How to port an OS to a SoC?

How to port an OS to a SoC?

Ensuring that a System on a Chip works with a particular OS is no simple task and requires specialized skills. We delve into details of how to port an OS to a SoC in more detail

Sudipto Chanda

Wednesday, December 17, 2008

Board Support Package
The BSP for an operating system is an implementation specific code for a given HW board containing the CPU and other peripheral devices such as memory and memory controllers, timers, interrupt controllers, USB, Ethernet etc. In case of a SoC, a large part of the 'board' is inside the IC, on the same silicon chip as the CPU core itself.

The BSP provides an abstraction of the hardware (CPU and peripherals) to the generic OS interfaces. It facilitates communication between the OS and target HW device and includes code to manage memory, handle interrupts, timers, power management, bus abstraction, generic I/O control codes (IOCTLs), and so on.

The BSP is one of the most complex parts of the OS porting and integration effort and contains code for the following essential OS services:

Memory Management: The OS may use virtual memory to manage the RAM. The BSP contains code to initialize the Memory Management Unit (MMU) and provide an interface to the higher level OS layers to setup the memory access data structures such as page tables.
Timer: The OS uses the timer for many functions including task scheduling – which is vital for its functioning. The BSP contains code to initialize the timer hardware as per the needs of the OS.
Interrupt Handler: All peripherals to the CPU, such as Serial Bus, USB, Camera Interface etc. typically use interrupts to signal the CPU about data availability or data transfer completions. The CPU then executes the appropriate Interrupt service routines (ISR) to take the necessary actions. The BSP initializes the Interrupt mechanism.
Power Management: The OS needs to conserve power on most devices by providing power saving modes such as Full Power, Hibernation, Sleep etc. When the system restarts, it needs to invoke the correct code to handle the wake-up. The BSP initializes the power management system of the SoC.

Many more functionalities related to the OS operations are a part of the BSP, which need to be implemented or ported in order to make the OS utilize all features of the underlying SoC.

Device Drivers
These are software components that provide an interface to the hardware peripherals connected to the CPU. Each peripheral will have its own device driver. The device driver allows the setting of various parameters associated with the operation of a peripheral, transfer data from CPU to the peripheral or vice-versa, provides polling or interrupts based signalling mechanism for initiation/completion of operation.

It also implements the ISR for the peripheral.For example, the device driver for a camera interface may implement the following functionality:

Setting the resolution: the camera may supports two resolutions – 320x240 pixels, 640x480 pixels, and the device driver will allow applications to choose between these two modes. For this purpose the driver usually accesses a HW register provided by the Camera Interface, which contains bits to set the resolution.

The driver in turn provides an API through the OS to the applications which is independent of the camera hardware.

Data transfer: copy the captured image data from the camera interface's memory to the CPU's main memory for further processing.
Interrupt the CPU, to indicate capture of a frame or multiple frames, and implement the ISR.
Developing a device driver requires a thorough understanding of the hardware peripheral, including its register set, the data transfer mechanism and the interrupt mechanism.

Drivers are critical components which operate very close to the OS kernel and run in privileged modes. Any bug in a device driver can cause unwanted operation or even crash the system. In addition, each OS provides its own device driver architecture and developers need to understand and adhere to these specifications.

Middlewar
Today's high level operating systems provide a rich set of middleware components which greatly shorten the time taken to develop complex applications. The Middleware components include: