Implement pico_bootsel_via_double_reset with the POWMAN DOUBLE_TAP fl…

…ag on RP2350.

The existing RAM implementation does not work because asserting the RUN pin holds SRAM in power-down.

src/rp2_common/pico_bootsel_via_double_reset/pico_bootsel_via_double_reset.c

@@ -13,6 +13,10 @@
 #include "pico/bootrom.h"
 #include "pico/binary_info.h"
 
+#if !PICO_RP2040
+#include "hardware/structs/powman.h"
+#endif
+
 // PICO_CONFIG: PICO_BOOTSEL_VIA_DOUBLE_RESET_TIMEOUT_MS, Window of opportunity for a second press of a reset button to enter BOOTSEL mode (milliseconds), type=int, default=200, group=pico_bootsel_via_double_reset
 #ifndef PICO_BOOTSEL_VIA_DOUBLE_RESET_TIMEOUT_MS
 #define PICO_BOOTSEL_VIA_DOUBLE_RESET_TIMEOUT_MS 200
@@ -41,12 +45,62 @@
 bi_decl(bi_program_feature("double reset -> BOOTSEL"));
 #endif
 
+#if PICO_RP2040
+
+// RP2040 stores a token in RAM, which is retained over assertion of the RUN pin.
+
 static const uint32_t magic_token[] = {
         0xf01681de, 0xbd729b29, 0xd359be7a,
 };
 
 static uint32_t __uninitialized_ram(magic_location)[count_of(magic_token)];
 
+static inline bool double_tap_flag_is_set(void) {
+    for (uint i = 0; i < count_of(magic_token); i++) {
+        if (magic_location[i] != magic_token[i]) {
+            return false;
+        }
+    }
+    return true;
+}
+
+static inline void set_double_tap_flag(void) {
+    for (uint i = 0; i < count_of(magic_token); i++) {
+        magic_location[i] = magic_token[i];
+    }
+}
+
+static inline void clear_double_tap_flag(void) {
+    magic_location[0] = 0;
+}
+
+#else
+
+// Newer microcontrollers have a purpose-made register which is retained over
+// RUN events, for detecting double-tap events. The ROM has built-in support
+// for this, but this library can also use the same hardware feature.
+// (Also, RAM is powered down when the RUN pin is asserted, so it's a bad
+// place to put the token!)
+//
+// Note if ROM support is also enabled (via DOUBLE_TAP in OTP BOOT_FLAGS) then
+// we never reach this point with the double tap flag still set. The window
+// is the sum of the delay added by this library and the delay added by the
+// ROM. It's not recommended to enable both, but it works.
+
+static inline bool double_tap_flag_is_set(void) {
+    return powman_hw->chip_reset & POWMAN_CHIP_RESET_DOUBLE_TAP_BITS;
+}
+
+static inline void set_double_tap_flag(void) {
+    hw_set_bits(&powman_hw->chip_reset, POWMAN_CHIP_RESET_DOUBLE_TAP_BITS);
+}
+
+static inline void clear_double_tap_flag(void) {
+    hw_clear_bits(&powman_hw->chip_reset, POWMAN_CHIP_RESET_DOUBLE_TAP_BITS);
+}
+
+#endif
+
 /* Check for double reset and enter BOOTSEL mode if detected
  *
  * This function is registered to run automatically before main(). The
@@ -62,19 +116,16 @@ static uint32_t __uninitialized_ram(magic_location)[count_of(magic_token)];
  * in place so that the second boot will go to the bootloader.
  */
 static void __attribute__((constructor)) boot_double_tap_check(void) {
-    for (uint i = 0; i < count_of(magic_token); i++) {
-        if (magic_location[i] != magic_token[i]) {
-            // Arm, wait, then disarm and continue booting
-            for (i = 0; i < count_of(magic_token); i++) {
-                magic_location[i] = magic_token[i];
-            }
-            busy_wait_us(PICO_BOOTSEL_VIA_DOUBLE_RESET_TIMEOUT_MS * 1000);
-            magic_location[0] = 0;
-            return;
-        }
+    if (!double_tap_flag_is_set()) {
+        // Arm, wait, then disarm and continue booting
+        set_double_tap_flag();
+        busy_wait_us(PICO_BOOTSEL_VIA_DOUBLE_RESET_TIMEOUT_MS * 1000);
+        clear_double_tap_flag();
+        return;
     }
+
     // Detected a double reset, so enter USB bootloader
-    magic_location[0] = 0;
+    clear_double_tap_flag();
 #ifdef PICO_BOOTSEL_VIA_DOUBLE_RESET_ACTIVITY_LED
     const uint32_t led_mask = 1u << PICO_BOOTSEL_VIA_DOUBLE_RESET_ACTIVITY_LED;
 #else