drivers: clocksource: add support for ARM architected timer event stream

The ARM architected timer can generate events (used for waking up
CPUs executing the wfe instruction) at a frequency represented as a
power-of-2 divisor of the clock rate.

An event stream might be used:
- To implement wfe-based timeouts for userspace locking implementations.
- To impose a timeout on a wfe for safeguarding against any programming
  error in case an expected event is not generated.

This patch computes the event stream frequency aiming for a period
of 100us between events. It uses ARM/ARM64 specific backends to configure
and enable the event stream.

Change-Id: I43906fa7d083a2c611c2047e15c34d68bb047ac7
Cc: Lorenzo Pieralisi <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Acked-by: Olof Johansson <[email protected]>
Signed-off-by: Will Deacon <[email protected]>
[sudeep: moving ARM/ARM64 changes into separate patches
         and adding Kconfig option]
Signed-off-by: Sudeep KarkadaNagesha <[email protected]>
Git-commit: 37f637767a82907efedda78d3ff405c34020075
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
[[email protected]: Fixed trivial merge conflicts]
Signed-off-by: Trilok Soni <[email protected]>

drivers/clocksource/Kconfig

@@ -67,6 +67,21 @@ config ARM_ARCH_TIMER
 	bool
 	select CLKSRC_OF if OF
 
+config ARM_ARCH_TIMER_EVTSTREAM
+	bool "Support for ARM architected timer event stream generation"
+	default y if ARM_ARCH_TIMER
+	help
+	  This option enables support for event stream generation based on
+	  the ARM architected timer. It is used for waking up CPUs executing
+	  the wfe instruction at a frequency represented as a power-of-2
+	  divisor of the clock rate.
+	  The main use of the event stream is wfe-based timeouts of userspace
+	  locking implementations. It might also be useful for imposing timeout
+	  on wfe to safeguard against any programming errors in case an expected
+	  event is not generated.
+	  This must be disabled for hardware validation purposes to detect any
+	  hardware anomalies of missing events.
+
 config CLKSRC_METAG_GENERIC
 	def_bool y if METAG
 	help

drivers/clocksource/arm_arch_timer.c

@@ -325,7 +325,20 @@ static void arch_counter_set_user_access(void)
 	arch_timer_set_cntkctl(cntkctl);
 }
 
-static int __cpuinit arch_timer_setup(struct clock_event_device *clk)
+static void arch_timer_configure_evtstream(void)
+{
+	int evt_stream_div, pos;
+
+	/* Find the closest power of two to the divisor */
+	evt_stream_div = arch_timer_rate / ARCH_TIMER_EVT_STREAM_FREQ;
+	pos = fls(evt_stream_div);
+	if (pos > 1 && !(evt_stream_div & (1 << (pos - 2))))
+		pos--;
+	/* enable event stream */
+	arch_timer_evtstrm_enable(min(pos, 15));
+}
+
+static int arch_timer_setup(struct clock_event_device *clk)
 {
 	__arch_timer_setup(ARCH_CP15_TIMER, clk);
 
@@ -338,6 +351,8 @@ static int __cpuinit arch_timer_setup(struct clock_event_device *clk)
 	}
 
 	arch_counter_set_user_access();
+	if (IS_ENABLED(CONFIG_ARM_ARCH_TIMER_EVTSTREAM))
+		arch_timer_configure_evtstream();
 
 	return 0;
 }

include/clocksource/arm_arch_timer.h

@@ -39,6 +39,8 @@
 #define ARCH_TIMER_USR_VT_ACCESS_EN	(1 << 8) /* virtual timer registers */
 #define ARCH_TIMER_USR_PT_ACCESS_EN	(1 << 9) /* physical timer registers */
 
+#define ARCH_TIMER_EVT_STREAM_FREQ	10000	/* 100us */
+
 #ifdef CONFIG_ARM_ARCH_TIMER
 
 extern u32 arch_timer_get_rate(void);