[PATCH v3] Force cppc_cpufreq to report values in KHz to fix user space reporting

[Al Stone]

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(-)

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));
+}
+
+
+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)
+{
+	/*
+	 * 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();
+
+	/* 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
 	default n
 	help
 	  This adds a CPUFreq driver which uses CPPC methods
-- 
1.8.3.1