• MyungJoo Ham's avatar
    PM: Introduce devfreq: generic DVFS framework with device-specific OPPs · a3c98b8b
    MyungJoo Ham authored
    With OPPs, a device may have multiple operable frequency and voltage
    sets. However, there can be multiple possible operable sets and a system
    will need to choose one from them. In order to reduce the power
    consumption (by reducing frequency and voltage) without affecting the
    performance too much, a Dynamic Voltage and Frequency Scaling (DVFS)
    scheme may be used.
    This patch introduces the DVFS capability to non-CPU devices with OPPs.
    DVFS is a techique whereby the frequency and supplied voltage of a
    device is adjusted on-the-fly. DVFS usually sets the frequency as low
    as possible with given conditions (such as QoS assurance) and adjusts
    voltage according to the chosen frequency in order to reduce power
    consumption and heat dissipation.
    The generic DVFS for devices, devfreq, may appear quite similar with
    /drivers/cpufreq.  However, cpufreq does not allow to have multiple
    devices registered and is not suitable to have multiple heterogenous
    devices with different (but simple) governors.
    Normally, DVFS mechanism controls frequency based on the demand for
    the device, and then, chooses voltage based on the chosen frequency.
    devfreq also controls the frequency based on the governor's frequency
    recommendation and let OPP pick up the pair of frequency and voltage
    based on the recommended frequency. Then, the chosen OPP is passed to
    device driver's "target" callback.
    When PM QoS is going to be used with the devfreq device, the device
    driver should enable OPPs that are appropriate with the current PM QoS
    requests. In order to do so, the device driver may call opp_enable and
    opp_disable at the notifier callback of PM QoS so that PM QoS's
    update_target() call enables the appropriate OPPs. Note that at least
    one of OPPs should be enabled at any time; be careful when there is a
    Signed-off-by: default avatarMyungJoo Ham <myungjoo.ham@samsung.com>
    Signed-off-by: default avatarKyungmin Park <kyungmin.park@samsung.com>
    Reviewed-by: default avatarMike Turquette <mturquette@ti.com>
    Acked-by: default avatarKevin Hilman <khilman@ti.com>
    Signed-off-by: default avatarRafael J. Wysocki <rjw@sisk.pl>
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