armv8: generic_timer: Use event stream for udelay

Polling cntpct_el0 in a tight loop for delays is inefficient.
This is particularly apparent on Arm FVPs, which do not simulate
real time, meaning that a 1s sleep can take a couple of orders
of magnitude longer to execute in wall time.

If running at EL2 or above (where CNTHCTL_EL2 is available), enable
the cntpct_el0 event stream temporarily and use wfe to implement
the delay more efficiently. The event period is chosen as a
trade-off between efficiency and the fact that Arm FVPs do not
typically simulate real time.

This is only implemented for Armv8 boards, where an architectural
timer exists, and only enabled by default for the ARCH_VEXPRESS64
board family.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>

arch/arm/cpu/armv8/Kconfig

@@ -191,6 +191,14 @@ config ARMV8_EA_EL3_FIRST
 	  Exception handling at all exception levels for External Abort and
 	  SError interrupt exception are taken in EL3.
 
+config ARMV8_UDELAY_EVENT_STREAM
+	bool "Use the event stream for udelay"
+	default y if ARCH_VEXPRESS64
+	help
+	  Use the event stream provided by the AArch64 architectural timer for
+	  delays. This is more efficient than the default polling
+	  implementation.
+
 menuconfig ARMV8_CRYPTO
 	bool "ARM64 Accelerated Cryptographic Algorithms"
 

arch/arm/cpu/armv8/generic_timer.c

@@ -114,3 +114,30 @@ ulong timer_get_boot_us(void)
 
 	return val / get_tbclk();
 }
+
+#if CONFIG_IS_ENABLED(ARMV8_UDELAY_EVENT_STREAM)
+void __udelay(unsigned long usec)
+{
+	u64 target = get_ticks() + usec_to_tick(usec);
+
+	/* At EL2 or above, use the event stream to avoid polling CNTPCT_EL0 so often */
+	if (current_el() >= 2) {
+		u32 cnthctl_val;
+		const u8 event_period = 0x7;
+
+		asm volatile("mrs %0, cnthctl_el2" : "=r" (cnthctl_val));
+		asm volatile("msr cnthctl_el2, %0" : : "r"
+			(cnthctl_val | CNTHCTL_EL2_EVNT_EN | CNTHCTL_EL2_EVNT_I(event_period)));
+
+		while (get_ticks() + (1ULL << event_period) <= target)
+			wfe();
+
+		/* Reset the event stream */
+		asm volatile("msr cnthctl_el2, %0" : : "r" (cnthctl_val));
+	}
+
+	/* Fall back to polling CNTPCT_EL0 */
+	while (get_ticks() <= target)
+		;
+}
+#endif

arch/arm/include/asm/system.h

@@ -69,8 +69,10 @@
 /*
  * CNTHCTL_EL2 bits definitions
  */
-#define CNTHCTL_EL2_EL1PCEN_EN	(1 << 1)  /* Physical timer regs accessible   */
-#define CNTHCTL_EL2_EL1PCTEN_EN	(1 << 0)  /* Physical counter accessible      */
+#define CNTHCTL_EL2_EVNT_EN	BIT(2)	     /* Enable the event stream       */
+#define CNTHCTL_EL2_EVNT_I(val)	((val) << 4) /* Event stream trigger bits     */
+#define CNTHCTL_EL2_EL1PCEN_EN	(1 << 1)     /* Physical timer regs accessible */
+#define CNTHCTL_EL2_EL1PCTEN_EN	(1 << 0)     /* Physical counter accessible   */
 
 /*
  * HCR_EL2 bits definitions