On 19-04-16, 16:12, Ashwin Chaugule wrote: > + Ryan > > Hi Al, > > On 18 April 2016 at 20:11, Al Stone <ahs3@xxxxxxxxxx> 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). > > > > While the investigation for a long term fix proceeds (several options > > are being explored, some of which may require spec changes or other > > much more invasive fixes), this patch forces the values read by CPPC > > to be read in KHz, regardless of what they actually represent. > > > > 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. This > > patch retrieves the first 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. Don't we have any big LITTLE ARM servers yet ? Or we will not have them at all ? > > For arm64 servers, this may be sufficient, but it does rely on > > firmware values being set correctly. Hence, other approaches are > > also being considered. > > > > This has been tested on three arm64 servers, with and without DMI, with > > and without CPPC support. > > > > 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 <ahs3@xxxxxxxxxx> > > This looks like a good short term solution. Does it make more sense to > move this to the cppc_cpufreq driver though? Since that ties more > closely into the cpufreq framework which requires the kHz values in > sysfs. That way we can keep the cppc_acpi.c shim compliant with the > ACPI spec. (i.e. values read in cppc structures remain abstract and > unitless). > > Rafael, Viresh, others, > > Any other ideas how to handle this better in the long term? > > - Decouple the cpufreq sysfs from the cppc driver and introduce its > own entries. Is it possibly to do this cleanly while still allowing > usage of cpufreq registration with existing governors? > > - Come up with a scaling factor using the PMU cycle counter at boot > before the CPPC drivers are initialized. This would use the current > freq set by some UEFI var. This would possibly require some messy > perfevents plumbing and added bootup time though. I may be missing the obvious, but can't we just create the cpufreq-table from this table in khz? We wouldn't require any further change then. -- viresh -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
