Modern Computer: Full Stack

ECE 440 · Week 4 · Lecture

How a driver meets its device

• The device tree describes a node: a compatible string, registers, interrupts.
• A platform driver lists the compatible strings it handles.
• When the two strings match, the kernel calls the driver's probe() with that device.
• No match, no probe, and no error either: silence is the first thing to debug.

ECE 440 · Week 4 · Lecture

The smallest platform driver

static const struct of_device_id mydev_ids[] = {
	{ .compatible = "vendor,mydev" },
	{ }
};
MODULE_DEVICE_TABLE(of, mydev_ids);

static struct platform_driver mydev_driver = {
	.probe  = mydev_probe,
	.driver = {
		.name = "mydev",
		.of_match_table = mydev_ids,
	},
};
module_platform_driver(mydev_driver);

ECE 440 · Week 4 · Lecture

probe(): map the registers, read one

static int mydev_probe(struct platform_device *pdev)
{
	void __iomem *base;
	u32 val;

	base = devm_platform_ioremap_resource(pdev, 0);
	if (IS_ERR(base))
		return PTR_ERR(base);

	val = readl(base);
	dev_info(&pdev->dev, "bound, reg0=0x%08x\n", val);
	return 0;
}

ECE 440 · Week 4 · Lecture

Reading the bind line

• dev_info() prefixes the driver name and the device name, so the line says who bound to what.
• dmesg shows it after insmod; rmmod then insmod again shows a second one.
• /sys/bus/platform/drivers/<name>/ lists the devices a driver holds.
• A negative return from probe() leaves an error in the log and no device in that directory.

ECE 440 · Week 4 · Lecture

What stays in userspace

Put in the kernel only what needs the hardware or its timing.