[ltp] WARNING: Lenovo ThinkPads and 65W power supplies

[Henrique de Moraes Holschuh]

This is a warning to owners of any Lenovo ThinkPads that own 65W AC power
adaptors.

Apparently, the 65W power supplies alone are NOT sufficient to power the
notebook in all scenarios.  The battery can be used to supply power
during demand peaks that the external power supply cannot meet, and the
firmware knows this.

As a safety measure, the firmware attempts to identify whether you have
a underpowered power supply or not, and will attempt to limit CPU power
usage (by limiting its maximum allowed clock frequency) if you try to
use such a power supply WITHOUT a battery.

We know for sure the Z61, T60 and T410 are affected.  That means there
is a very high probability that just about every recent Lenovo ThinkPad
have the same limitation, except maybe the ones with ultra-low-voltage
processors.

In order for the protection mechanism to work correctly, it is required
that the operating system correctly process the PPC ACPI mechanism, and
that cpufreq support is active and working.

You are advised to:

  1.  Use high-powered power supplies (90W seems enough) with your
      Lenovo ThinkPad during high power-draw workloads (i.e. loads where
      there is heavy demand of the CPU or discrete GPU).  This allows
      the ThinkPad to operate on its higher speeds during the entire
      workload.

  2.  Never use the 65W power adapter *WITHOUT* a battery, just in case
      the operating system is not doing its job properly;

  3.  ALWAYS use the latest firmware (EC and BIOS) available.  You have
      been warned.

If your usage patterns does include the use of a 65W power adapter
*WITHOUT* a charged battery:

  1.  Use the latest Linux stable kernels, as there were recent fixes
      to the ACPI PPC handling;

  2.  Make sure the CPUFREQ system is working (i.e. cpu frequency limits
      are available), ACPI is operational (NEVER operate any ThinkPad
      manufactured in the last 5 years with ACPI deactivated), and if
      at all possible, that CPUIDLE (ACPI C states) are also working.
      "powertop" will show both CPUFREQ and CPUIDLE operation.

  3.  Never *EVER* tell the kernel ACPI drivers to ignore the ACPI PPC
      method in a ThinkPad (i.e. never use the ignore_ppc=1 kernel
      parameter).  If you have any PPC-related problem, *report a bug*,
      don't just try to work around it.

It is worth noting that, if the operating system does its job correctly
(and you did not braindamage it on purpose by using certain kernel
parameters) your data is NEVER in a dangerous situation, as the laptop
firmware will take steps to make sure it will have enough power to
operate properly.

This has been brought to our attention due to a bug in the Linux kernel
ACPI handling of the "_PPC" method which interacted badly with the T410
firmware.  I am not sure what the situation with very old kernels is,
but for 2.6.32 and later kernels, you are strongly advised to use the
latest stable kernels, which have been fixed to handle _PPC correctly.

As a general rule, don't use any recent Lenovo ThinkPad with a 65W power
adapter without a battery.  These boxes really want a high-power (90W or
more) power adapter.

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh