root Access or Custom ROMs)
Create a custom permission in the manifest that your application uses:
<permission android:name="com.yourdomain.your_app.SERIAL_PORT_ACCESS"
android:protectionLevel="signature" />
<uses-permission android:name="com.yourdomain.your_app.SERIAL_PORT_ACCESS" />
You would then need to modify the system configuration to grant this permission to your application. This usually requires modifying the system-level configuration files or potentially rooting the device.
Device Tree Modifications (Advanced)
For deeper control, you can modify the device tree, which defines the hardware configuration for the i.MX6. This will allow you to manage the ownership of the serial ports more precisely. This is a more advanced approach and requires in-depth knowledge of the device tree format and the specific hardware configuration.
Understanding Device Ownership and Group Membership
The Linux operating system manages access to devices through file permissions, device ownership, and group membership.
Identifying the Owner and Group of Serial Port Devices
Use the ls -l /dev/ttymxc0 command (replace ttymxc0 with your target serial port) to view the owner and group of the serial port device. The output will resemble:
crw-rw---- 1 root dialout ...
This example shows the owner as root and the group as dialout. Your application must be a member of the same group or have the required permissions to access the device.
Adding Your Application’s User to the Appropriate Group
This is a crucial step. You can add your application’s user to the dialout (or the relevant group) using the following steps:
Identifying Your Application’s User ID: You can determine the user ID of your application. The
run-ascommand can be helpful if you have root access on the device.Using
adb shellandusermod(Root Required): Connect to your device via ADB and use theadb shellcommand to gain shell access. As root, execute the following command (replace<your_user_id>with your user ID):usermod -a -G dialout <your_user_id>Important: Root access is often required to modify user groups.
Alternative approach: The
init.rcfile (Advanced): Modifying the device’sinit.rcfile can ensure the correct group membership is configured on boot. This requires a rooted device.
Code-Level Considerations and Best Practices
Your C++ code must be written to manage serial port communication effectively.
Using the Correct open() Flags
When calling open(), make sure you’re using the correct flags. The flags you should use will depend on what operations you expect your application to perform.
int fd = open("/dev/ttymxc0", O_RDWR | O_NOCTTY | O_NDELAY);
O_RDWR: Opens the port for reading and writing.O_NOCTTY: Prevents the serial port from becoming the controlling terminal.O_NDELAY: Allows non-blocking operation.
Configuring Serial Port Settings with termios
After successfully opening the serial port, you must configure its settings using the termios structure and related functions.
#include <termios.h>
#include <unistd.h>
struct termios options;
tcgetattr(fd, &options); // Get the current configuration
// Set baud rate
cfsetispeed(&options, B115200); // Set the input baud rate
cfsetospeed(&options, B115200); // Set the output baud rate
// Configure data bits, parity, stop bits
options.c_cflag &= ~PARENB; // Disable parity
options.c_cflag &= ~CSTOPB; // 1 stop bit
options.c_cflag &= ~CSIZE; // Clear the character size mask
options.c_cflag |= CS8; // 8 data bits
// Enable the receiver and set local mode
options.c_cflag |= (CLOCAL | CREAD);
// Apply the settings
tcsetattr(fd, TCSANOW, &options); // Apply immediately
Non-Blocking I/O
Consider using non-blocking I/O to prevent your application from hanging while waiting for data.
fcntl(fd, F_SETFL, FNDELAY); // Set non-blocking mode
Closing the Serial Port Properly
Always close the serial port when you’re finished with it.
close(fd);
Troubleshooting and Further Investigation
If, despite implementing the above steps, you still encounter issues, you will need to conduct further investigations.
Verifying Hardware Connections
Ensure all hardware connections are correct and secure.
Checking the RS232 Cable and Connectors
Double-check the physical connections of your RS232 cable. Ensure it’s connected correctly and that the connectors are properly seated.
Testing with a Serial Port Loopback
Perform a loopback test by connecting the transmit (TX) and receive (RX) pins on the serial port. This will help to determine if the hardware itself is functioning correctly.
Exploring Kernel Driver Issues
The kernel drivers are critical.
Recompiling the Kernel (Advanced)
In some cases, you may need to recompile the kernel.
Loading and Unloading Kernel Modules
You might attempt to unload and reload the kernel modules related to the serial port devices (again, be cautious).
Testing with a Simple Terminal Application
Verify the serial port communication by using a simple terminal application to communicate with your device.
Using minicom or screen (Linux-Based Systems)
Use a terminal emulator like minicom or screen to test serial port communication.
Testing from Android Command Line
Test using a simple serial port tool directly from the Android command line.
Addressing Potential Issues
Beyond the main topics, address some potential issues.
Power Cycling the Device
Power cycle the Wandboard i.MX6. This can help resolve transient hardware or software glitches.
Firmware Updates
Ensure your device’s firmware is up to date.
Conclusion: Achieving Robust Serial Port Communication
By meticulously following the steps outlined in this comprehensive guide, you’ll be well-equipped to overcome the “permission denied” errors and establish seamless serial port communication on your Android-based Wandboard i.MX6 system. Remember that persistent troubleshooting, careful analysis, and a deep understanding of the underlying system are the keys to success. Armed with this knowledge and a methodical approach, you’ll be able to read RS232 data confidently.