Summary

A buggy BIOS can cause many different and subtle problems to Linux, ranging from reboot problems, incorrect battery power readings, suspend/resume not working correctly, and strange ACPI issues.

This page describes some of the issues which can cause problems and may help you to troubleshoot broken BIOS issues.

Buggy DSDT

Sometimes a BIOS will contain a buggy Differentiated System Description Table (DSDT)and can cause subtle problems for the Linux ACPI driver.

The (DSDT) can be disassembled using the Intel iasl disassembler tool. Install iasl using:

sudo apt-get install iasl acpidump

and get the DSDT data and disassemble it as follows:

sudo acpidump -o acpidump.txt
acpixtract acpidump.txt
iasl -d dsdt.dat ssdt*.dat

Note that acpixtract can produce additional files. Often those are SSDT#.dat files which are suplemental system description tables which can hold additions to the DSDT. The files can be disassembled the same way the main DSDT.dat is done.

More information about the DSDT can be found in the ACPI specification, which can be downloaded from http://www.acpi.info/

Although understanding the disassembled DSDT can be challenging, it can be worth the effort to understand what the ASL code is trying to do as sometimes there are coding errors which lead to very subtle ACPI problems.

Using the Intel DSDT disassembler has proved very revealing in some cases. For example, one BIOS had a race condition in the initialisation of the Embedded Controller. The Linux ACPI driver was reading values form the Embedded Controller before the controller was fully ready, and hence incorrect values were being returned to the Linux kernel for various devices, such as the smart battery etc..

Another bug found in the DSDT was from the sloppy use of the ASL Acquire() operator used to obtain a mutex. The Acquire() operator has a timeout argument, which can be 0xFFFF (wait forever to acquire a mutex) or a timeout in milliseconds. For the non-infinite wait there should be a check to see if the Acquire worked correctly or timed-out. We have seen code that does not check the mutex timeout and hence race conditions have occurred causing the corruption of settings in the embedded controller and misreadings by the Linux ACPI driver.

Rule: Always check return values from mutex Acquire() operators when using finite timeouts.

BIOS checking tools

If you suspect your BIOS is not behaving correctly, then it is worth using the Firmware Test Suite (fwts) to automatically check for incorrect BIOS behaviour.

To install, use:

sudo apt-get install fwts

..and to run the automated batch tests use:

sudo fwts

The test results will be automatically appended to a log file "results.log"

We suggest running this tool and carefully looking at the log of errors it reports as a first pass in seeing any glaring problems.

DSDT handling Operating System Variants

Most BIOS code check OS variants keyed off the _OSI and _OS objects. Most BIOS that invoke OSI(Linux) do nothing with it, but others that do cause Linux to break in different ways (supsend/resume, Video reposting, etc). The Linux ACPI driver disables OSI(Linux) by default, with the hope that it discourage BIOS writers from using it.

Linux will continue to claim OSI compatibility with Windows until the day when the majority of Linux systems have passed a Linux compatibility test rather than a Windows compatibility test.

ACPI _BIF method

The ACPI _BIF method provides the kernel with battery specific information. Hence if it is incorrect it can fool high level applications such as gnome-power-manager to shut a system down when it believes power has reached a critically low point.

A broken _BIF method has been known to caused some power management headaches. For example, it is important that the "Design Capacity of Warning" and "Design Capacity of Low" are non-zero, otherwise the gnome-power-manager cannot easily determine a correct strategy for hibernating or shutting down a PC when the battery becomes critically low.

The Firmware Test Suite "method" test will will sanity check many of the common ACPI methods, such as _BIF. To run this test, use:

sudo fwts method

Generally, packages should have all their fields set without relying on any defaults to zero. It is a sloppy BIOS practice to omit the setting of fields in packages - fields need to be set - missing fields lead to bugs which can fool the kernel or applications that rely on specific ACPI information being provided correctly.

Reboot Methods

A PC can be rebooted by Linux using several different strategies, selectable by the kernel reboot= boot option. These strategies are:

• Putting the processor back into real mode and jumping to the BIOS reset address (reboot=b)
• Keyboard controller reset by writing 0xfe to port 0x64 (reboot=k)
• Forcing the processor to triple fault (reboot=t)
• By forcing a Intel PCI reset by writing 0x2 and then 0x04 to port 0xCF9 (reboot=p)
• By writing a magic value to a port/register, as specified by ACPI FACP values RESET_VALUE and RESET_REG (an "ACPI reset") (reboot=a)

The last method ("ACPI reset") has shown to be problematic with one particular BIOS, as the RESET_VALUE and RESET_REG were values 0x06 and 0xCF9 which tries to do a Intel PCI style reset but only worked in 90% of reboots. This is because the reset should be performed in two stages (a write of 0x2, a port delay, and then a write of 0x04) rather than just one write of (0x02 | 0x04).

For debugging purpose it is possible to add "reboot=<letter>" to the kernel command line in order to enforce a particular reboot method)

Suspend/Resume and Hibernate/Resume

BIOS issues can cause suspend/resume to fail. You can use the Firmware Test Suite to soak test Suspend/Resume (S3) and Hibernate/Resume (S4). Use the --s3-multiple and --s4-multiple options to specify the number of iterations. For example..

Suspend Resume:

sudo fwts s3 --s3-multiple=20

Hibernate Resume:

sudo fwts s4 --s4-multiple=20

Debugging Suspend/Resume is now explained at: https://wiki.ubuntu.com/DebuggingKernelSuspend.

Debugging Hibernate/Resume is now explained at: https://wiki.ubuntu.com/DebuggingKernelHibernate.

See Also

• https://wiki.ubuntu.com/Kernel/Reference/fwts

• https://wiki.ubuntu.com/DebuggingACPI

• ftp://ftp.suse.com/pub/people/trenn/ACPI_BIOS_on_Linux_guide/acpi_guideline_for_vendors.pdf

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