Date:	Thu, 19 May 2016 15:12:44 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Al Stone <ahs3@...hat.com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Ashwin Chaugule <ashwin.chaugule@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v3] Force cppc_cpufreq to report values in KHz to fix user
 space reporting

On Thu, May 19, 2016 at 1:41 AM, Al Stone <ahs3@...hat.com> 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@...hat.com>
> ---
>  drivers/acpi/cppc_acpi.c    | 96 ++++++++++++++++++++++++++++++++++++++++++---
>  drivers/cpufreq/Kconfig.arm |  1 +
>  2 files changed, 92 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index 8adac69..56a46e6 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -40,6 +40,9 @@
>  #include <linux/cpufreq.h>
>  #include <linux/delay.h>
>  #include <linux/ktime.h>
> +#include <linux/dmi.h>
> +
> +#include <asm/unaligned.h>
>
>  #include <acpi/cppc_acpi.h>
>  /*
> @@ -709,6 +712,55 @@ static int cpc_write(struct cpc_reg *reg, u64 val)
>         return ret_val;
>  }
>
> +static u64 cppc_dmi_khz;
> +
> +static void cppc_find_dmi_mhz(const struct dmi_header *dm, void *private)
> +{
> +       u16 *mhz = (u16 *)private;
> +       const u8 *dmi_data = (const u8 *)dm;
> +
> +       if (dm->type == DMI_ENTRY_PROCESSOR && dm->length >= 48)
> +               *mhz = (u16)get_unaligned((const u16 *)(dmi_data + 0x14));

Is the offset standardized across architectures (I can't recall ATM)?
If so, maybe #define a symbol for it and add a comment saying that
next to its definition?

> +}
> +
> +
> +static u64 cppc_get_dmi_khz(void)
> +{
> +       u16 mhz;
> +
> +       dmi_walk(cppc_find_dmi_mhz, &mhz);
> +
> +       /*
> +        * Real stupid fallback value, just in case there is no
> +        * actual value set.
> +        */
> +       mhz = mhz ? mhz : 1;
> +
> +       return (1000 * mhz);
> +}
> +
> +static u64 cppc_unitless_to_khz(u64 min_in, u64 max_in, u64 val)

Is the "unitless" part of the name really necessary?

> +{
> +       /*
> +        * The incoming val should be min <= val <= max.  Our
> +        * job is to convert that to KHz so it can be properly
> +        * reported to user space via cpufreq_policy.
> +        */
> +       u64 curval = val;
> +       u64 maxf = max_in;
> +       u64 minf = min_in;
> +
> +       if (!cppc_dmi_khz)
> +               cppc_dmi_khz = cppc_get_dmi_khz();

I don't like hidden initializations like this if they are avoidable
and it very much looks like it is avoidable here.

Also you seem to be using the same cppc_dmi_khz value for all
processors handled by this driver.  Is that really guaranteed to be
correct?

> +
> +       /* range check the input values */
> +       curval = curval < minf ? minf : curval;
> +       curval = curval > maxf ? maxf : curval;
> +       minf = minf >= maxf ? maxf - 1 : minf;
> +
> +       return ((curval - minf) * cppc_dmi_khz) / (maxf - minf);
> +}
> +
>  /**
>   * cppc_get_perf_caps - Get a CPUs performance capabilities.
>   * @cpunum: CPU from which to get capabilities info.
> @@ -748,17 +800,51 @@ int cppc_get_perf_caps(int cpunum, struct cppc_perf_caps *perf_caps)
>                 }
>         }
>
> +       /*
> +        * Since these values in perf_caps will be used in setting
> +        * up the cpufreq policy, they must always be stored in units
> +        * of KHz.  If they are not, user space tools will become very
> +        * confused since they assume these are in KHz when reading
> +        * sysfs.
> +        *
> +        * NB: there may be better approaches to this problem that, as
> +        * of this writing, are still being explored.  Ideally, this is
> +        * a short term solution since correlating CPPC abstract values
> +        * with CPU frequency may or may not reflect actual performance.
> +        *
> +        * The reason longer term solutions are being explored is because
> +        * this solution requires we make the following 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 any
> +        *        relative differences.  This code 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.
> +        *
> +        *    (3) It assumes that performance and frequency both scale
> +        *        linearly.
> +        *
> +        * None of these are particularly horrible assumptions.  But, they
> +        * are assumptions and ultimately we'd like to be able to report
> +        * performance without quite so many of them.
> +        *
> +        */
>         cpc_read(&highest_reg->cpc_entry.reg, &high);
> -       perf_caps->highest_perf = high;
> -
>         cpc_read(&lowest_reg->cpc_entry.reg, &low);
> -       perf_caps->lowest_perf = low;
> +
> +       perf_caps->highest_perf = cppc_unitless_to_khz(low, high, high);
> +       perf_caps->lowest_perf = cppc_unitless_to_khz(low, high, low);
>
>         cpc_read(&ref_perf->cpc_entry.reg, &ref);
> -       perf_caps->reference_perf = ref;
> +       perf_caps->reference_perf = cppc_unitless_to_khz(low, high, ref);
>
>         cpc_read(&nom_perf->cpc_entry.reg, &nom);
> -       perf_caps->nominal_perf = nom;
> +       perf_caps->nominal_perf = cppc_unitless_to_khz(low, high, nom);
>
>         if (!ref)
>                 perf_caps->reference_perf = perf_caps->nominal_perf;
> diff --git a/drivers/cpufreq/Kconfig.arm b/drivers/cpufreq/Kconfig.arm
> index 14b1f93..0573982 100644
> --- a/drivers/cpufreq/Kconfig.arm
> +++ b/drivers/cpufreq/Kconfig.arm
> @@ -255,6 +255,7 @@ config ACPI_CPPC_CPUFREQ
>         tristate "CPUFreq driver based on the ACPI CPPC spec"
>         depends on ACPI
>         select ACPI_CPPC_LIB
> +       select DMI

What if there are unmet dependencies for DMI?  Or is that not possible?

>         default n
>         help
>           This adds a CPUFreq driver which uses CPPC methods
> --

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