RE: [PATCH 2/2] intel_idle: Introduce 'states_off' module parameter

[David Laight]

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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