Re: [PATCH] arm/dt: Don't add disabled CPUs to system topology

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Fri, Jun 07, 2013 at 03:20:20PM +0100, Rob Herring wrote:
> On 06/07/2013 05:23 AM, Lorenzo Pieralisi wrote:
> > Hi James,
> > 
> > On Thu, Jun 06, 2013 at 06:11:25PM +0100, James King wrote:
> >> If CPUs are marked as disabled in the devicetree, make sure they do
> >> not exist in the system CPU information and CPU topology information.
> >> In this case these CPUs will not be able to be added to the system later
> >> using hot-plug. This allows a single chip with many CPUs to be easily
> >> used in a variety of hardware devices where they may have different
> >> actual processing requirements (eg for thermal/cost reasons).
> >>
> >> - Change devicetree.c to ignore any cpu nodes marked as disabled,
> >>   this effectively limits the number of active cpu cores so no need
> >>   for the max_cpus=x in the chosen node.
> >> - Change topology.c to ignore any cpu nodes marked as disabled, this
> >>   is where the scheduler would learn about big/LITTLE cores so this
> >>   effectively keeps the scheduler in sync.
> >>
> > 
> > I have two questions:
> > 
> > 1) Since with this approach the DT should change anyway if on different
> >    hardware devices based on the same chip you want to allow booting a
> >    different number of CPUs, why do not we remove the cpu nodes instead of
> >    disabling them ? Put it another way: cpu nodes define a cpu as
> >    possible (currently), we can simply remove the node if we do not want
> >    that cpu to be seen by the kernel.
> > 2) If we go for the "status" property, why do not we use it to set present
> >    mask ? That way the cpu is possible but not present, you cannot
> >    hotplug it in. It is a bit of a stretch, granted, the cpu _is_ present,
> >    we just want to disable it, do not know how this is handled in x86
> >    and other archs though.
> 
> The meaning of disabled for cpus in ePAPR is that the cpu is offline
> (i.e. in a spinloop or wfi), not that the cpu is unavailable. This is a
> bit of a departure and inconsistency from how status for devices are
> used. That would imply that we should be setting status to disabled for
> all secondary cores and that possibly the status value should get
> updated to reflect the state of the cpu.

Yes, that's what I understood from the ePAPR as well. According to
the ePAPR, as you say, a cpu with its status property == "disabled" is a
possible CPU, since it can be enabled (through a specific enable-method).

I am not sure "status" can be reused for the purpose this patch was developed
for without changing the bindings in the ePAPR (ie if DT parsing skips
cpu nodes with status == "disabled", this is a significant departure
from what ePAPR defines, and it would force us to define an enable-method
to enable/online those CPUs which is not what this patch was developed for).

How was PowerPC tackling the problem James set about solving ?

Lorenzo