From: Rafael J. Wysocki

Sent: 30 January 2020 14:47

In certain system configurations it may not be desirable to use some
C-states assumed to be available by intel_idle and the driver needs
to be prevented from using them even before the cpuidle sysfs
interface becomes accessible to user space.  Currently, the only way
to achieve that is by setting the 'max_cstate' module parameter to a
value lower than the index of the shallowest of the C-states in
question, but that may be overly intrusive, because it effectively
makes all of the idle states deeper than the 'max_cstate' one go
away (and the C-state to avoid may be in the middle of the range
normally regarded as available).

To allow that limitation to be overcome, introduce a new module
parameter called 'states_off' to represent a list of idle states to
be disabled by default in the form of a bitmask and update the
documentation to cover it.

The problem I see is that there are (at least) 3 different ways of
referring to the C-States:

1) The state names, C1, C1E, C3, C7 etc.
   I'm not sure these are visible outside intel_idle.c.
2) The maximum allowed latency in us.
3) The index into the cpu-dependant tables in intel_idle.c.

Boot parameters that set 3 are completely hopeless for normal
users. The C-state names might be - but they aren't documented.

Unless you know exactly which cpu table is being used the
only constraint a user can request is the latency.

(I've had the misfortune to read intel_idle.c in the last week.
Almost impenetrable TLA ridden uncommented code.)

...

+ * The positions of the bits that are set in the two's complement representation
+ * of this value are the indices of the idle states to be disabled by default
+ * (as reflected by the names of the corresponding idle state directories in
+ * sysfs, "state0", "state1" ... "state<i>" ..., where <i> is the index of the
+ * given state).

What has 'two's complement' got to do with anything?

...

+The value of the ``states_off`` module parameter (0 by default) represents a
+list of idle states to be disabled by default in the form of a bitmask.  Namely,
+the positions of the bits that are set in the two's complement representation of
+that value are the indices of idle states to be disabled by default (as
+reflected by the names of the corresponding idle state directories in ``sysfs``,
+:file:`state0`, :file:`state1` ... :file:`state<i>` ..., where ``<i>`` is the
+index of the given idle state; see :ref:`idle-states-representation` in
+:doc:`cpuidle`).  For example, if ``states_off`` is equal to 3, the driver will
+disable idle states 0 and 1 by default, and if it is equal to 8, idle state 3
+will be disabled by default and so on (bit positions beyond the maximum idle
+state index are ignored).  The idle states disabled this way can be enabled (on
+a per-CPU basis) from user space via ``sysfs``.

A few line breaks would make that easier to read.

	David

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