Actually install modules to root manually, irq failed attempt

README.md

@@ -303,6 +303,7 @@ But TODO I don't think you can see where you are in the kernel source code and l
 1.  Examples
     1.  [Host](host/)
     1.  Buildroot
+        1.  [module_init](kernel_module/module_init.c)
         1.  Debugging
             1.  [hello](kernel_module/hello.c)
             1.  [hello2](kernel_module/hello2.c)
@@ -318,3 +319,4 @@ But TODO I don't think you can see where you are in the kernel source code and l
             1. [kthread_uninterruptible](kernel_module/kthread_uninterruptible.c)
             1. [timer](kernel_module/timer.c)
             1. [work_from_work](kernel_module/work_from_work.c)
+        1.  [irq](irq.c)

kernel_module/external.mk

@@ -13,6 +13,12 @@ define KERNEL_MODULE_BUILD_CMDS
 endef
 
 define KERNEL_MODULE_INSTALL_TARGET_CMDS
+	# The modules are already installed by the kernel-module package type
+	# under /lib/modules/**, but let's also copy the modules to the root
+	# for insmod convenience.
+	#
+	# Modules can be still be easily inserted with "modprobe module" however.
+	$(INSTALL) -D -m 0655 $(@D)/*.ko '$(TARGET_DIR)'
 	$(INSTALL) -D -m 0755 $(@D)/test/*.out '$(TARGET_DIR)'
 endef
 

kernel_module/irq.c

@@ -0,0 +1,42 @@
+/*
+TODO: get handler running multiple times on some existing interrupt from /proc/interrupts.
+*/
+
+#include <linux/interrupt.h>
+#include <linux/jiffies.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+
+MODULE_LICENSE("GPL");
+
+/**
+ * Return value from kernel docs:*
+ *
+ * enum irqreturn
+ * @IRQ_NON         interrupt was not from this device or was not handled
+ * @IRQ_HANDLED     interrupt was handled by this device
+ * @IRQ_WAKE_THREAD handler requests to wake the handler thread
+ */
+static irqreturn_t handler(int i, void *v)
+{
+	pr_info("handler %llu\n", (unsigned long long)jiffies);
+	return IRQ_HANDLED;
+}
+
+int init_module(void)
+{
+	irqreturn_t r;
+	r = request_irq(
+		1,
+		handler,
+		IRQF_SHARED,
+		"myirqhandler0",
+		0
+	);
+	pr_info("request_irq %llu\n", (unsigned long long)r);
+	return 0;
+}
+
+void cleanup_module(void)
+{
+}

kernel_module/module_init.c

@@ -0,0 +1,26 @@
+/*
+Hello world with module_init / exit macros.
+
+TODO: vs direct init_module and cleanup_module.
+
+- modprobe only works with the macros. Try "modprobe module_init".
+*/
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+
+MODULE_LICENSE("GPL");
+
+static int module_init_init(void)
+{
+	pr_info("module_init init\n");
+	return 0;
+}
+
+static void module_init_exit(void)
+{
+	pr_info("module_exit cleanup\n");
+}
+
+module_init(module_init_init)
+module_exit(module_init_exit)