TF-A GIC driver: Add barrier before eoi

It is desired to have the peripheral writes completed to clear the
interrupt condition and de-assert the interrupt request to GIC before
EOI write. Failing which spurious interrupt will occurred.

A barrier is needed to ensure peripheral register write transfers are
complete before EOI is done.

GICv2 memory mapped DEVICE nGnR(n)E writes are ordered from core point
of view. However these writes may pass over different interconnects,
bridges, buffers leaving some rare chances for the actual write to
complete out of order.

GICv3 ICC EOI system register writes have no ordering against nGnR(n)E
memory writes as they are over different interfaces.

Hence a dsb can ensure from core no writes are issued before the previous
writes are *complete*.
Signed-off-by: Sandeep Tripathy <sandeep.tripathy@broadcom.com>
Change-Id: Ie6362009e2f91955be99dca8ece14ade7b4811d6

drivers/arm/gic/v2/gicv2_main.c

@@ -247,6 +247,15 @@ void gicv2_end_of_interrupt(unsigned int id)
 	assert(driver_data != NULL);
 	assert(driver_data->gicc_base != 0U);
 
+	/*
+	 * Ensure the write to peripheral registers are *complete* before the write
+	 * to GIC_EOIR.
+	 *
+	 * Note: The completion gurantee depends on various factors of system design
+	 * and the barrier is the best core can do by which execution of further
+	 * instructions waits till the barrier is alive.
+	 */
+	dsbishst();
 	gicc_write_EOIR(driver_data->gicc_base, id);
 }
 

include/drivers/arm/gicv3.h

@@ -332,6 +332,18 @@ static inline uint32_t gicv3_get_pending_interrupt_id_sel1(void)
 
 static inline void gicv3_end_of_interrupt_sel1(unsigned int id)
 {
+	/*
+	 * Interrupt request deassertion from peripheral to GIC happens
+	 * by clearing interrupt condition by a write to the peripheral
+	 * register. It is desired that the write transfer is complete
+	 * before the core tries to change GIC state from 'AP/Active' to
+	 * a new state on seeing 'EOI write'.
+	 * Since ICC interface writes are not ordered against Device
+	 * memory writes, a barrier is required to ensure the ordering.
+	 * The dsb will also ensure *completion* of previous writes with
+	 * DEVICE nGnRnE attribute.
+	 */
+	dsbishst();
 	write_icc_eoir1_el1(id);
 }
 
@@ -345,6 +357,18 @@ static inline uint32_t gicv3_acknowledge_interrupt(void)
 
 static inline void gicv3_end_of_interrupt(unsigned int id)
 {
+	/*
+	 * Interrupt request deassertion from peripheral to GIC happens
+	 * by clearing interrupt condition by a write to the peripheral
+	 * register. It is desired that the write transfer is complete
+	 * before the core tries to change GIC state from 'AP/Active' to
+	 * a new state on seeing 'EOI write'.
+	 * Since ICC interface writes are not ordered against Device
+	 * memory writes, a barrier is required to ensure the ordering.
+	 * The dsb will also ensure *completion* of previous writes with
+	 * DEVICE nGnRnE attribute.
+	 */
+	dsbishst();
 	return write_icc_eoir0_el1(id);
 }