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

On 02.10.14 01:28, Scott Wood wrote:

On Thu, 2014-10-02 at 01:21 +0200, Alexander Graf wrote:

quoted

On 02.10.14 00:39, Scott Wood wrote:

quoted

On Wed, 2014-10-01 at 15:27 +0200, Alexander Graf wrote:

quoted

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.

True, and it is slightly ugly to expose a specific SoC's guts in our
generic pv machine type.

quoted

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.

I agree. Today, only ARM has a comparable mechanism for drivers to hook
a reset handler into the machine specific reset path.

I'd say let's wait for Guenter to finish the power off rework and then
tackle a generic reset call chain based approach next.


Alex