linux-kernel - Re: [PATCH 01/44] kernel: Add support for poweroff handler call chain

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Date:	Tue, 7 Oct 2014 11:39:03 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Guenter Roeck <linux@...ck-us.net>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/44] kernel: Add support for poweroff handler call
 chain

On Mon,  6 Oct 2014 22:28:03 -0700
Guenter Roeck <linux@...ck-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.

have_kernel_poweroff() has a similar problem - the answer isn't always
valid by the time the call returns.

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 ?

Alan
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