[PATCH 00/20] powerpc: Convert power off logic to pm_power_off

Scott Wood scottwood at freescale.com
Thu Oct 2 09:28:47 EST 2014

On Thu, 2014-10-02 at 01:21 +0200, Alexander Graf wrote:
> On 02.10.14 00:39, Scott Wood wrote:
> > On Wed, 2014-10-01 at 15:27 +0200, Alexander Graf wrote:
> >> The generic Linux framework to power off the machine is a function pointer
> >> called pm_power_off. The trick about this pointer is that device drivers can
> >> potentially implement it rather than board files.
> >>
> >> Today on PowerPC we set pm_power_off to invoke our generic full machine power
> >> off logic which then calls ppc_md.power_off to invoke machine specific power
> >> off.
> >>
> >> However, when we want to add a power off GPIO via the "gpio-poweroff" driver,
> >> this card house falls apart. That driver only registers itself if pm_power_off
> >> is NULL to ensure it doesn't override board specific logic. However, since we
> >> always set pm_power_off to the generic power off logic (which will just not
> >> power off the machine if no ppc_md.power_off call is implemented), we can't
> >> implement power off via the generic GPIO power off driver.
> >>
> >> To fix this up, let's get rid of the ppc_md.power_off logic and just always use
> >> pm_power_off as was intended. Then individual drivers such as the GPIO power off
> >> driver can implement power off logic via that function pointer.
> >>
> >> With this patch set applied and a few patches on top of QEMU that implement a
> >> power off GPIO on the virt e500 machine, I can successfully turn off my virtual
> >> machine after halt.
> > 
> > Are there any plans to handle restart similarly?
> I don't see an immediate need for it, as we do have access to reset via
> the guts device.

The guts device is problematic from a hardware description perspective,
as we advertise a bunch of things to the guest that we don't implement.
E.g. the only reason we don't currently have a problem with Linux trying
to use guts to sync the timebase across cores is that by chance we
happen to expose a guts that corresponds to a single-cpu SoC.

> I also don't see any generic driver in Linux that would implement reset
> via a gpio controller device, so apparently it's not incredibly common
> to implement reset via gpio.

There wasn't a generic gpio-power-off driver either until a couple years
ago...  Regardless of whether it's done via gpio, using ppc_md for it is
bad for the same reasons as for power-off.


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