From 9006123157886acc47bdc17e26c933d710b8d6cd Mon Sep 17 00:00:00 2001 From: "Gautham R. Shenoy" Date: Wed, 22 Mar 2017 20:34:15 +0530 Subject: [PATCH] powerpc/powernv/smp: Add busy-wait loop as fall back for CPU-Hotplug Currently, the powernv cpu-offline function assumes that platform idle states such as stop on POWER9, winkle/sleep/nap on POWER8 are always available. On POWER8, it picks nap as the default state if other deep idle states like sleep/winkle are not available and enabled in the platform. On POWER9, nap is not available and all idle states are managed by STOP instruction. The parameters to the idle state are passed through processor stop status control register (PSSCR). Hence as such executing STOP would take parameters from current PSSCR. We do not want to make any assumptions in kernel on what STOP states and PSSCR features are configured by the platform. Ideally platform will configure a good set of stop states that can be used in the kernel. We would like to start with a clean slate, if the platform choose to not configure any state or there is an error in platform firmware that lead to no stop states being configured or allowed to be requested. This patch adds a fallback method for CPU-Hotplug that is similar to snooze loop at idle where the threads are left to spin at low priority and hence reduce the cycles consumed. This is a safe fallback mechanism in the case when no stop state would be requested if the platform firmware did not configure them most likely due to an error condition. Requesting a stop state when the platform has not configured them or enabled them would lead to further error conditions which could be difficult to debug. [Changelog written with inputs from svaidy@linux.vnet.ibm.com] Reviewed-by: Nicholas Piggin Signed-off-by: Gautham R. Shenoy Signed-off-by: Michael Ellerman --- arch/powerpc/platforms/powernv/idle.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/arch/powerpc/platforms/powernv/idle.c b/arch/powerpc/platforms/powernv/idle.c index 419edffec516..f335e0f13192 100644 --- a/arch/powerpc/platforms/powernv/idle.c +++ b/arch/powerpc/platforms/powernv/idle.c @@ -283,8 +283,16 @@ unsigned long pnv_cpu_offline(unsigned int cpu) } else if ((idle_states & OPAL_PM_SLEEP_ENABLED) || (idle_states & OPAL_PM_SLEEP_ENABLED_ER1)) { srr1 = power7_sleep(); - } else { + } else if (idle_states & OPAL_PM_NAP_ENABLED) { srr1 = power7_nap(1); + } else { + /* This is the fallback method. We emulate snooze */ + while (!generic_check_cpu_restart(cpu)) { + HMT_low(); + HMT_very_low(); + } + srr1 = 0; + HMT_medium(); } return srr1; -- 2.20.1