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    SubjectRe: Question about userspace-consumer
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    On Tue, 2009-08-11 at 10:40 +0100, Mark Brown wrote:
    > On Tue, Aug 11, 2009 at 08:44:42AM +0300, Felipe Balbi wrote:
    > > On Mon, Aug 10, 2009 at 10:58:01PM +0100, Mark Brown wrote:
    >
    > > > Looking at that I'm not sure why you wish to push this into user space?
    >
    > > we need some daemon monitoring battery statuses and taking actions on
    > > that. Imagine, for example, usb charging where we can:
    >
    > > a. charge up to 100mA when unconfigured
    > > b. charge up to 500mA when configured
    > > c. charge up to 2.5A when with dedicated charger
    > > d. charge up to 2.5mA when bus is suspended
    >
    > It's more complex than that - those are the limits at the USB port that
    > define the power that can be drawn by the system. The actual power
    > available to the battery subsytem will be less since the rest of the
    > system needs to be powered. In many cases even with 500mA available
    > the battery will need to be used to provide additional power in order to
    > keep the system operational rather than being charged.
    >
    > For USB powered operation at least some of the management here would
    > usually be implemented in hardware to provide the responsiveness
    > required. Waiting for software to get involved would often allow the
    > main system supply to collapse.
    >
    > > handling all of those cases on kernel space seems a little bit odd,
    > > especially because we still need to take care of state-of-charge,
    > > pack temperature, time-to-charge, etc etc etc.
    >
    > > a big looping polling for that stuff in kernel space didn't seem ok to
    > > me.
    >
    > No matter what you're still going to need at least some of the code
    > in-kernel in order to handle the monitoring daemon exiting. For
    > example, if the battery is in fast charge then something will need to
    > back the charger off at least as the charge completes (if not
    > immediately user space exits) otherwise the battery or entire system is
    > likely to be damaged.
    >
    > Like I say some user space control does seem reasonable but I'd not
    > expect an entirely user space implementation.

    I agree, I think this probably deserves both user and kernel space
    components although the dividing line between them is a little uncertain
    atm.

    Generally, I'd expect the kernel side to provide a guaranteed *safe*
    environment for charging wrt system stability and battery status. A
    simple state machine would probably suffice.

    I think userspace is where we would manage policy. We would also store
    past battery history in order to better manage future charging and
    charge level estimation.

    Liam



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