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    [ACPI/CPUFREQ] Introduce bios_limit per cpu cpufreq sysfs interface · e2f74f35
    Thomas Renninger authored
    
    
    This interface is mainly intended (and implemented) for ACPI _PPC BIOS
    frequency limitations, but other cpufreq drivers can also use it for
    similar use-cases.
    
    Why is this needed:
    
    Currently it's not obvious why cpufreq got limited.
    People see cpufreq/scaling_max_freq reduced, but this could have
    happened by:
      - any userspace prog writing to scaling_max_freq
      - thermal limitations
      - hardware (_PPC in ACPI case) limitiations
    
    Therefore export bios_limit (in kHz) to:
      - Point the user that it's the BIOS (broken or intended) which limits
        frequency
      - Export it as a sysfs interface for userspace progs.
        While this was a rarely used feature on laptops, there will appear
        more and more server implemenations providing "Green IT" features like
        allowing the service processor to limit the frequency. People want
        to know about HW/BIOS frequency limitations.
    
    All ACPI P-state driven cpufreq drivers are covered with this patch:
      - powernow-k8
      - powernow-k7
      - acpi-cpufreq
    
    Tested with a patched DSDT which limits the first two cores (_PPC returns 1)
    via _PPC, exposed by bios_limit:
    # echo 2200000 >cpu2/cpufreq/scaling_max_freq
    # cat cpu*/cpufreq/scaling_max_freq
    2600000
    2600000
    2200000
    2200000
    # #scaling_max_freq shows general user/thermal/BIOS limitations
    
    # cat cpu*/cpufreq/bios_limit
    2600000
    2600000
    2800000
    2800000
    # #bios_limit only shows the HW/BIOS limitation
    
    CC: Pallipadi Venkatesh <venkatesh.pallipadi@intel.com>
    CC: Len Brown <lenb@kernel.org>
    CC: davej@codemonkey.org.uk
    CC: linux@dominikbrodowski.net
    
    Signed-off-by: default avatarThomas Renninger <trenn@suse.de>
    Signed-off-by: default avatarDave Jones <davej@redhat.com>
    e2f74f35