cpuidle: Ignore interval prediction result when timer is shorter

This patch prevents cpuidle menu governor from using repeating interval
prediction result if the idle period predicted is longer than the one
allowed by shortest running timer.

Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
cpuidle: Rearrange code and comments in get_typical_interval()

This patch rearranges a if-return-elsif-goto-fi-return sequence into
if-return-fi-if-return-fi-goto sequence. The functionality remains the
same. Also, a lengthy comment that did not describe the functionality
in the order it occurs is split into half and top half is moved closer
to actual implementation it describes.

Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
cpuidle: Check called function parameter in get_typical_interval()

get_typical_interval() uses int_sqrt() in calculation of standard
deviation. The formal parameter of int_sqrt() is unsigned long, which
may on some platforms be smaller than the 64 bit unsigned integer used
as the actual parameter. The overflow can occur frequently when actual
idle period lengths are in hundreds of milliseconds.

This patch adds a check for such overflow and rejects the candidate
average when an overflow would occur.

Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
cpuidle: CodingStyle: Break up multiple assignments on single line

The function get_typical_interval() initializes a number of variables
that are immediately after declarations assigned constant values.
In addition, there are multiple assignments on a single line, which
is explicitly forbidden by Documentation/CodingStyle.

This patch removes redundant initial values for the variables and
breaks up the multiple assignment line.

Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
cpuidle: Fix menu_device->intervals type

Struct menu_device member intervals is declared as u32, but the value
stored is (unsigned) int. The type is changed to match the value being
stored.

Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
cpuidle: Fix variable domains in get_typical_interval()

The menu governor uses a static function get_typical_interval() to
try to detect a repeating pattern of wakeups. The previous interval
durations are stored as an array of unsigned ints, but the arithmetic
in the function is performed exclusively as 64 bit values, even when
the value stored in a variable is known not to exceed unsigned int,
which may be smaller and more efficient on some platforms.

This patch changes the types of varibles used to store some
intermediates, the maximum and and the cutoff threshold to unsigned
ints. Average and standard deviation are still treated as 64 bit values,
even when the values are known to be within the domain of unsigned int,
to avoid casts to ensure correct integer promotion for arithmetic
operations.

Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
cpuidle: Add a comment warning about possible overflow

The menu governor has a number of tunable constants that may be changed
in the source. If certain combination of values are chosen, an overflow
is possible when the correction_factor is being recalculated.

This patch adds a warning regarding this possibility and describes the
change needed for fixing the issue. The change should not be permanently
enabled, as it will hurt performance when it is not needed.

Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
cpuidle: Change struct menu_device field types

Field predicted_us value can never exceed expected_us value, but it has
a potentially larger type. As there is no need for additional 32 bits of
zeroes on 32 bit plaforms, change the type of predicted_us to match the
type of expected_us.

Field correction_factor is used to store a value that cannot exceed the
product of RESOLUTION and DECAY (default 1024*8 = 8192). The constants
cannot in practice be incremented to such values, that they'd overflow
unsigned int even on 32 bit systems, so the type is changed to avoid
unnecessary 64 bit arithmetic on 32 bit systems.

One multiplication of (now) 32 bit values needs an added cast to avoid
truncation of the result and has been added.

In order to avoid another multiplication from 32 bit domain to 64 bit
domain, the new correction_factor calculation has been changed from
new = old * (DECAY-1) / DECAY
to
new = old - old / DECAY,
which with infinite precision would yeild exactly the same result, but
now changes the direction of rounding. The impact is not significant as
the maximum accumulated difference cannot exceed the value of DECAY,
which is relatively small compared to product of RESOLUTION and DECAY
(8 / 8192).

Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Merge branch 'pm-cpuidle-next' into bleeding-edge

* pm-cpuidle-next:
  cpuidle: Change struct menu_device field types
  cpuidle: Add a comment warning about possible overflow
  cpuidle: Fix variable domains in get_typical_interval()
  cpuidle: Fix menu_device->intervals type
  cpuidle: CodingStyle: Break up multiple assignments on single line
  cpuidle: Check called function parameter in get_typical_interval()
  cpuidle: Rearrange code and comments in get_typical_interval()
  cpuidle: Ignore interval prediction result when timer is shorter