dev pci min works, non pci dev doesnt

kernel_module/hw_min.c

@@ -0,0 +1,75 @@
+/*
+TODO: how to setup a device with magic addresses without PCI?
+https://stackoverflow.com/questions/26965239/board-files-for-x86-based-systems
+
+request_mem_region returns NULL.
+
+TODO: which physical address is QEMU mapping?
+*/
+
+#include <asm/uaccess.h>
+#include <linux/cdev.h>
+#include <linux/errno.h>
+#include <linux/fs.h>
+#include <linux/init.h>
+#include <linux/interrupt.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <uapi/linux/stat.h> /* S_IRUSR | S_IWUSR */
+
+#define CDEV_NAME "lkmc_hw_min"
+#define MMIO_LEN 8
+#define IRQ_RAISE 0
+#define IRQ_ACK 4
+
+MODULE_LICENSE("GPL");
+
+static unsigned long mmio_addr = 0;
+module_param(mmio_addr, ulong, S_IRUSR | S_IWUSR);
+
+static int major;
+static int irq;
+static void __iomem *mmio;
+
+static struct file_operations fops = {
+	.owner   = THIS_MODULE,
+};
+
+static irqreturn_t irq_handler(int irq, void *dev)
+{
+	pr_info("irq_handler irq = %d dev = %d\n", irq, *(int *)dev);
+	iowrite32(0, mmio + IRQ_ACK);
+	return IRQ_HANDLED;
+}
+
+static int myinit(void)
+{
+	pr_info("init\n");
+	major = register_chrdev(0, CDEV_NAME, &fops);
+	mmio = (void __iomem *)mmio_addr;
+	if (request_mem_region(mmio_addr, MMIO_LEN, CDEV_NAME) == NULL) {
+		pr_info("request_mem_region\n");
+		goto error;
+	}
+	ioremap(mmio_addr, 8);
+	irq = 10;
+	if (request_irq(irq, irq_handler, IRQF_SHARED, CDEV_NAME, &major) < 0) {
+		pr_info("request_irq\n");
+		goto error;
+	}
+	iowrite32(0x12345678, mmio + IRQ_RAISE);
+	return 0;
+error:
+	return -ENODEV;
+}
+
+static void myexit(void)
+{
+	pr_info("exit\n");
+	free_irq(irq, &major);
+	release_mem_region(mmio_addr, MMIO_LEN);
+	unregister_chrdev(major, CDEV_NAME);
+}
+
+module_init(myinit);
+module_exit(myexit);