David Gibson - IBM

Linux on the eLAP

Paper (PDF) and Paper (TeX) and

This paper looks at Linux running on IBM's PowerPC 405LP PDA Reference Design (also known as the Embedded Linux Application Platform or eLAP). We look at some of the features of the CPU and hardware, and some of the issues involved in running Linux on such a device.

The 405LP is an embedded CPU designed especially for handheld applications. Like other PowerPC 4xx CPUs, it includes a number of peripheral devices built into the CPU die itself. In the case of the 405LP, these include an LCD controller, serial ports, an audio codec interface and an I2C interface amongst others. The 405LP also includes a number of novel power management features: as well as being able to power on and off the various chip components as necessary, it can adjust the CPU core and internal bus frequencies with very low latency allowing the the chip to provide high performance when necessary, but low-power operation the rest of the time.

The eLAP is intended to run Linux, and it is expected that IBM partners will make and sell PDAs based on this design. As a development device, it includes quite a number of external interfaces, including PCMCIA/Compact Flash, USB (both as a gadget and a host), Ethernet and SDIO.

We will examine what is involved in getting Linux to take advantage of the features of this hardware, focusing primarily on the kernel. We also look at what's necessary to deal with the peculiarities of such a device, where the hardware is very different from a typical PC or server. We pay particular attention to power management, which is clearly of great importance in a handheld device. We also look at how future work with the new Linux unified device model can simplify the handling of embedded devices.