Remove toplevel trash, add debugfs and kernel config fragment

Config.in

@@ -1,6 +0,0 @@
-config BR2_PACKAGE_KERNEL_MODULE
-	bool "kernel_module"
-	help
-	  Kernel module hello world.
-
-	  http://example.com/

Makefile

@@ -1,10 +0,0 @@
-obj-m += hello.o
-ccflags-y := -Wno-declaration-after-statement -std=gnu99
-
-.PHONY: all clean
-
-all:
-	$(MAKE) -C '$(LINUX_DIR)' M='$(PWD)' modules
-
-clean:
-	$(MAKE) -C '$(LINUX_DIR)' M='$(PWD)' clean

README.md

@@ -30,3 +30,4 @@ The Linux kernel version can be found with:
     1.  Buildroot
         1. [hello](kernel_module/package/kernel_module/src/hello.c)
         1. [hello2](kernel_module/package/kernel_module/src/hello2.c)
+        1. [debugfs](kernel_module/package/kernel_module/src/debugfs.c)

TODO.md

@@ -1,7 +0,0 @@
-# TODO
-
-- build kernel module against a given kernel tree
-- build out of tree kernel module into the kernel image
-    - http://stackoverflow.com/questions/7353851/insert-linux-kernel-module-statically
-    - http://stackoverflow.com/questions/8421970/compiling-a-driver-as-a-part-of-a-kernel-not-as-a-module
-- .ko file format vs .o http://stackoverflow.com/questions/10476990/difference-between-o-and-ko-file

external.desc

@@ -1 +0,0 @@
-name: KERNEL_MODULE

external.mk

@@ -1,20 +0,0 @@
-################################################################################
-#
-# kernel_module
-#
-################################################################################
-
-KERNEL_MODULE_VERSION = 1.0
-KERNEL_MODULE_SITE = ..
-KERNEL_MODULE_SITE_METHOD = local
-
-define KERNEL_MODULE_BUILD_CMDS
-	$(MAKE) -C '$(@D)' LINUX_DIR='$(LINUX_DIR)' PWD='$(@D)' CC='$(TARGET_CC)' LD='$(TARGET_LD)'
-endef
-
-define KERNEL_MODULE_INSTALL_TARGET_CMDS
-	$(INSTALL) -D -m 0755 '$(@D)/kernel_module.ko' '$(TARGET_DIR)/kernel_module.ko'
-endef
-
-$(eval $(kernel-module))
-$(eval $(generic-package))

hello.c

@@ -1,13 +0,0 @@
-#include <linux/module.h>
-#include <linux/kernel.h>
-
-int init_module(void)
-{
-	printk(KERN_INFO "hello init\n");
-	return 0;
-}
-
-void cleanup_module(void)
-{
-	printk(KERN_INFO "hello cleanup\n");
-}

kernel-config-frag

@@ -0,0 +1,4 @@
+# If you change this file, you need to run:
+#     rm -rf buildroot/output/build/linux-*.*.*/
+# before ./run
+CONFIG_DEBUG_FS=y

kernel_module/package/kernel_module/src/Makefile

@@ -1,4 +1,4 @@
-obj-m += hello.o hello2.o
+obj-m += debugfs.o hello.o hello2.o
 ccflags-y := -Wno-declaration-after-statement -std=gnu99
 
 .PHONY: all clean

kernel_module/package/kernel_module/src/debugfs.c

@@ -0,0 +1,45 @@
+/*
+Adapted from: https://github.com/chadversary/debugfs-tutorial/blob/47b3cf7ca47208c61ccb51b27aac6f9f932bfe0b/example1/debugfs_example1.c
+
+Requires CONFIG_DEBUG_FS.
+
+	mkdir /debugfs
+	mount -t debugfs none /sys/kernel/debug
+	insmod /debugfs.ko
+	cd /debugfs/kernel_module_cheat
+	cat myfile
+
+Output:
+
+	42
+*/
+
+#include <linux/debugfs.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+
+MODULE_LICENSE("GPL");
+
+static struct dentry *dir = 0;
+static u32 value = 42;
+
+int init_module(void)
+{
+    struct dentry *file;
+    dir = debugfs_create_dir("kernel_module_cheat", 0);
+    if (!dir) {
+        printk(KERN_ALERT "debugfs_create_dir failed");
+        return -1;
+    }
+    file = debugfs_create_u32("myfile", 0666, dir, &value);
+    if (!file) {
+        printk(KERN_ALERT "debugfs_create_u32 failed");
+        return -1;
+    }
+    return 0;
+}
+
+void cleanup_module(void)
+{
+    debugfs_remove_recursive(dir);
+}

kernel_module/package/kernel_module/src/hello.c

@@ -1,5 +1,11 @@
 /*
 Hello world module.
+
+	dmesg -c
+	insmod hello.ko
+	dmesg -c | grep 'hello init'
+	rmmod hello.ko
+	dmesg -c | grep 'hello cleanup'
 */
 
 #include <linux/module.h>

run

@@ -2,7 +2,9 @@
 set -e
 cd buildroot
 make BR2_EXTERNAL="$(pwd)/../kernel_module" qemu_x86_64_defconfig
-echo 'BR2_PACKAGE_KERNEL_MODULE=y' >> .config
-# kernel_module-reconfigure rebuilds the modules if they were modified.
-env -u LD_LIBRARY_PATH make BR2_JLEVEL="$(($(nproc) - 2))" kernel_module-reconfigure all
+echo '
+BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES="../kernel-config-frag"
+BR2_PACKAGE_KERNEL_MODULE=y
+' >> .config
+env -u LD_LIBRARY_PATH make BR2_JLEVEL="$(($(nproc) - 2))" kernel_module-rebuild all
 qemu-system-x86_64 -M pc -kernel output/images/bzImage -drive file=output/images/rootfs.ext2,if=virtio,format=raw -append root=/dev/vda -net nic,model=virtio -net user