[PATCH v2 1/2] arm_arch_timer: Expose event stream status

[Julien Thierry]

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 concider the event stream as
properly configured.

Signed-off: Julien Thierry <julien.thierry at arm.com>
Cc: Mark Rutland <mark.rutland at arm.com>
Cc: Marc Zyngier <marc.zyngier at arm.com>
Cc: Russell King <linux at armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will.deacon at arm.com>
---
 arch/arm/include/asm/arch_timer.h    |  1 +
 arch/arm64/include/asm/arch_timer.h  |  1 +
 drivers/clocksource/arm_arch_timer.c | 32 ++++++++++++++++++++++++++++++--
 include/clocksource/arm_arch_timer.h |  6 ++++++
 4 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/arch_timer.h b/arch/arm/include/asm/arch_timer.h
index d4ebf56..0b6e104 100644
--- a/arch/arm/include/asm/arch_timer.h
+++ b/arch/arm/include/asm/arch_timer.h
@@ -106,6 +106,7 @@ static inline u32 arch_timer_get_cntkctl(void)
 static inline void arch_timer_set_cntkctl(u32 cntkctl)
 {
 	asm volatile("mcr p15, 0, %0, c14, c1, 0" : : "r" (cntkctl));
+	isb();
 }

 #endif
diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
index 74d08e4..26b3376 100644
--- a/arch/arm64/include/asm/arch_timer.h
+++ b/arch/arm64/include/asm/arch_timer.h
@@ -144,6 +144,7 @@ static inline u32 arch_timer_get_cntkctl(void)
 static inline void arch_timer_set_cntkctl(u32 cntkctl)
 {
 	write_sysreg(cntkctl, cntkctl_el1);
+	isb();
 }

 static inline u64 arch_counter_get_cntpct(void)
diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
index aae87c4..866bbf8 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -77,6 +77,7 @@ struct arch_timer {
 static bool arch_counter_suspend_stop;
 static bool vdso_default = true;

+static DEFINE_PER_CPU(bool, evtstrm_available) = false;
 static bool evtstrm_enable = IS_ENABLED(CONFIG_ARM_ARCH_TIMER_EVTSTREAM);

 static int __init early_evtstrm_cfg(char *buf)
@@ -736,6 +737,7 @@ static void arch_timer_evtstrm_enable(int divider)
 #ifdef CONFIG_COMPAT
 	compat_elf_hwcap |= COMPAT_HWCAP_EVTSTRM;
 #endif
+	__this_cpu_write(evtstrm_available, true);
 }

 static void arch_timer_configure_evtstream(void)
@@ -860,6 +862,26 @@ u32 arch_timer_get_rate(void)
 	return arch_timer_rate;
 }

+bool arch_timer_evtstrm_available(void)
+{
+	/*
+	 * This function might get called outside of a preempt disable context.
+	 * This is not an issue because the result of this function will stay
+	 * valid for the CPU the caller task will be running on, even if
+	 * preempted. The possible case are as follow:
+	 *   - evtstrm_available == false -> either event stream is disabled
+	 *     system-wide or event stream is momentarily disabled on this CPU
+	 *     meaning we won't be scheduling in/out tasks on this CPU until it
+	 *     is reconfigured. Current task is made aware it cannot rely on
+	 *     event stream.
+	 *   - evtstrm_available == true -> task was running on a CPU with event
+	 *     stream enabled, if it gets preempted, it will be resumed on a CPU
+	 *     where event stream is configured properly. The task can use the
+	 *     event stream.
+	 */
+	return this_cpu_read(evtstrm_available);
+}
+
 static u64 arch_counter_get_cntvct_mem(void)
 {
 	u32 vct_lo, vct_hi, tmp_hi;
@@ -925,6 +947,8 @@ static int arch_timer_dying_cpu(unsigned int cpu)
 {
 	struct clock_event_device *clk = this_cpu_ptr(arch_timer_evt);

+	__this_cpu_write(evtstrm_available, false);
+
 	arch_timer_stop(clk);
 	return 0;
 }
@@ -934,10 +958,14 @@ static int arch_timer_dying_cpu(unsigned int cpu)
 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)
+		__this_cpu_write(evtstrm_available, false);
+	}
+	else if (action == CPU_PM_ENTER_FAILED || action == CPU_PM_EXIT) {
 		arch_timer_set_cntkctl(__this_cpu_read(saved_cntkctl));
+		__this_cpu_write(evtstrm_available, !!(elf_hwcap & HWCAP_EVTSTRM));
+	}
 	return NOTIFY_OK;
 }

diff --git a/include/clocksource/arm_arch_timer.h b/include/clocksource/arm_arch_timer.h
index cc805b7..4e28283 100644
--- a/include/clocksource/arm_arch_timer.h
+++ b/include/clocksource/arm_arch_timer.h
@@ -93,6 +93,7 @@ struct arch_timer_mem {
 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

@@ -106,6 +107,11 @@ static inline u64 arch_timer_read_counter(void)
 	return 0;
 }

+static inline bool arch_timer_evtstrm_available(void)
+{
+	return false;
+}
+
 #endif

 #endif
--
1.9.1