Add gpio_get_out_level() accessor, and correct SIO GPIO_OUT struct ty… (#247)

* Add gpio_get_out_level() accessor, and correct SIO GPIO_OUT struct type from WO to RW

* Clean up ambiguous-looking parentheses

src/rp2040/hardware_structs/include/hardware/structs/sio.h

@@ -17,7 +17,7 @@ typedef struct {
     io_ro_32 gpio_hi_in;
     uint32_t _pad;
 
-    io_wo_32 gpio_out;
+    io_rw_32 gpio_out;
     io_wo_32 gpio_set;
     io_wo_32 gpio_clr;
     io_wo_32 gpio_togl;
@@ -27,7 +27,7 @@ typedef struct {
     io_wo_32 gpio_oe_clr;
     io_wo_32 gpio_oe_togl;
 
-    io_wo_32 gpio_hi_out;
+    io_rw_32 gpio_hi_out;
     io_wo_32 gpio_hi_set;
     io_wo_32 gpio_hi_clr;
     io_wo_32 gpio_hi_togl;

src/rp2_common/hardware_gpio/include/hardware/gpio.h

@@ -415,6 +415,26 @@ static inline void gpio_put(uint gpio, bool value) {
         gpio_clr_mask(mask);
 }
 
+/*! \brief Determine whether a GPIO is currently driven high or low
+ *  \ingroup hardware_gpio
+ *
+ * This function returns the high/low output level most recently assigned to a
+ * GPIO via gpio_put() or similar. This is the value that is presented outward
+ * to the IO muxing, *not* the input level back from the pad (which can be
+ * read using gpio_get()).
+ *
+ * To avoid races, this function must not be used for read-modify-write
+ * sequences when driving GPIOs -- instead functions like gpio_put() should be
+ * used to atomically update GPIOs. This accessor is intended for debug use
+ * only.
+ *
+ * \param gpio GPIO number
+ * \return true if the GPIO output level is high, false if low.
+ */
+static inline bool gpio_get_out_level(uint gpio) {
+    return !!(sio_hw->gpio_out & (1u << gpio));
+}
+
 // ----------------------------------------------------------------------------
 // Direction
 // ----------------------------------------------------------------------------