sam0_common: Use Single-cycle I/O Port for GPIO when available

The Cortex-m0 based ATSAM devices can use the Single-cycle I/O Port for
GPIO. This commit modifies the gpio_t type to use this port when
available. It is mapped back to the peripheral memory space for
configuration access.  When it is not available, the _port_iobus() and
_port() functions behave identical, which is the case for the samd51.

cpu/sam0_common/include/periph_cpu_common.h

@@ -75,6 +75,8 @@ typedef uint32_t gpio_t;
  */
 #ifdef CPU_FAM_SAML11
 #define GPIO_PIN(x, y)      (((gpio_t)(&PORT_SEC->Group[x])) | y)
+#elif defined(PORT_IOBUS)   /* Use IOBUS access when available */
+#define GPIO_PIN(x, y)      (((gpio_t)(&PORT_IOBUS->Group[x])) | y)
 #else
 #define GPIO_PIN(x, y)      (((gpio_t)(&PORT->Group[x])) | y)
 #endif

cpu/sam0_common/periph/gpio.c

@@ -21,6 +21,19 @@
  * @author      Kaspar Schleiser <kaspar@schleiser.de>
  * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
  *
+ * The GPIO implementation here support the PERIPH_GPIO_FAST_READ pseudomodule
+ * on the cortex-m0/m23 based devices.  This trades an increase in power
+ * consumption for a decrease in GPIO pin read latency. Enable this in your
+ * makefile with:
+ *
+ * ```
+ * FEATURES_OPTIONAL += periph_gpio_fast_read
+ * ```
+ *
+ * This switches the GPIO reads to use the Cortex-M0+ single-cycle I/O port
+ * instead of the regular APB acces. The single-cycle I/O port is always used
+ * for writes when it is available on the device.
+ *
  * @}
  */
 
@@ -75,11 +88,29 @@ typedef enum {
 static gpio_isr_ctx_t gpio_config[NUMOF_IRQS];
 #endif /* MODULE_PERIPH_GPIO_IRQ */
 
-static inline PortGroup *_port(gpio_t pin)
+/* The Cortex-m0 based ATSAM devices can use the Single-cycle I/O Port for GPIO.
+ * When used, the gpio_t is mapped to the IOBUS area and must be mapped back to
+ * the peripheral memory space for configuration access. When it is not
+ * available, the _port_iobus() and _port() functions behave identical.
+ */
+static inline PortGroup *_port_iobus(gpio_t pin)
 {
     return (PortGroup *)(pin & ~(0x1f));
 }
 
+static inline PortGroup *_port(gpio_t pin)
+{
+#ifdef PORT_IOBUS
+    /* Shift the PortGroup address back from the IOBUS region to the peripheral
+     * region
+     */
+    return (PortGroup *)((uintptr_t)_port_iobus(pin) -
+                         (uintptr_t)PORT_IOBUS + (uintptr_t)PORT);
+#else
+    return _port_iobus(pin);
+#endif
+}
+
 static inline int _pin_pos(gpio_t pin)
 {
     return (pin & 0x1f);
@@ -121,9 +152,15 @@ int gpio_init(gpio_t pin, gpio_mode_t mode)
 
     /* set pin direction */
     if (mode & 0x2) {
+        if (IS_ACTIVE(MODULE_PERIPH_GPIO_FAST_READ)) {
+            port->CTRL.reg |= pin_mask;
+        }
         port->DIRCLR.reg = pin_mask;
     }
     else {
+        if (IS_ACTIVE(MODULE_PERIPH_GPIO_FAST_READ)) {
+            port->CTRL.reg &= ~pin_mask;
+        }
         port->DIRSET.reg = pin_mask;
     }
 
@@ -143,9 +180,16 @@ int gpio_init(gpio_t pin, gpio_mode_t mode)
 
 int gpio_read(gpio_t pin)
 {
-    PortGroup *port = _port(pin);
+    PortGroup *port;
     int mask = _pin_mask(pin);
 
+    if (IS_ACTIVE(MODULE_PERIPH_GPIO_FAST_READ)) {
+        port = _port_iobus(pin);
+    }
+    else {
+        port = _port(pin);
+    }
+
     if (port->DIR.reg & mask) {
         return (port->OUT.reg & mask) ? 1 : 0;
     }
@@ -156,25 +200,25 @@ int gpio_read(gpio_t pin)
 
 void gpio_set(gpio_t pin)
 {
-    _port(pin)->OUTSET.reg = _pin_mask(pin);
+    _port_iobus(pin)->OUTSET.reg = _pin_mask(pin);
 }
 
 void gpio_clear(gpio_t pin)
 {
-    _port(pin)->OUTCLR.reg = _pin_mask(pin);
+    _port_iobus(pin)->OUTCLR.reg = _pin_mask(pin);
 }
 
 void gpio_toggle(gpio_t pin)
 {
-    _port(pin)->OUTTGL.reg = _pin_mask(pin);
+    _port_iobus(pin)->OUTTGL.reg = _pin_mask(pin);
 }
 
 void gpio_write(gpio_t pin, int value)
 {
     if (value) {
-        _port(pin)->OUTSET.reg = _pin_mask(pin);
+        _port_iobus(pin)->OUTSET.reg = _pin_mask(pin);
     } else {
-        _port(pin)->OUTCLR.reg = _pin_mask(pin);
+        _port_iobus(pin)->OUTCLR.reg = _pin_mask(pin);
     }
 }