rp2/modrp2: Support reading the BOOTSEL button on the Pico board.

And any other board that exposes this pin with a button.

ports/rp2/modrp2.c

@@ -27,6 +27,11 @@
 #include "py/runtime.h"
 #include "drivers/dht/dht.h"
 #include "modrp2.h"
+#include "pico/stdlib.h"
+#include "hardware/gpio.h"
+#include "hardware/sync.h"
+#include "hardware/structs/ioqspi.h"
+#include "hardware/structs/sio.h"
 
 #if MICROPY_PY_NETWORK_CYW43
 #include "extmod/modnetwork.h"
@@ -36,11 +41,59 @@
 MP_DECLARE_CONST_FUN_OBJ_VAR_BETWEEN(mod_network_country_obj);
 #endif
 
+// Picoboard has a button attached to the flash CS pin, which the bootrom
+// checks, and jumps straight to the USB bootcode if the button is pressed
+// (pulling flash CS low). We can check this pin in by jumping to some code in
+// SRAM (so that the XIP interface is not required), floating the flash CS
+// pin, and observing whether it is pulled low.
+//
+// This doesn't work if others are trying to access flash at the same time,
+// e.g. XIP streamer, or the other core.
+// https://github.com/raspberrypi/pico-examples/blob/master/picoboard/button/button.c
+
+bool __no_inline_not_in_flash_func(get_bootsel_button)() {
+    const uint CS_PIN_INDEX = 1;
+
+    // Must disable interrupts, as interrupt handlers may be in flash, and we
+    // are about to temporarily disable flash access!
+    uint32_t flags = save_and_disable_interrupts();
+
+    // Set chip select to Hi-Z
+    hw_write_masked(&ioqspi_hw->io[CS_PIN_INDEX].ctrl,
+        GPIO_OVERRIDE_LOW << IO_QSPI_GPIO_QSPI_SS_CTRL_OEOVER_LSB,
+            IO_QSPI_GPIO_QSPI_SS_CTRL_OEOVER_BITS);
+
+    // Note we can't call into any sleep functions in flash right now
+    for (volatile int i = 0; i < 1000; ++i) {;
+    }
+
+    // The HI GPIO registers in SIO can observe and control the 6 QSPI pins.
+    // Note the button pulls the pin *low* when pressed.
+    bool button_state = !(sio_hw->gpio_hi_in & (1u << CS_PIN_INDEX));
+
+    // Need to restore the state of chip select, else we are going to have a
+    // bad time when we return to code in flash!
+    hw_write_masked(&ioqspi_hw->io[CS_PIN_INDEX].ctrl,
+        GPIO_OVERRIDE_NORMAL << IO_QSPI_GPIO_QSPI_SS_CTRL_OEOVER_LSB,
+            IO_QSPI_GPIO_QSPI_SS_CTRL_OEOVER_BITS);
+
+    restore_interrupts(flags);
+
+    return button_state;
+}
+
+STATIC mp_obj_t rp2_bootsel_button(void) {
+    return MP_OBJ_NEW_SMALL_INT(get_bootsel_button());
+}
+MP_DEFINE_CONST_FUN_OBJ_0(rp2_bootsel_button_obj, rp2_bootsel_button);
+
+
 STATIC const mp_rom_map_elem_t rp2_module_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR___name__),            MP_ROM_QSTR(MP_QSTR_rp2) },
     { MP_ROM_QSTR(MP_QSTR_Flash),               MP_ROM_PTR(&rp2_flash_type) },
     { MP_ROM_QSTR(MP_QSTR_PIO),                 MP_ROM_PTR(&rp2_pio_type) },
     { MP_ROM_QSTR(MP_QSTR_StateMachine),        MP_ROM_PTR(&rp2_state_machine_type) },
+    { MP_ROM_QSTR(MP_QSTR_bootsel_button),      MP_ROM_PTR(&rp2_bootsel_button_obj) },
 
     #if MICROPY_PY_NETWORK_CYW43
     // Deprecated (use network.country instead).