Porting LinuxBIOS to the AMD SC520
A portion of the LinuxBIOS directory tree structure is shown in Figure 1. Starting at the top of the tree, there are three main directories: src, targets and util. The src directory contains all the source for all the BIOSes—all mainboards, all CPUs, all devices and so on. You build a specific BIOS in the target directory using a config file. For example, for our project, we built our BIOS in the targets/digitallogic/msm586seg directory, using the file Config.lb in that directory. Finally, the util directory contains many utilities used to create BIOS files or to burn the BIOS image into the motherboard Flash part.
Configuration files in LinuxBIOS describe resources and how they are used in the construction of a target. Each resource can have a set of options defined for it. The set of all available options is defined in one file, src/config/Options.lb; only options defined in that file may be used or set in configuration files. Once a resource is named in a configuration file, resources defined within the scope of that resource inherit the options settings for that resource. The options have lexical scope; once the block for the resource ends, the options revert to values they had before the block was started. Options may have a default value set in the Options.lb file, or it may not be set; they may have a default value set in the mainboard configuration file; or they may be set in the target configuration file. To avoid the confusion we saw in earlier versions of the configuration tool, options may be set in only a few places: the target file, the mainboard file and CPU files. Options may be set only once. Thus, an option may have a default value, which can be changed once and only once in a configuration file. Forcing the set-once rule avoids problems we saw earlier with dueling configuration files.
A full writeup on the configuration language would consume this entire article. Therefore, this article touches on the important points, but we cannot cover all the aspects of the configuration language.
In all mainboards, some resource hardware can be queried to determine what other resources it needs, for example, how much memory and I/O space it needs. There also is hardware that cannot be queried, such as the wires that wire a PCI slot to an interrupt controller. For the latter type of resource, the only way to tell the BIOS about it is to put the information directly into the BIOS. Unfortunately, this information is contained in many places in PC BIOSes. Interrupt routing may be found in the $PIR (uniprocessor), _MP_ (multiprocessor or IO-APIC) or ACPI tables. The configuration tool must generate these tables, but the user in turn must tell the tool what values go in the tables.
Super I/O chips cannot be queried dynamically, and the location in I/O space and type of Super I/O chip must be specified in the mainboard configuration file.
Newer PC mainboards are harder to figure out at runtime. For example, Opteron processors have three HyperTransport ports that can be wired in arbitrary configurations on different mainboards. The configuration file for a mainboard has to specify how these ports are wired.
On modern systems, with Synchronous DRAM chips, the memory is not accessible until a lot of setup has been done. The size and parameters of the DRAM are read in over a two-wire bus called the SMBUS. Thus, in order to establish working memory, the BIOS has to:
Turn on the chipset to some extent.
Enable the SMBUS, usually on a Super I/O or southbridge.
Read in parameters of DRAM over SMBUS; more than 20 in some cases.
Perform complex calculations to determine timing.
Initialize DRAM control registers with proper values.
Perform a complex sequence of reads not writes from DRAM to get it running.
All this has to be done without a stack, which means that function calls and variables are almost impossible to use. Without memory, programming is limited to the registers. Function calls can be made only one level deep. In the bad old days, a big, bad ball of assembly code was used to get this work done. Expert assembly code writers used every trick in the book to get this code working. Writing this code is the single hardest part of any BIOS.
In 2002, Eric Biederman of Linux NetworX developed a compiler called romcc. romcc is a simple optimizing C compiler—one file, 25,043 lines of code—that uses only registers, not memory. The compiler can use extended register sets such as MMX, SSI or 3DNOW. romcc allowed us to junk almost all of the assembly code in LinuxBIOS, so that even the earliest code, run with no working DRAM, can be written in C.
romcc is used only for early, pre-memory code. For code that runs after memory comes up, we use GCC.