platform_device（Platform Device）

Platform Device

Introduction

The platform_device is an essential concept in Linux kernel programming. It provides a mechanism for device drivers to register their devices with the kernel. This allows the kernel to manage and interact with the devices in a standardized and efficient manner. In this article, we will explore the platform_device framework, its features, and how it is used in Linux kernel development.

Understanding Platform Devices

Platform devices are a type of device found in embedded systems that do not conform to traditional device buses like PCI or USB. Instead, these devices are built directly onto the system's platform, usually as part of the system-on-chip (SoC) design. Examples of platform devices include sensors, I/O controllers, and display interfaces.

The platform_device framework provides a way for these devices to be identified, configured, and accessed by the kernel and other device drivers. It allows device drivers to register their devices with the kernel, specifying various attributes like memory ranges, IRQs, and platform-specific data that are necessary for proper device operation. Once registered, the kernel can manage these devices and allow other drivers to interact with them.

Registering Platform Devices

To register a platform device, a device driver must follow a few steps. First, it needs to define a platform_driver structure that specifies the device driver's name, probe and remove callback functions, and any necessary flags or device-specific data.

Next, the platform driver needs to call the platform_driver_register() function, passing in the platform_driver structure as an argument. This function registers the platform driver with the kernel, allowing it to match and bind with any corresponding platform devices.

Once the platform driver is registered, the driver's probe() function will be called whenever a matching platform device is found. In this callback function, the driver can initialize the device, allocate resources, and perform any necessary configuration. Conversely, the remove() function is called when the platform device is removed or during system shutdown, allowing the driver to clean up and release any resources used by the device.

Platform Data and Device Trees

Platform data is a common mechanism used by device drivers to provide platform-specific information to the kernel. It allows for customization and configuration of devices on a platform-by-platform basis. Platform data can include information such as GPIO pins, IRQ numbers, and other device-specific settings.

Device trees, on the other hand, provide a more flexible and portable way of describing the hardware configuration of a platform. They are typically used in embedded systems and are written in a device tree language (DTL) that describes the relationships and properties of platform devices. The device tree is compiled into a binary form by the device tree compiler (dtc) and is loaded by the bootloader or firmware at system boot.

Both platform data and device trees can be used to provide platform-specific information to the kernel during device registration. The platform_device structure contains a pointer to the platform-specific data and can be accessed by a device driver using the dev_get_platdata() function. This allows the driver to retrieve any necessary configuration or settings required for proper device operation.

Conclusion

The platform_device framework is a crucial part of Linux kernel programming, especially in embedded systems. It allows device drivers to register their devices with the kernel and enables standardized and efficient device management. Understanding the platform_device framework and how to use it is essential for developers working on embedded Linux systems.

In this article, we explored the concept of platform devices, the registration process for platform drivers, and the role of platform data and device trees in device configuration. Armed with this knowledge, developers can effectively leverage the platform_device framework to build robust and reliable device drivers for their embedded systems.