arm_arch_timer: Expose event stream status

The arch timer configuration for a CPU might get reset after suspending
said CPU.

In order to reliably use the event stream in the kernel (e.g. for delays),
we keep track of the state where we can safely consider the event stream as
properly configured. After writing to cntkctl, we issue an ISB to ensure
that subsequent delay loops can rely on the event stream being enabled.

Signed-off-by: Julien Thierry <[e-mail]>
Acked-by: Mark Rutland <[e-mail]>
Cc: Marc Zyngier <[e-mail]>
Cc: Russell King <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

arch/arm/include/asm/arch_timer.h

···
 static inline void arch_timer_set_cntkctl(u32 cntkctl)
 {
 	asm volatile("mcr p15, 0, %0, c14, c1, 0" : : "r" (cntkctl));
+	isb();
 }
 
 #endif

arch/arm64/include/asm/arch_timer.h

···
 static inline void arch_timer_set_cntkctl(u32 cntkctl)
 {
 	write_sysreg(cntkctl, cntkctl_el1);
+	isb();
 }
 
 static inline u64 arch_counter_get_cntpct(void)

drivers/clocksource/arm_arch_timer.c

···
 static bool arch_counter_suspend_stop;
 static bool vdso_default = true;
 
+static cpumask_t evtstrm_available = CPU_MASK_NONE;
 static bool evtstrm_enable = IS_ENABLED(CONFIG_ARM_ARCH_TIMER_EVTSTREAM);
 
 static int __init early_evtstrm_cfg(char *buf)
···
 #ifdef CONFIG_COMPAT
 	compat_elf_hwcap |= COMPAT_HWCAP_EVTSTRM;
 #endif
+	cpumask_set_cpu(smp_processor_id(), &evtstrm_available);
 }
 
 static void arch_timer_configure_evtstream(void)
···
 	return arch_timer_rate;
 }
 
+bool arch_timer_evtstrm_available(void)
+{
+	/*
+	 * We might get called from a preemptible context. This is fine
+	 * because availability of the event stream should be always the same
+	 * for a preemptible context and context where we might resume a task.
+	 */
+	return cpumask_test_cpu(raw_smp_processor_id(), &evtstrm_available);
+}
+
 static u64 arch_counter_get_cntvct_mem(void)
 {
 	u32 vct_lo, vct_hi, tmp_hi;
···
 {
 	struct clock_event_device *clk = this_cpu_ptr(arch_timer_evt);
 
+	cpumask_clear_cpu(smp_processor_id(), &evtstrm_available);
+
 	arch_timer_stop(clk);
 	return 0;
 }
···
 static int arch_timer_cpu_pm_notify(struct notifier_block *self,
 				    unsigned long action, void *hcpu)
 {
-	if (action == CPU_PM_ENTER)
+	if (action == CPU_PM_ENTER) {
 		__this_cpu_write(saved_cntkctl, arch_timer_get_cntkctl());
-	else if (action == CPU_PM_ENTER_FAILED || action == CPU_PM_EXIT)
+
+		cpumask_clear_cpu(smp_processor_id(), &evtstrm_available);
+	} else if (action == CPU_PM_ENTER_FAILED || action == CPU_PM_EXIT) {
 		arch_timer_set_cntkctl(__this_cpu_read(saved_cntkctl));
+
+		if (elf_hwcap & HWCAP_EVTSTRM)
+			cpumask_set_cpu(smp_processor_id(), &evtstrm_available);
+	}
 	return NOTIFY_OK;
 }
 
···
 	err = arch_timer_cpu_pm_init();
 	if (err)
 		goto out_unreg_notify;
-
 
 	/* Register and immediately configure the timer on the boot CPU */
 	err = cpuhp_setup_state(CPUHP_AP_ARM_ARCH_TIMER_STARTING,

include/clocksource/arm_arch_timer.h

···
 extern u32 arch_timer_get_rate(void);
 extern u64 (*arch_timer_read_counter)(void);
 extern struct arch_timer_kvm_info *arch_timer_get_kvm_info(void);
+extern bool arch_timer_evtstrm_available(void);
 
 #else
 
···
 static inline u64 arch_timer_read_counter(void)
 {
 	return 0;
+}
+
+static inline bool arch_timer_evtstrm_available(void)
+{
+	return false;
 }
 
 #endif