[PATCH 3/5] ARM: MCPM: make internal helpers private to the core code

[Nicolas Pitre]

On Fri, 24 Jul 2015, Chen-Yu Tsai wrote:

> Hi,
> 
> On Sat, May 2, 2015 at 12:06 AM, Nicolas Pitre <nicolas.pitre at linaro.org> wrote:
> > This concerns the following helpers:
> >
> >         __mcpm_cpu_going_down()
> >         __mcpm_cpu_down()
> >         __mcpm_outbound_enter_critical()
> >         __mcpm_outbound_leave_critical()
> >         __mcpm_cluster_state()
> >
> > They are and should only be used by the core code now.  Therefore their
> > declarations are removed from mcpm.h and their definitions are made
> > static, hence the need to move them before their users which accounts
> > for the bulk of this patch.
> 
> I'm looking for some advice. On the Allwinner A80, at least on mainline,
> there is no external PMU or embedded controller in charge of power
> controls. What this means is that I'm doing power sequencing in the
> kernel as part of the MCPM calls, specifically powering down cores and
> clusters in the .wait_for_powerdown callback. (I don't think it's
> reasonable or even possible to power down stuff in .*_powerdown_prepare)

Can you tell me more about the power control knobs at your disposal?  Do 
power gates become effective immediately or only when WFI is asserted? 

And can you configure things so a core may be powered up asynchronously 
from an IRQ?

> Previously I was using __mcpm_cluster_state() to check if the last core
> in a cluster was to be powered off, and thus the whole cluster could be
> turned off as well.
> I could also check if the individual power gates or
> resets are asserted, but if a core was already scheduled to be brought
> up, and MCPM common framework didn't call .cluster_powerup, there might
> be a problem.

I fail to see how a core could be scheduled to be brought up without 
deasserting its reset line somehow though.

> Any suggestions? Maybe export __mcpm_cluster_state() so platform code
> can know what's going to happen?

The cluster state may change unexpectedly.  There is a special locking 
sequence and state machine needed to make this information reliable.  
Simply returning the current state wouldn't be enough to ensure 
it can be used race free.

As Dave stated, we might have to supplement the MCPM core code with 
special methods involving a surviving CPU to perform the power-down 
operation on the dying CPU's behalf.  Doing this in .wait_for_powerdown 
is just an abuse of the API.

It also brings up the question if MCPM is actually necessary in that 
case or if you can do without its complexity.  For example, you may look 
at commit 905cdf9dda5d for such a case.  It mainly depends on whether or 
not cores (and the cluster) may be awaken asynchronously upon assertion 
of an IRQ in the context of cpuidle. If the hardware doesn't support 
that then MCPM doesn't bring you any actual benefit.

So it depends on your hardware capabilities.


Nicolas