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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/