cpufreq: Introduce an optional cpuinfo_avg_freq sysfs entry

Currently the CPUFreq core exposes two sysfs attributes that can be used
to query current frequency of a given CPU(s): namely cpuinfo_cur_freq
and scaling_cur_freq. Both provide slightly different view on the
subject and they do come with their own drawbacks.

cpuinfo_cur_freq provides higher precision though at a cost of being
rather expensive. Moreover, the information retrieved via this attribute
is somewhat short lived as frequency can change at any point of time
making it difficult to reason from.

scaling_cur_freq, on the other hand, tends to be less accurate but then
the actual level of precision (and source of information) varies between
architectures making it a bit ambiguous.

The new attribute, cpuinfo_avg_freq, is intended to provide more stable,
distinct interface, exposing an average frequency of a given CPU(s), as
reported by the hardware, over a time frame spanning no more than a few
milliseconds. As it requires appropriate hardware support, this
interface is optional.

Note that under the hood, the new attribute relies on the information
provided by arch_freq_get_on_cpu, which, up to this point, has been
feeding data for scaling_cur_freq attribute, being the source of
ambiguity when it comes to interpretation. This has been amended by
restoring the intended behavior for scaling_cur_freq, with a new
dedicated config option to maintain status quo for those, who may need
it.

CC: Jonathan Corbet <corbet@lwn.net>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Ingo Molnar <mingo@redhat.com>
CC: Borislav Petkov <bp@alien8.de>
CC: Dave Hansen <dave.hansen@linux.intel.com>
CC: H. Peter Anvin <hpa@zytor.com>
CC: Phil Auld <pauld@redhat.com>
CC: x86@kernel.org
CC: linux-doc@vger.kernel.org

Signed-off-by: Beata Michalska <beata.michalska@arm.com>
Reviewed-by: Prasanna Kumar T S M <ptsm@linux.microsoft.com>
Reviewed-by: Sumit Gupta <sumitg@nvidia.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20250131162439.3843071-3-beata.michalska@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 16 insertions, 1 deletions

diff --git a/Documentation/admin-guide/pm/cpufreq.rst b/Documentation/admin-guide/pm/cpufreq.rst
index a21369eba034..3950583f2b15 100644
--- a/Documentation/admin-guide/pm/cpufreq.rst
+++ b/Documentation/admin-guide/pm/cpufreq.rst
@@ -248,6 +248,20 @@ are the following:
 	If that frequency cannot be determined, this attribute should not
 	be present.
 
+``cpuinfo_avg_freq``
+        An average frequency (in KHz) of all CPUs belonging to a given policy,
+        derived from a hardware provided feedback and reported on a time frame
+        spanning at most few milliseconds.
+
+        This is expected to be based on the frequency the hardware actually runs
+        at and, as such, might require specialised hardware support (such as AMU
+        extension on ARM). If one cannot be determined, this attribute should
+        not be present.
+
+        Note, that failed attempt to retrieve current frequency for a given
+        CPU(s) will result in an appropriate error, i.e: EAGAIN for CPU that
+        remains idle (raised on ARM).
+
 ``cpuinfo_max_freq``
 	Maximum possible operating frequency the CPUs belonging to this policy
 	can run at (in kHz).
@@ -293,7 +307,8 @@ are the following:
 	Some architectures (e.g. ``x86``) may attempt to provide information
 	more precisely reflecting the current CPU frequency through this
 	attribute, but that still may not be the exact current CPU frequency as
-	seen by the hardware at the moment.
+	seen by the hardware at the moment. This behavior though, is only
+	available via c:macro:``CPUFREQ_ARCH_CUR_FREQ`` option.
 
 ``scaling_driver``
 	The scaling driver currently in use.