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Using External Toolchain Option 1: Give tarball URL Specify URL for the tarball in BR_TOOLCHAIN_EXTERNAL_URL Example: BR_TOOLCHAIN_EXTERNAL_URL=http://192.168.101.52:8082/artifactory/toolchain.tar.xz In this case you will have to deselect BR2_PRIMARY_SITE_ONLY option Option 2: Give tarball relative dl path If BR2_PRIMARY_SITE_ONLY option is selected then you have to keep the toolchain inside dl/toolchain-external-custom/ directory and pass the name of tarball to BR_TOOLCHAIN_EXTERNAL_URL Example: BR2_PRIMARY_SITE="http://192.168.101.52:8082/artifactory/dl" BR2_PRIMARY_SITE_ONLY=y BR_TOOLCHAIN_EXTERNAL_URL=nvrx-rk3588-toolcahin.tar.xz This will extract the toolchain to buildroot’s build directory output/host/opt/ext-toolchain ...

Overview A relocatable toolchain/SDK is a self-contained set of cross-compilation tools that can be moved to different locations without breaking dependencies. Buildroot provides an option to generate such a toolchain, allowing developers to use it for cross-compiling applications without depending on a fixed absolute path. Prepare Relocatable SDK Configure Buildroot for SDK Generation Disable BusyBox and set /bin/sh to None under System configuration. This prevents unnecessary shell dependencies within the SDK, ensuring better relocatability. ...

Truth Table X Y X & Y X | Y X ^ Y 0 0 0 0 0 0 1 0 1 1 1 0 0 1 1 1 1 1 1 0 Points to Remember The left-shift and right-shift operators should not be used for negative numbers Left Shift(<<) just means multiply by 2. Similarly >> results division by 2. XOR results 0 if both bits are same. So a^1=~a , a^0=a and a^a=0. Questions How to toggle or flip a particular bit in a number? To toggle any bit in a variable, Use (^) exclusive OR operator. #define togglebit(data, bit) (data* = data ^ (1<<bit)) Write MACRO to Swap the bytes in 16bit Integer Variable. #define ByteSwap16(Value) ((Value & 0x00FF) << 8) | ((Value & 0xFF00) >> 8) #define ByteSwap32(Value) ((Value & 0x000000FF) << 24) | ((Value & 0x0000FF00U) << 8) | ((Value & 0x00FF0000U) >> 8) | ((Value & 0xFF000000U) >> 24) Count the number of set bits in a number unsigned int countSetBits( unsigned int number ) { unsigned int count = 0; while( number != 0) { count++; number &= (number-1); } return count; } Swap 2 bits of given integer int swapBits(unsigned int n, unsigned int p1, unsigned int p2) { unsigned int bit1 = (n >> p1) & 1; /* Move p1'th to rightmost side */ unsigned int bit2 = (n >> p2) & 1; /* Move p2'th to rightmost side */ unsigned int x = (bit1 ^ bit2); /* XOR the two bits */ /* Put the xor bit back to their original positions */ x = (x << p1) | (x << p2); /* XOR 'x' with the original number so that the two sets are swapped */ unsigned int result = n ^ x; }

Introduction GPIO (General Purpose Input/Output) is a fundamental interface in embedded systems and Linux-based platforms. Linux provides multiple methods to control GPIOs, including the deprecated /sys/class/gpio/ interface and the modern libgpiod (GPIO character device) API. This document provides a comprehensive guide to managing GPIOs in Linux. GPIO Interfaces in Linux Linux provides three primary ways to manage GPIOs: Legacy Sysfs Interface (/sys/class/gpio/) - Deprecated but still present on some systems. GPIO Character Device (/dev/gpiochipX) - The recommended approach using libgpiod. Direct Kernel Access - Through kernel drivers or device tree configurations. 1. Sysfs GPIO Interface (Deprecated) The sysfs-based interface was historically used to control GPIOs but has been marked as deprecated in favor of gpiod. If still available, it can be accessed via /sys/class/gpio/. ...

Overview A Bayer filter is a color filter array (CFA) that arranges RGB color filters on a square grid of photosensors. Color Sensitivity Digital image sensors can only detect brightness, not color. To produce color sensors, a color filter is applied to each pixel. Generally the sensor is divided into 50% green, 25% red, and 25% blue photosites. G R G R B G B G G R G R B G B G Demosaicing Each pixel is filtered to record only one of three colors, so the data from each pixel cannot fully specify each of the red, green, and blue values on its own. To obtain a full-color image, demosaicing algorithms are used to interpolate a set of complete red, green, and blue values for each pixel. The color of each pixel is interpolated by those around it. ...

SPI (Serial Peripheral Interface) Overview Synchronous, full-duplex serial bus. Master-slave architecture (1 master, multiple slaves). Uses 4 wires: SCLK (clock), MOSI (Master Out Slave In), MISO (Master In Slave Out), SS/CS (Slave Select). Physical Layer Push-pull outputs (faster than open-drain). Each slave requires a dedicated SS line. Data Frame Structure No start/stop bits – continuous stream synchronized to SCLK. Data sampled on clock edges defined by CPOL (clock polarity) and CPHA (clock phase): Mode 0: CPOL=0 (idle low), CPHA=0 (sample on rising edge). Mode 3: CPOL=1 (idle high), CPHA=1 (sample on falling edge). SCLK | MOSI (Data from Master) | MISO (Data from Slave) | CS (Active Low) Key Features Full-duplex communication (simultaneous MOSI/MISO). No addressing – slaves selected via SS lines. Speeds: Up to 100+ Mbps (depends on hardware). Pros & Cons Pros Cons High-speed communication High pin count (n+3 for n slaves) Simple protocol, flexible modes No built-in error detection Full-duplex support No multi-master support Use Cases High-speed sensors (e.g., IMUs). Display controllers (OLED, TFT). SD cards, NOR flash memory. Comparison Table Feature UART I2C SPI Clock None (async) Shared (SCL) Shared (SCLK) Duplex Full-duplex Half-duplex Full-duplex Topology Point-to-point Multi-device Master-slave Speed Low (≤115kbps) Moderate (≤3.4Mbps) High (≥10Mbps) Addressing None 7/10-bit Hardware (SS lines) Pins 2 (TX/RX) 2 (SCL/SDA) 4 + n (SS per slave) Error Handling Parity bit ACK/NACK None

UART (Universal Asynchronous Receiver-Transmitter) UART is a simple, asynchronous serial communication protocol used for full-duplex communication between two devices. Key Features: Asynchronous: No clock signal – relies on pre-agreed baud rate (e.g., 9600, 115200 bps). Uses two main lines: TX (Transmit) and RX (Receive). Configurable baud rate (e.g., 9600, 115200 bps). Error detection: Parity bit (optional). Flow control: Hardware (RTS/CTS) or software (XON/XOFF). No addressing – only two devices per bus. Data Frame Structure Start bit (1 bit, logic low). Data bits (5–9 bits, LSB-first). Parity bit (optional, even/odd/none). Stop bit(s) (1 or 2 bits, logic high). Start Bit | Data Bits (5-9) | Parity Bit (Optional) | Stop Bit (1-2) Points to Remember If the baud rate is set as 115200, then the recever will expect stop bit that is high state for 1 baud period(generally). Usage in Linux Kernel: #include <linux/serial_core.h> struct uart_port *port; uart_write(port, "Hello", 5); Use Cases Debugging consoles (e.g., Linux kernel printk via UART). GPS modules, Bluetooth/Wi-Fi modules.

1. Introduction In a multitasking environment, multiple processes and threads may need to access shared resources concurrently. Without proper synchronization, race conditions, deadlocks, and data corruption can occur. The Linux kernel provides various synchronization primitives to ensure safe concurrent access while maintaining performance. 2. Spinlocks Spinlocks are busy-waiting locks used in scenarios where critical sections are short and must be protected from concurrent access. Key Features: Suitable for short, critical sections. Does not sleep, making it ideal for use in interrupt handlers. If contention occurs, the CPU spins in a loop until the lock is available. Usage: spinlock_t my_lock; spin_lock_init(&my_lock); spin_lock(&my_lock); /* Critical section */ spin_unlock(&my_lock); Types of Spinlocks: ...

Monolithic Kernel All core OS services (memory management, process scheduling, file systems, drivers) reside in kernel space. Example: Linux Kernel. Pros: Fast performance, directaccess to hardware. Cons: Large codebase, difficult debugging, crashes can affect the whole system. Microkernel Minimal core kernel, with most services running in user space. Example: QNX, Minix. Pros: Stability, modularity, better security. Cons: Performance overhead due to inter-process communication (IPC).

Overview Kernel Space: This is where the Linux kernel executes and provides low-level access to hardware, system memory management, process scheduling, and device drivers. Kernel space has privileged access to system resources and is protected from direct user interference. For example, when a user requests data from a hardware sensor, the kernel driver handles communication with the hardware, processes the request, and returns the data to user space through system calls. User Space: This is where applications and system utilities run. User-space processes operate with restricted privileges and interact with the kernel via system calls, libraries, and IPC mechanisms. For example, a user-space daemon may monitor the watchdog status by writing to /dev/watchdog, or a mobile app may read light intensity from /sys/bus/i2c/devices/1-0039/lux. Communication Methods between Kernel and User Space There are several ways to facilitate communication between user space and kernel space in an embedded Linux environment: ...