Demo module, initially for RPI

demo_module/Makefile

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+# This is a standard makefile for building a kernel module.
+# It looks a bit backwards compared to a normal makefile, but that's because
+# the kernel supplies its own sub-makefile that we use to build the module.
+# 
+# The M= option tells the kernel's makefile where to find the module's source
+# files, and the -C option sets the context directory for make.
+#
+# The obj-m variable tells the kernel's makefile what the module's object file
+# should be called. In this case, it's demo_module.o. This is how the sub-makefile
+# infers the name of the module's source file.
+# 
+# This build is highly sensitive to compiler version, and requires the system gcc
+# to be the same version as the gcc that built the current kernel.
+#
+# For more info, see: https://www.kernel.org/doc/html/latest/kbuild/modules.html
+
+obj-m = demo_module.o
+
+all:
+	make -C /lib/modules/$(shell uname -r)/build/ M=$(PWD) modules
+
+clean:
+	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

demo_module/demo_module.c

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+#include <linux/gpio/consumer.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+
+/*
+ * Last compiled with Linux 6.14.5-300.fc42.x86_64
+ */
+
+/* Meta Info */
+MODULE_DESCRIPTION("Demo kernel module.");
+MODULE_VERSION("0.0.1");
+MODULE_AUTHOR("Imbus");
+MODULE_LICENSE("GPL");
+
+/*
+ * Hard coded for RPI3
+ * Usually retrieved via device tree
+ */
+#define IO_LED 21
+#define IO_BUTTON 20
+#define IO_OFFSET 0
+
+static struct gpio_desc *led, *button;
+
+/**
+ * @brief Called on load
+ */
+static int __init demo_mod_init(void) {
+  printk(KERN_INFO "Demo module loaded successfully...");
+
+  int status;
+
+  led = gpio_to_desc(IO_LED + IO_OFFSET);
+  if (!led) {
+    printk("gpioctrl - Error getting pin 21\n");
+    return -ENODEV;
+  }
+
+  button = gpio_to_desc(IO_BUTTON + IO_OFFSET);
+  if (!button) {
+    printk("gpioctrl - Error getting pin 20\n");
+    return -ENODEV;
+  }
+
+  status = gpiod_direction_output(led, 0);
+  if (status) {
+    printk("gpioctrl - Error setting pin 20 to output\n");
+    return status;
+  }
+
+  status = gpiod_direction_input(button);
+  if (status) {
+    printk("gpioctrl - Error setting pin 21 to input\n");
+    return status;
+  }
+
+  gpiod_set_value(led, 1);
+
+  printk("gpioctrl - Button is %spressed\n",
+         gpiod_get_value(button) ? " " : "not ");
+
+  return 0;
+}
+
+/**
+ * @brief Called on unload
+ */
+static void __exit demo_mod_exit(void) {
+  printk(KERN_INFO "Demo module unloaded successfully...");
+}
+
+module_init(demo_mod_init);
+module_exit(demo_mod_exit);