Using Linux:Managing the Kernel






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Enabling Advanced Power Management
Advanced Power Management provides a means to reduce the power a PC consumes, and controls features such as software power off as well. APM was around a long time before the standard was implemented, with significant 16-bit underpinnings, which makes support difficult in 32-bit operating systems. To make matters worse, desktop machines’ “Green” BIOSes often don’t comply fully with the established APM standard. My opinion is that it’s interesting to try APM even with a desktop machine—just make sure that you have a backup kernel that does not have APM enabled in case things go awry.

The APM driver for Linux was designed specifically for notebook PCs to use the CPU’s power-saving features. The driver does not provide support for hard disk spin-down (this is provided separately by the utility hdparm(8)), or VESA–compliant monitor power-saving, which can be enabled by editing a kernel source file. Notebooks, more so than desktop PCs, vary widely in their configuration and adherence to PC standards. This means that your mileage on this information will vary, probably not only from machine to machine, but also depending on the BIOS revision of a given line of machines from the same manufacturer.
Enabling APM is a simple case of checking the Enable Advanced Power Management under the Character Devices section of make xconfig. After you enable APM, there are a few additional choices to make:

•  Ignore User Suspend turns off the code that responds to the user suspend APM signal. This must be turned off if the machine hangs or the Linux kernel panics/OOPes (you know you’re there when your screen is full of hexadecimal numbers). The documentation specifically mentions the NEC Versa M series notebooks as qualifying for this special setting.
•  Enable PM at Boot Time makes the Linux kernel turn on APM as soon as the machine boots. Most machines do this automatically, but it’s worth experimenting with if power-saving does not seem to work. NEC Ultralite Versa 33/C and Toshiba 410CDT machines seem to experience problems when this is enabled, and this option is off by default.
•  Make CPU Idle calls when Idle makes the Linux kernel call the APM CPU idle routines when not busy. This setting results in significant power savings on machines where this works; it allows the APM BIOS to do things such as slow the CPU clock after the machine has been idle or at a low load for some time.
•  Enable Console Blanking Using APM allows the LCD backlight on your notebook PC’s LCD screen to be turned off when the stock Linux screen-blanker blanks the screen. This option has known problems with the gpm utility, and if you’re using this feature you should disable gpm using the command chkconfig ---del gpm.
•  Power Off on Shutdown causes your machine to turn off when the system’s halt command is issued, typically by the shutdown script. This feature is possibly of interest to owners of later desktop machines that support soft power-off, if their system has a compliant BIOS.

The main point to keep in mind here is that with the wide variety of implementations of APM in PC systems today, some or all of the features may not work with your particular PC. If you’re comfortable with reading C and assembler source code, the drivers/char/apm_bios.c file makes for interesting reading (this is subjective, of course) and contains plenty of notes about how varying machines implement APM.



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