On 09/13/2016 07:09 PM, Rafael J. Wysocki wrote:

On Wednesday, July 20, 2016 03:10:04 PM Al Stone wrote:

quoted

When CPPC is being used by ACPI on arm64, user space tools such as
cpupower report CPU frequency values from sysfs that are incorrect.

What the driver was doing was reporting the values given by ACPI tables
in whatever scale was used to provide them.  However, the ACPI spec
defines the CPPC values as unitless abstract numbers.  Internal kernel
structures such as struct perf_cap, in contrast, expect these values
to be in KHz.  When these struct values get reported via sysfs, the
user space tools also assume they are in KHz, causing them to report
incorrect values (for example, reporting a CPU frequency of 1MHz when
it should be 1.8GHz).

The downside is that this approach has some assumptions:

   (1) It relies on SMBIOS3 being used, *and* that the Max Frequency
   value for a processor is set to a non-zero value.

   (2) It assumes that all processors run at the same speed, or that
   the CPPC values have all been scaled to reflect relative speed.
   This patch retrieves the largest CPU Max Frequency from a type 4 DMI
   record that it can find.  This may not be an issue, however, as a
   sampling of DMI data on x86 and arm64 indicates there is often only
   one such record regardless.  Since CPPC is relatively new, it is
   unclear if the ACPI ASL will always be written to reflect any sort
   of relative performance of processors of differing speeds.

   (3) It assumes that performance and frequency both scale linearly.

For arm64 servers, this may be sufficient, but it does rely on
firmware values being set correctly.  Hence, other approaches will
be considered in the future.

This has been tested on three arm64 servers, with and without DMI, with
and without CPPC support.

Changes for v5:
    -- Move code to cpufreq/cppc_cpufreq.c from acpi/cppc_acpi.c to keep
       frequency-related code together, and keep the CPPC abstract scale
       in ACPI (Prashanth Prakash)
    -- Fix the scaling to remove the incorrect assumption that frequency
       was always a range from zero to max; as a practical matter, it is
       not (Prasanth Prakash); this also allowed us to remove an over-
       engineered function to do this math.

Changes for v4:
    -- Replaced magic constants with #defines (Rafael Wysocki)
    -- Renamed cppc_unitless_to_khz() to cppc_to_khz() (Rafael Wysocki)
    -- Replaced hidden initialization with a clearer form (Rafael Wysocki)
    -- Instead of picking up the first Max Speed value from DMI, we will
       now get the largest Max Speed; still an approximation, but slightly
       less subject to error (Rafael Wysocki)
    -- Kconfig for cppc_cpufreq now depends on DMI, instead of selecting
       it, in order to make sure DMI is set up properly (Rafael Wysocki)

Changes for v3:
    -- Added clarifying commentary re short-term vs long-term fix (Alexey
       Klimov)
    -- Added range checking code to ensure proper arithmetic occurs,
       especially no division by zero (Alexey Klimov)

Changes for v2:
    -- Corrected thinko: needed to have DEPENDS on DMI in Kconfig.arm,
       not SELECT DMI (found by build daemon)

Signed-off-by: Al Stone <redacted>
Signed-off-by: Prashanth Prakash <redacted>

Applied.

Thanks,
Rafael

I've been on vacation so just now am seeing this.  Thanks, Rafael!

-- 
ciao,
al
-----------------------------------
Al Stone
Software Engineer
Red Hat, Inc.
ahs3@redhat.com
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