N00b Asks: Supporting Linux Power Management HOWTO?

From: Leo L. Schwab (ewhac_at_best.com)
Date: 03/12/05

  • Next message: Kumar Gala: "[PATCH] ppc32: Remove SPR short-hand defines" 
    Date:	Fri, 11 Mar 2005 15:54:12 -0800
To: linux-kernel@vger.kernel.org

            I'm currently working on driver development for an embedded/handheld
    system. We're currently using kernel 2.6.11 on an ARM-based platform, and
    I've already got a handful of drivers running (anyone need an I2C bus driver
    for a Freescale MX21?). Earlier this week, I was asked to begin work on
    power management -- shutting off unused devices, placing the system in
    suspend/standby/sleep, etc.

            So I started Googling around for info on Linux power management and,
    near as I can tell, there doesn't appear to be a cohesive approach to the
    issue. What I've managed to turn up so far is:

            - APM
              An older power management system, supplanted by ACPI. It seems to
              enjoy the most mature support in Linux, if for no other reason
              than it's been around so much longer. It lacks flexibility,
              however.
            - ACPI
              The current "standard" for power management. Support even on x86-
              platforms seems highly uneven, however, and the ACPI support code
              still seems to be undergoing major revisions. It also appears to
              be widely regarded as an unholy mess of a spec. I've also gleaned
              hints from LKML discussons that ACPI contains enough x86-specific
              material to make it a questionable choice on other platforms.
            - DPM (Dynamic Power Management, http://dynamicpower.sf.net/ )
              A proposal put forward by MontaVista and IBM, primarily for
              embedded systems (although an Intel Speedstep implementation is
              available). While the reference implementation is being kept
              current, it doesn't seem to have a lot of activity surrounding it,
              despite being two years old.

            The functionality in each of these facilities seems to overlap
    the others. DPM in particular seems to heavily overlap services provided by
    ACPI and exposed by 'cpufreq'. This suggests to me that one ends up
    redundantly implementing power support across three APIs, or that one is
    supposed to select a single API and stick with that. How one makes this
    selection is not at all clear.

            Ideally, what I'd like is "the" approach that integrates most
    cleanly into existing frameworks. That is, I'd like to be able to write the
    power management support into the drivers and platform-specific kernel
    files, then essentially drop a stock distro on top of it and have the
    largest number of scripts and tools Just Work. I would prefer, if possible,
    to write support for one interface rather than three (APM, ACPI, DPM)
    especially when two of them are still moving targets.

            So, I guess what I'm asking for at the moment is:
            - A brief overview of the current state and maturity of power
              management support in the 2.6.x kernel series,
            - the currently available facilities for implementing and supporting
              power management,
            - pointers to documents describing the facilities in detail and how
              to write driver and platform support for them.

            I realize I well may be asking for too much -- the following LKML
    thread suggests that Linux power management remains a problem under active
    discussion with fragmented solutions:

            http://groups-beta.google.com/group/fa.linux.kernel/browse_thread/thread/cab82562215fde4/4660291cd90cf2d2?q=power#4660291cd90cf2d2

            Nevertheless, any and all guidance would be greatly appreciated.
    Please Cc: list responses to this address. My sincerest thanks in advance. 

    --
Leo L. Schwab -- Digital Spellweaver                   ewhac@best.com  ..or..
 \_ -_   http://ewhac.best.vwh.net/                    ewhac@well.com
O----^o  Recumbent Bikes: The Only Way To Fly.         (pronounced "EH-wack")
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