Refactor build-kernel.sh for root-directory execution

- Removed dependency on separate build directory by using script-relative paths.
- Simplified script structure by consolidating functions into a linear flow.
- Enhanced error handling with streamlined trap mechanisms.
- Automated patch application by iterating over all patches in the patches/ directory.
- Improved logging setup for better build process visibility.
- Cleaned up unnecessary comments and redundant code for better readability.
- Ensured all paths are absolute based on the script's location for portability.

.gitignore

@@ -3,6 +3,5 @@ cache
 logs
 output
 *swp
-build/build_kernel.log
-build/linux/
+*.log
 linux/

build-kernel.sh

@@ -0,0 +1,219 @@
+#!/bin/bash
+
+# Build script for cross-compiling the Linux kernel for Raspberry Pi CM4/RPI4
+# and packaging it into a Debian package
+# Target: ARM64, Distribution: Debian Bookworm
+# Author: Jerry Olla <jerryolla@gmail.com>
+
+set -euo pipefail  # Enable strict error handling
+
+# Enable detailed logging
+LOG_FILE="build_kernel.log"
+exec > >(tee -i "$LOG_FILE") 2>&1
+
+# Determine the directory where the script resides
+BASE_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+# Configuration Variables
+KERNEL_REPO="https://github.com/raspberrypi/linux.git"
+KERNEL_BRANCH="rpi-6.12.y"
+KERNEL_SRC_DIR="$BASE_DIR/linux"
+OUTPUT_PATH="$BASE_DIR/output"
+CROSS_COMPILE="aarch64-linux-gnu-"
+ARCH="arm64"
+BASE_CONFIG="bcm2711_defconfig"
+CUSTOM_CONFIG="$BASE_DIR/wlanpi_v8_defconfig"
+PATCHES_DIR="$BASE_DIR/patches"
+PACKAGE_DIR="$BASE_DIR/wlanpi-kernel-package"
+NUM_CORES=$(nproc)
+
+# Derived Variables
+KERNEL_IMAGE_NAME="wlanpi-kernel8.img"
+IMAGE_OUTPUT="$OUTPUT_PATH/boot/firmware/$KERNEL_IMAGE_NAME"
+DTB_OUTPUT_DIR="$OUTPUT_PATH/boot/firmware/"
+DTBO_OUTPUT_DIR="$OUTPUT_PATH/boot/firmware/overlays/"
+MODULES_OUTPUT_DIR="$OUTPUT_PATH/lib/modules"
+
+# Debian Package Metadata
+PACKAGE_NAME="wlanpi-kernel-bookworm"
+
+# Trap for error handling
+trap 'echo "Error encountered at line $LINENO. Exiting."; exit 1' ERR
+
+# Initialize output directories
+echo "Creating output directories..."
+mkdir -p "$(dirname "$IMAGE_OUTPUT")" \
+         "$DTB_OUTPUT_DIR" \
+         "$DTBO_OUTPUT_DIR" \
+         "$MODULES_OUTPUT_DIR"
+
+# Clone or update the kernel source repository
+if [ ! -d "$KERNEL_SRC_DIR" ]; then
+    echo "Cloning kernel source from $KERNEL_REPO..."
+    git clone --depth=1 -b "$KERNEL_BRANCH" "$KERNEL_REPO" "$KERNEL_SRC_DIR"
+else
+    echo "Kernel source directory exists. Updating..."
+    cd "$KERNEL_SRC_DIR"
+    git fetch origin "$KERNEL_BRANCH"
+    git checkout "$KERNEL_BRANCH"
+    git reset --hard "origin/$KERNEL_BRANCH"
+    cd "$BASE_DIR"
+fi
+
+# Configure the kernel
+echo "Configuring the kernel..."
+cd "$KERNEL_SRC_DIR"
+
+export ARCH="$ARCH"
+export CROSS_COMPILE="$CROSS_COMPILE"
+
+echo "Loading base config: $BASE_CONFIG..."
+make "$BASE_CONFIG"
+
+echo "Merging custom config: $(basename "$CUSTOM_CONFIG")..."
+if [ -f "$CUSTOM_CONFIG" ]; then
+    ./scripts/kconfig/merge_config.sh "arch/$ARCH/configs/$BASE_CONFIG" "$CUSTOM_CONFIG"
+    make olddefconfig
+else
+    echo "ERROR: Custom config file $CUSTOM_CONFIG not found."
+    exit 1
+fi
+
+# Apply patches
+echo "Applying patches from $PATCHES_DIR..."
+for patch in "$PATCHES_DIR"/*.patch; do
+    if [ -f "$patch" ]; then
+        echo "Applying patch: $(basename "$patch")"
+        patch -p1 -N < "$patch"
+    else
+        echo "No patches found in $PATCHES_DIR."
+    fi
+done
+
+# Build the kernel, modules, and DTBs
+echo "Starting kernel build..."
+
+echo "Building Image..."
+make -j"$NUM_CORES" Image
+
+echo "Building modules..."
+make -j"$NUM_CORES" modules
+
+echo "Installing modules to $MODULES_OUTPUT_DIR..."
+make INSTALL_MOD_PATH="$OUTPUT_PATH" modules_install
+
+echo "Building Device Tree Blobs (DTBs)..."
+make -j"$NUM_CORES" dtbs
+
+# Collect build artifacts
+echo "Collecting build artifacts..."
+
+if [ ! -f "arch/arm64/boot/Image" ]; then
+    echo "ERROR: Kernel image not found!"
+    exit 1
+fi
+
+cp arch/arm64/boot/Image "$IMAGE_OUTPUT"
+
+find arch/arm64/boot/dts/ -name '*.dtb' -exec cp {} "$DTB_OUTPUT_DIR" \;
+find arch/arm64/boot/dts/overlays/ -name '*.dtbo' -exec cp {} "$DTBO_OUTPUT_DIR" \;
+
+# Prepare Debian package
+echo "Preparing Debian package..."
+
+# Retrieve kernel version and set package version
+KERNEL_VERSION=$(make kernelrelease)
+BUILD_DATE=$(date +%Y%m%d)
+PACKAGE_VERSION="${KERNEL_VERSION}-${BUILD_DATE}"
+
+echo "Kernel Version: $KERNEL_VERSION"
+echo "Build Date: $BUILD_DATE"
+echo "Package Name: $PACKAGE_NAME"
+echo "Package Version: $PACKAGE_VERSION"
+
+# Clean previous package directory
+rm -rf "$PACKAGE_DIR"
+mkdir -p "$PACKAGE_DIR/DEBIAN" \
+         "$PACKAGE_DIR/usr/local/lib/wlanpi-kernel/boot/firmware/overlays" \
+         "$PACKAGE_DIR/lib/modules/$KERNEL_VERSION"
+
+# Copy files to package directory
+cp "$IMAGE_OUTPUT" "$PACKAGE_DIR/usr/local/lib/wlanpi-kernel/boot/firmware/"
+cp "$DTB_OUTPUT_DIR"*.dtb "$PACKAGE_DIR/usr/local/lib/wlanpi-kernel/boot/firmware/"
+cp "$DTBO_OUTPUT_DIR"*.dtbo "$PACKAGE_DIR/usr/local/lib/wlanpi-kernel/boot/firmware/overlays/"
+cp -r "$MODULES_OUTPUT_DIR/$KERNEL_VERSION" "$PACKAGE_DIR/lib/modules/."
+
+# Create DEBIAN/control file
+cat <<EOF > "$PACKAGE_DIR/DEBIAN/control"
+Package: $PACKAGE_NAME
+Version: $PACKAGE_VERSION
+Section: kernel
+Priority: optional
+Architecture: arm64
+Maintainer: Jerry Olla <jerryolla@gmail.com>
+Conflicts: wlanpi-kernel
+Replaces: wlanpi-kernel
+Depends: libc6 (>= 2.29)
+Description: Custom Linux kernel for Raspberry Pi CM4/RPI4 with WLAN Pi v8 configuration for Debian Bookworm
+ This package contains a custom-built Linux kernel image, Device Tree Blobs (DTBs),
+ and kernel modules tailored for the WLAN Pi v8 configuration on Raspberry Pi CM4/RPI4 running Debian Bookworm.
+EOF
+
+# Create DEBIAN/postinst script
+cat <<'EOF' > "$PACKAGE_DIR/DEBIAN/postinst"
+#!/bin/bash
+set -e
+
+# Variables
+FIRMWARE_DIR="/boot/firmware"
+PACKAGE_KERNEL_DIR="/usr/local/lib/wlanpi-kernel/boot/firmware"
+CONFIG_TXT="$FIRMWARE_DIR/config.txt"
+KERNEL_IMAGE="wlanpi-kernel8.img"
+
+echo "Post-installation: Installing kernel image and DTBs to $FIRMWARE_DIR..."
+
+# Ensure the firmware directory exists
+if [ ! -d "$FIRMWARE_DIR" ]; then
+    echo "ERROR: Firmware directory $FIRMWARE_DIR does not exist."
+    exit 1
+fi
+
+# Copy kernel image
+echo "Copying kernel image..."
+cp -f "$PACKAGE_KERNEL_DIR/$KERNEL_IMAGE" "$FIRMWARE_DIR/"
+
+# Copy DTBs
+echo "Copying DTBs..."
+cp -f "$PACKAGE_KERNEL_DIR/"*.dtb "$FIRMWARE_DIR/"
+cp -f "$PACKAGE_KERNEL_DIR/overlays/"*.dtbo "$FIRMWARE_DIR/overlays/"
+
+# Update config.txt with the new kernel
+echo "Updating $CONFIG_TXT with the new kernel parameter..."
+if grep -q "^kernel=" "$CONFIG_TXT"; then
+    sed -i "s|^kernel=.*|kernel=$KERNEL_IMAGE|" "$CONFIG_TXT"
+else
+    echo "kernel=$KERNEL_IMAGE" >> "$CONFIG_TXT"
+fi
+
+echo "Kernel image and DTBs installed successfully."
+
+# Optionally, prompt for a reboot
+# read -p "Reboot now to apply the new kernel? [y/N] " confirm && [[ $confirm == [yY] ]] && reboot
+
+exit 0
+EOF
+
+# Make postinst script executable
+chmod 755 "$PACKAGE_DIR/DEBIAN/postinst"
+
+# Build the Debian package
+echo "Building Debian package..."
+dpkg-deb --build "$PACKAGE_DIR" "$OUTPUT_PATH/${PACKAGE_NAME}_${PACKAGE_VERSION}_arm64.deb"
+
+echo "Debian package ${PACKAGE_NAME}_${PACKAGE_VERSION}_arm64.deb created successfully in $OUTPUT_PATH."
+
+# Clean up temporary package directory
+echo "Cleaning up temporary package directory..."
+rm -rf "$PACKAGE_DIR"
+
+echo "Kernel build, module installation, and package creation completed successfully."