On 09/13/2016 07:09 PM, Rafael J. Wysocki wrote:
> On Wednesday, July 20, 2016 03:10:04 PM Al Stone wrote:
>> 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
>> -- 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 <a...@redhat.com>
>> Signed-off-by: Prashanth Prakash <pprak...@codeaurora.org>
I've been on vacation so just now am seeing this. Thanks, Rafael!
Red Hat, Inc.