Re: [PATCH] arm/dt: Don't add disabled CPUs to system topology
From: Lorenzo Pieralisi
Date: Fri Jun 07 2013 - 12:14:12 EST
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 ?
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