esp32/machine_pin: Add new hold keyword argument and remove PULL_HOLD.

The current pull=Pin.PULL_HOLD argument doesn't make a lot of sense in the
context of what it actually does vs what the ESP32 quickref document says
it does.

This commit removes PULL_HOLD and adds a new hold=True|False keyword
argument to Pin()/Pin.init().  Setting this to True will cause the ESP32 to
lock the configuration of the pin – including direction, output value,
drive strength, pull-up/-down – such that it can't be accidentally changed
and will be retained through a watchdog or internal reset.

Fixes issue #8283, and see also #8284.

docs/esp32/quickref.rst

@@ -177,6 +177,14 @@ safe maximum source/sink currents and approximate internal driver resistances:
  - ``Pin.DRIVE_2``: 20mA / 30 ohm (default strength if not configured)
  - ``Pin.DRIVE_3``: 40mA / 15 ohm
 
+The ``hold=`` keyword argument to ``Pin()`` and ``Pin.init()`` will enable the
+ESP32 "pad hold" feature. When set to ``True``, the pin configuration
+(direction, pull resistors and output value) will be held and any further
+changes (including changing the output level) will not be applied. Setting
+``hold=False`` will immediately apply any outstanding pin configuration changes
+and release the pin. Using ``hold=True`` while a pin is already held will apply
+any configuration changes and then immediately reapply the hold.
+
 Notes:
 
 * Pins 1 and 3 are REPL UART TX and RX respectively
@@ -549,6 +557,10 @@ deep-sleep mode::
     pin.init(pull=None)
     machine.deepsleep(10000)
 
+Output-configured RTC pins will also retain their output direction and level in
+deep-sleep if pad hold is enabled with the ``hold=True`` argument to
+``Pin.init()``.
+
 Non-RTC GPIO pins will be disconnected by default on entering deep-sleep.
 
 SD card

ports/esp32/machine_pin.c

@@ -47,7 +47,6 @@
 // Used to implement a range of pull capabilities
 #define GPIO_PULL_DOWN (1)
 #define GPIO_PULL_UP   (2)
-#define GPIO_PULL_HOLD (4)
 
 #if CONFIG_IDF_TARGET_ESP32
 #define GPIO_FIRST_NON_OUTPUT (34)
@@ -253,14 +252,15 @@ STATIC void machine_pin_print(const mp_print_t *print, mp_obj_t self_in, mp_prin
     mp_printf(print, "Pin(%u)", self->id);
 }
 
-// pin.init(mode=None, pull=-1, *, value, drive)
+// pin.init(mode=None, pull=-1, *, value, drive, hold)
 STATIC mp_obj_t machine_pin_obj_init_helper(const machine_pin_obj_t *self, size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args) {
-    enum { ARG_mode, ARG_pull, ARG_value, ARG_drive };
+    enum { ARG_mode, ARG_pull, ARG_value, ARG_drive, ARG_hold };
     static const mp_arg_t allowed_args[] = {
         { MP_QSTR_mode, MP_ARG_OBJ, {.u_obj = mp_const_none}},
         { MP_QSTR_pull, MP_ARG_OBJ, {.u_obj = MP_OBJ_NEW_SMALL_INT(-1)}},
         { MP_QSTR_value, MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_obj = MP_OBJ_NULL}},
         { MP_QSTR_drive, MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_obj = MP_OBJ_NULL}},
+        { MP_QSTR_hold, MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_obj = MP_OBJ_NULL}},
     };
 
     // parse args
@@ -325,10 +325,15 @@ STATIC mp_obj_t machine_pin_obj_init_helper(const machine_pin_obj_t *self, size_
         } else {
             gpio_pullup_dis(self->id);
         }
-        if (mode & GPIO_PULL_HOLD) {
+    }
-            gpio_hold_en(self->id);
+
-        } else if (GPIO_IS_VALID_OUTPUT_GPIO(self->id)) {
+    // configure pad hold
+    if (args[ARG_hold].u_obj != MP_OBJ_NULL && GPIO_IS_VALID_OUTPUT_GPIO(self->id)) {
+        // always disable pad hold to apply outstanding config changes
         gpio_hold_dis(self->id);
+        // (re-)enable pad hold if requested
+        if (mp_obj_is_true(args[ARG_hold].u_obj)) {
+            gpio_hold_en(self->id);
         }
     }
 
@@ -480,7 +485,6 @@ STATIC const mp_rom_map_elem_t machine_pin_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_OPEN_DRAIN), MP_ROM_INT(GPIO_MODE_INPUT_OUTPUT_OD) },
     { MP_ROM_QSTR(MP_QSTR_PULL_UP), MP_ROM_INT(GPIO_PULL_UP) },
     { MP_ROM_QSTR(MP_QSTR_PULL_DOWN), MP_ROM_INT(GPIO_PULL_DOWN) },
-    { MP_ROM_QSTR(MP_QSTR_PULL_HOLD), MP_ROM_INT(GPIO_PULL_HOLD) },
     { MP_ROM_QSTR(MP_QSTR_IRQ_RISING), MP_ROM_INT(GPIO_PIN_INTR_POSEDGE) },
     { MP_ROM_QSTR(MP_QSTR_IRQ_FALLING), MP_ROM_INT(GPIO_PIN_INTR_NEGEDGE) },
     { MP_ROM_QSTR(MP_QSTR_WAKE_LOW), MP_ROM_INT(GPIO_PIN_INTR_LOLEVEL) },