Re: [PATCH v3 1/5] x86/mwait-idle: add 'preferred-cstates' command line option

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Aug 18, 2022 at 03:03:33PM +0200, Jan Beulich wrote:
> From: Artem Bityutskiy <artem.bityutskiy@xxxxxxxxxxxxxxx>
> 
> On Sapphire Rapids Xeon (SPR) the C1 and C1E states are basically mutually
> exclusive - only one of them can be enabled. By default, 'intel_idle' driver
> enables C1 and disables C1E. However, some users prefer to use C1E instead of
> C1, because it saves more energy.
> 
> This patch adds a new module parameter ('preferred_cstates') for enabling C1E
> and disabling C1. Here is the idea behind it.
> 
> 1. This option has effect only for "mutually exclusive" C-states like C1 and
>    C1E on SPR.
> 2. It does not have any effect on independent C-states, which do not require
>    other C-states to be disabled (most states on most platforms as of today).
> 3. For mutually exclusive C-states, the 'intel_idle' driver always has a
>    reasonable default, such as enabling C1 on SPR by default. On other
>    platforms, the default may be different.
> 4. Users can override the default using the 'preferred_cstates' parameter.
> 5. The parameter accepts the preferred C-states bit-mask, similarly to the
>    existing 'states_off' parameter.
> 6. This parameter is not limited to C1/C1E, and leaves room for supporting
>    other mutually exclusive C-states, if they come in the future.
> 
> Today 'intel_idle' can only be compiled-in, which means that on SPR, in order
> to disable C1 and enable C1E, users should boot with the following kernel
> argument: intel_idle.preferred_cstates=4
> 
> Signed-off-by: Artem Bityutskiy <artem.bityutskiy@xxxxxxxxxxxxxxx>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Origin: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 
> da0e58c038e6
> 
> Enable C1E (if requested) not only on the BSP's socket / package. Alter
> command line option to fit our model, and extend it to also accept
> string form arguments.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

With the indentation fixes.

Thanks, Roger.