This series is an attempt at fixing the maxcpus=n behavior on arm64. So far we have disabled hotplugging a CPU > n, when maxcpus=n is in effect, due to following reasons.
1) Possible cpu feature incompatibilities with the new CPU in heterogeneous systems. 2) New CPU requiring an errata work around which was not detected (and the code patched in) at boot time. 3) Failure to initialise the PMU in case the supported CPUs are not online while probing the PMU. (1) has been mostly solved with our early CPU feature verification support. This series tries to address (2) & (3). (2) is solved by iterating over the known erratas and checking if the new CPU requires an errata not set in the cpu_hwcaps, failing which, we kill the CPU. We plan to fix this properly by retaining the CPU errata work arounds and apply the required at runtime. (3) is ignored and will not be fixed as there is no reliable way of knowing if there would be a CPU that will be online to support the PMU. In the process, also restores the capability to check GIC interface settings by the firmware on individual CPUs. Tested on Juno with maxcpus=2 (enables A57 cores and A57-PMU) and maxcpus=1 (disables both A57 cores and A57-PMU). This series applies on 4.6-rc3 + "arm64: Support for systems without AArch32 state" [1]. The tree is available here : git://linux-arm.org/linux-skp.git maxcpus/v2 [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-April/422979.html Changes since V1: - Define default scope for capabilities. - WARN_ON !SCOPE_CPU for midr_check. Marc Zyngier (2): arm64: Allow a capability to be checked on a single CPU irqchip/gic: Restore CPU interface checking Suzuki K Poulose (3): arm64: cpufeature: Add scope for capability check arm64: Verify CPU errata work arounds on hotplugged CPU arm64: Fix behavior of maxcpus=N arch/arm64/include/asm/cpufeature.h | 12 +++++++- arch/arm64/kernel/cpu_errata.c | 24 ++++++++++++++- arch/arm64/kernel/cpufeature.c | 58 +++++++++++++++++++++++++---------- arch/arm64/kernel/smp.c | 18 +---------- drivers/irqchip/irq-gic.c | 5 ++- 5 files changed, 78 insertions(+), 39 deletions(-) -- 1.7.9.5