Problem with CPU frequency scaling

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Symptom

If the battery pack is removed and the laptop is powered by AC only, the CPU downclocks and remains locked in low speed. This happens irrespective of the scaling method in use (kernelspace or userspace) and of the frequency governor selected (ondemand, performance, etc.).

Output of cpufreq-info (on AC power without battery):

 analyzing CPU 0:
 driver: centrino
 CPUs which need to switch frequency at the same time: 0 1
 hardware limits: 1000 MHz - 1.83 GHz
 available frequency steps: 1.83 GHz, 1.33 GHz, 1000 MHz
 available cpufreq governors: ondemand, userspace, powersave, performance
 current policy: frequency should be within 1000 MHz and 1000 MHz.
                 The governor "ondemand" may decide which speed to use
                 within this range.
 current CPU frequency is 1000 MHz (asserted by call to hardware).
 
 analyzing CPU 1:
 driver: centrino
 CPUs which need to switch frequency at the same time: 0 1
 hardware limits: 1000 MHz - 1.83 GHz
 available frequency steps: 1.83 GHz, 1.33 GHz, 1000 MHz
 available cpufreq governors: ondemand, userspace, powersave, performance
 current policy: frequency should be within 1000 MHz and 1000 MHz.
                 The governor "ondemand" may decide which speed to use
                 within this range.
 current CPU frequency is 1000 MHz (asserted by call to hardware).

Output of cpufreq-info (on AC power with battery plugged in):

 analyzing CPU 0:
 driver: centrino
 CPUs which need to switch frequency at the same time: 0 1
 hardware limits: 1000 MHz - 1.83 GHz
 available frequency steps: 1.83 GHz, 1.33 GHz, 1000 MHz
 available cpufreq governors: ondemand, userspace, powersave, performance
 current policy: frequency should be within 1000 MHz and 1.83 GHz.
                 The governor "ondemand" may decide which speed to use
                 within this range.
 current CPU frequency is 1000 MHz (asserted by call to hardware).
 
 analyzing CPU 1:
 driver: centrino
 CPUs which need to switch frequency at the same time: 0 1
 hardware limits: 1000 MHz - 1.83 GHz
 available frequency steps: 1.83 GHz, 1.33 GHz, 1000 MHz
 available cpufreq governors: ondemand, userspace, powersave, performance
 current policy: frequency should be within 1000 MHz and 1.83 GHz.
                 The governor "ondemand" may decide which speed to use
                 within this range.
 current CPU frequency is 1000 MHz (asserted by call to hardware).

Solution

With the battery pack inserted, the CPU frequency scales up/down as expected.