On 18-05-16, 17:41, 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). > > 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, or that > the CPPC values have all been scaled to reflect relative 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. 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 are > also being considered. > > This has been tested on three arm64 servers, with and without DMI, with > and without CPPC support. > > 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 <ahs3@xxxxxxxxxx> > --- > drivers/acpi/cppc_acpi.c | 96 ++++++++++++++++++++++++++++++++++++++++++--- > drivers/cpufreq/Kconfig.arm | 1 + > 2 files changed, 92 insertions(+), 5 deletions(-) I can't keep it unread in my Inbox forever and so I am replying here to mark it read :) I don't have enough knowledge of this stuff and so I leave it for Ashwin and Rafael to do it. :) -- 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
