rp2/machine_spi: Allow MISO to be unspecified.

It's common with write-only SPI displays for MISO to be repurposed as a
register select or data/command pin.

While that was possible by setting up the pin after a call to
`machine.SPI()` this change makes `machine.SPI(miso=None)` explicit.

Signed-off-by: Phil Howard <github@gadgetoid.com>

ports/rp2/machine_spi.c

@@ -80,6 +80,9 @@
 
 #endif
 
+// Assume we won't get an RP2-compatible with 255 GPIO pins
+#define MICROPY_HW_SPI_PIN_UNUSED UINT8_MAX
+
 // SPI0 can be GP{0..7,16..23}, SPI1 can be GP{8..15,24..29}.
 #define IS_VALID_PERIPH(spi, pin)   ((((pin) & 8) >> 3) == (spi))
 // GP{2,6,10,14,...}
@@ -120,9 +123,14 @@ static machine_spi_obj_t machine_spi_obj[] = {
 
 static void machine_spi_print(const mp_print_t *print, mp_obj_t self_in, mp_print_kind_t kind) {
     machine_spi_obj_t *self = MP_OBJ_TO_PTR(self_in);
-    mp_printf(print, "SPI(%u, baudrate=%u, polarity=%u, phase=%u, bits=%u, sck=%u, mosi=%u, miso=%u)",
+    mp_printf(print, "SPI(%u, baudrate=%u, polarity=%u, phase=%u, bits=%u, sck=%u, mosi=%u, miso=",
         self->spi_id, self->baudrate, self->polarity, self->phase, self->bits,
-        self->sck, self->mosi, self->miso);
+        self->sck, self->mosi);
+    if (self->miso == MICROPY_HW_SPI_PIN_UNUSED) {
+        mp_printf(print, "None)");
+    } else {
+        mp_printf(print, "%u)", self->miso);
+    }
 }
 
 mp_obj_t machine_spi_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *all_args) {
@@ -140,7 +148,7 @@ mp_obj_t machine_spi_make_new(const mp_obj_type_t *type, size_t n_args, size_t n
         { MP_QSTR_firstbit, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = DEFAULT_SPI_FIRSTBIT} },
         { MP_QSTR_sck,      MICROPY_SPI_PINS_ARG_OPTS | MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_rom_obj = MP_ROM_NONE} },
         { MP_QSTR_mosi,     MICROPY_SPI_PINS_ARG_OPTS | MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_rom_obj = MP_ROM_NONE} },
-        { MP_QSTR_miso,     MICROPY_SPI_PINS_ARG_OPTS | MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_rom_obj = MP_ROM_NONE} },
+        { MP_QSTR_miso,     MICROPY_SPI_PINS_ARG_OPTS | MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_rom_obj = MP_ROM_INT(-1)} },
     };
 
     // Parse the arguments.
@@ -171,7 +179,10 @@ mp_obj_t machine_spi_make_new(const mp_obj_type_t *type, size_t n_args, size_t n
         }
         self->mosi = mosi;
     }
-    if (args[ARG_miso].u_obj != mp_const_none) {
+
+    if (args[ARG_miso].u_obj == mp_const_none) {
+        self->miso = MICROPY_HW_SPI_PIN_UNUSED;
+    } else if (args[ARG_miso].u_obj != MP_OBJ_NEW_SMALL_INT(-1)) {
         int miso = mp_hal_get_pin_obj(args[ARG_miso].u_obj);
         if (!IS_VALID_MISO(self->spi_id, miso)) {
             mp_raise_ValueError(MP_ERROR_TEXT("bad MISO pin"));
@@ -194,7 +205,9 @@ mp_obj_t machine_spi_make_new(const mp_obj_type_t *type, size_t n_args, size_t n
         self->baudrate = spi_set_baudrate(self->spi_inst, self->baudrate);
         spi_set_format(self->spi_inst, self->bits, self->polarity, self->phase, self->firstbit);
         gpio_set_function(self->sck, GPIO_FUNC_SPI);
-        gpio_set_function(self->miso, GPIO_FUNC_SPI);
+        if (self->miso != MICROPY_HW_SPI_PIN_UNUSED) {
+            gpio_set_function(self->miso, GPIO_FUNC_SPI);
+        }
         gpio_set_function(self->mosi, GPIO_FUNC_SPI);
     }