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    SubjectRe: [PATCH 01/44] kernel: Add support for poweroff handler call chain
    On Tue, Oct 07, 2014 at 11:39:03AM +0100, One Thousand Gnomes wrote:
    > On Mon, 6 Oct 2014 22:28:03 -0700
    > Guenter Roeck <linux@roeck-us.net> wrote:
    >
    > > Various drivers implement architecture and/or device specific means to
    > > remove power from the system. For the most part, those drivers set the
    > > global variable pm_power_off to point to a function within the driver.
    > >
    > > This mechanism has a number of drawbacks. Typically only one scheme
    > > to remove power is supported (at least if pm_power_off is used).
    > > At least in theory there can be multiple means remove power, some of
    > > which may be less desirable. For example, some mechanisms may only
    > > power off the CPU or the CPU card, while another may power off the
    > > entire system. Others may really just execute a restart sequence
    > > or drop into the ROM monitor. Using pm_power_off can also be racy
    > > if the function pointer is set from a driver built as module, as the
    > > driver may be in the process of being unloaded when pm_power_off is
    > > called. If there are multiple poweroff handlers in the system, removing
    > > a module with such a handler may inadvertently reset the pointer to
    > > pm_power_off to NULL, leaving the system with no means to remove power.
    > >
    > > Introduce a system poweroff handler call chain to solve the described
    > > problems. This call chain is expected to be executed from the
    > > architecture specific machine_power_off() function. Drivers providing
    > > system poweroff functionality are expected to register with this call chain.
    > > By using the priority field in the notifier block, callers can control
    > > poweroff handler execution sequence and thus ensure that the poweroff
    > > handler with the optimal capabilities to remove power for a given system
    > > is called first.
    >
    > Nice...
    >
    > register_poweroff_handler_simple isn't threadsafe. I'm not sure it
    > matters as we should only have one attempt per platform to use it anyway.
    >
    Yes, I know. Agreed, it should not matter, but maybe it can be solved by
    using raw notifiers and spinlocks for protection.

    > have_kernel_poweroff() has a similar problem - the answer isn't always
    > valid by the time the call returns.
    >
    This is an interesting one. Logically the answer can not be guaranteed to be
    correct by the time it is evaluated in the calling code, no matter how much
    protection I add around it. Not sure if there is anything I can or should do
    about that.

    > The actual poweroff logic is more of a problem - several of the Intel
    > PMICs are on i2c bus, so are not going to be happy in an atomic context
    > so I wonder if that is storing up problems for the future ?
    >
    Yes, Philippe brought that up as well. I think I may have to use raw
    notifiers, and use spinlocks for protection. Would this work ?

    Thanks,
    Guenter


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