Main Page | Namespace List | Class Hierarchy | Class List | File List | Namespace Members | Class Members | File Members | Related Pages

Implementation of Workarounds and Exceptions

Overview

A sad fact of life is that software has bugs. The system BIOS is no exception, and facilities must be designed in from the ground up to deal with exceptions in SMBIOS table where important and critical data in the SMBIOS table is wrong. If the design does not take this fact into account, then over time the design will be compromised as workaround after workaround are inserted into the code to fixup problems.

It is with this in mind that a standard way to fixup buggy and broken tables was designed. A few points were kept in mind when doing the design:

Prevention is the best medicine

One outcome of the libsmbios project is a tool that can be used to validate BIOS conformance to all of the relevant specifications. This tool, validateBios (on Linux) and validateBios.exe (on Windows) can be used to verify that all BIOS properly implements all specifications.

The validateBios executable is a natural outgrowth of the unit test suite. When first written, the purpose of the unit test suite was to find bugs in the libsmbios code and prevent them from progressing beyond an individual developer workstation. This approach was highly successfull, but after a certain point, the unit test suite went beyond finding bugs in libsmbios code and started finding bugs in the BIOS test data sets that were used.

At this point, it was easy to just paste in the failing functions into a separate executable that can be distributed to the BIOS team. This has been done, and work to add more tests to this executable are ongoing. We hope that the BIOS team adopts the libsmbios validateBios executable as part of their standard testing methodology for testing all potential BIOS releases.

Theory of Operation

The fundamental point upon which the workaround concept is based is that the ISmbiosTable object will normally have a buffer of the pristine SMBIOS table, as read from memory. The ISmbiosItem class gets its own copy of its individual data buffer when it is instantiated from the table. The workaround framework takes advantage of this fact to fixup the raw ISmbiosItem data after it is instantiated, but before the client code has a chance to ever look at potentially wrong data in the Item.

This insertion into the instantiation, and fixup of the item's raw data meets most of the design objectives for the workaround design.

Implementation

The ISmbiosTable object contains a Workaround object. This workaround object is initialized at the same time that the table is created. The workaround initialization process uses the raw table data to initialize enough of itself to select the appropriate set of workarounds for the system it is running on. After the workaround object has been initialized, workarounds are active.

Creation of ISmbiosItem objects is centrally controlled by the ISmbiosTable::makeItem() function. It is here where the workaround fixups are applied. First, the item is created normally. Next, the item's ->fixup() method is called with the table's workaround instance as a parameter.

The next step is the ISmbiosItem calls the workaround object's ->fixupItem() function call. The parameters are the item's "this" pointer, a pointer to the item's raw buffer, and the size of the raw buffer.

The purpose of this circuitous chain of function calls is to ensure the following:


Generated on Wed Apr 11 16:25:11 2007 for SMBIOS Library by doxygen 1.3.5