add irq_get_priority method - improve efficiency of irq_init_priorities (#609)

src/rp2_common/hardware_irq/include/hardware/irq.h

@@ -119,6 +119,10 @@ extern "C" {
  */
 typedef void (*irq_handler_t)(void);
 
+static inline void check_irq_param(__unused uint num) {
+    invalid_params_if(IRQ, num >= NUM_IRQS);
+}
+
 /*! \brief Set specified interrupt's priority
  *  \ingroup hardware_irq
  *
@@ -133,6 +137,21 @@ typedef void (*irq_handler_t)(void);
  */
 void irq_set_priority(uint num, uint8_t hardware_priority);
 
+/*! \brief Get specified interrupt's priority
+ *  \ingroup hardware_irq
+ *
+ * Numerically-lower values indicate a higher priority. Hardware priorities
+ * range from 0 (highest priority) to 255 (lowest priority) though only the
+ * top 2 bits are significant on ARM Cortex-M0+. To make it easier to specify
+ * higher or lower priorities than the default, all IRQ priorities are
+ * initialized to PICO_DEFAULT_IRQ_PRIORITY by the SDK runtime at startup.
+ * PICO_DEFAULT_IRQ_PRIORITY defaults to 0x80
+ *
+ * \param num Interrupt number
+ * \return the IRQ priority
+ */
+uint irq_get_priority(uint num);
+
 /*! \brief Enable or disable a specific interrupt on the executing core
  *  \ingroup hardware_irq
  *

src/rp2_common/hardware_irq/irq.c

@@ -33,10 +33,6 @@ static void set_raw_irq_handler_and_unlock(uint num, irq_handler_t handler, uint
     spin_unlock(spin_lock_instance(PICO_SPINLOCK_ID_IRQ), save);
 }
 
-static inline void check_irq_param(__unused uint num) {
-    invalid_params_if(IRQ, num >= NUM_IRQS);
-}
-
 void irq_set_enabled(uint num, bool enabled) {
     check_irq_param(num);
     irq_set_mask_enabled(1u << num, enabled);
@@ -364,6 +360,14 @@ void irq_set_priority(uint num, uint8_t hardware_priority) {
     *p = (*p & ~(0xffu << (8 * (num & 3u)))) | (((uint32_t) hardware_priority) << (8 * (num & 3u)));
 }
 
+uint irq_get_priority(uint num) {
+    check_irq_param(num);
+
+    // note that only 32 bit reads are supported
+    io_rw_32 *p = (io_rw_32 *)((PPB_BASE + M0PLUS_NVIC_IPR0_OFFSET) + (num & ~3u));
+    return (uint8_t)(*p >> (8 * (num & 3u)));
+}
+
 #if !PICO_DISABLE_SHARED_IRQ_HANDLERS
 // used by irq_handler_chain.S to remove the last link in a handler chain after it executes
 // note this must be called only with the last slot in a chain (and during the exception)
@@ -400,8 +404,11 @@ void irq_add_tail_to_free_list(struct irq_handler_chain_slot *slot) {
 
 void irq_init_priorities() {
 #if PICO_DEFAULT_IRQ_PRIORITY != 0
-    for (uint irq = 0; irq < NUM_IRQS; irq++) {
+    static_assert(!(NUM_IRQS & 3), "");
-        irq_set_priority(irq, PICO_DEFAULT_IRQ_PRIORITY);
+    uint32_t prio4 = (PICO_DEFAULT_IRQ_PRIORITY & 0xff) * 0x1010101u;
+    io_rw_32 * p = (io_rw_32 *)(PPB_BASE + M0PLUS_NVIC_IPR0_OFFSET);
+    for (uint i = 0; i < NUM_IRQS / 4; i++) {
+        *p++ = prio4;
     }
 #endif
 }