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

[Viresh Kumar]

Cc'ing Ashwin.

-- 
viresh

On 15-04-16, 16:16, 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.  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.
> 
> 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.
> 
> Signed-off-by: Al Stone <ahs3@xxxxxxxxxx>
> ---
>  drivers/acpi/cppc_acpi.c    | 61 +++++++++++++++++++++++++++++++++++++++++----
>  drivers/cpufreq/Kconfig.arm |  1 +
>  2 files changed, 57 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index 8adac69..049dced 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 +712,47 @@ 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, u64 max, 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.
> +	 */
> +
> +	if (!cppc_dmi_khz)
> +		cppc_dmi_khz = cppc_get_dmi_khz();
> +
> +	return ((val - min) * cppc_dmi_khz) / (max - min);
> +}
> +
>  /**
>   * cppc_get_perf_caps - Get a CPUs performance capabilities.
>   * @cpunum: CPU from which to get capabilities info.
> @@ -748,17 +792,24 @@ 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.
> +	 */
>  	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
> -- 
> 2.5.5