Add board.DISPLAY to MagTag. Fix luma computation

* Initialize the EPaper display on the MagTag at start.
* Tweak the display send to take a const buffer.
* Correct Luma math
  * Multiply the blue component, not add.
  * Add all of the components together before dividing. This
    reduces the impact of truncated division.

5 files changed, 9 insertions, 5 deletions

diff --git a/shared-bindings/displayio/EPaperDisplay.h b/shared-bindings/displayio/EPaperDisplay.h
index 352de899a..9a2d93bac 100644
--- a/shared-bindings/displayio/EPaperDisplay.h
+++ b/shared-bindings/displayio/EPaperDisplay.h
@@ -39,7 +39,7 @@ extern const mp_obj_type_t displayio_epaperdisplay_type;
 #define NO_COMMAND 0x100
 
 void common_hal_displayio_epaperdisplay_construct(displayio_epaperdisplay_obj_t* self,
-        mp_obj_t bus, uint8_t* start_sequence, uint16_t start_sequence_len, uint8_t* stop_sequence, uint16_t stop_sequence_len,
+        mp_obj_t bus, const uint8_t* start_sequence, uint16_t start_sequence_len, const uint8_t* stop_sequence, uint16_t stop_sequence_len,
         uint16_t width, uint16_t height, uint16_t ram_width, uint16_t ram_height, int16_t colstart, int16_t rowstart, uint16_t rotation,
         uint16_t set_column_window_command, uint16_t set_row_window_command,
         uint16_t set_current_column_command, uint16_t set_current_row_command,
diff --git a/shared-bindings/displayio/FourWire.h b/shared-bindings/displayio/FourWire.h
index ac186d2c3..6f6b528e7 100644
--- a/shared-bindings/displayio/FourWire.h
+++ b/shared-bindings/displayio/FourWire.h
@@ -48,7 +48,8 @@ bool common_hal_displayio_fourwire_bus_free(mp_obj_t self);
 
 bool common_hal_displayio_fourwire_begin_transaction(mp_obj_t self);
 
-void common_hal_displayio_fourwire_send(mp_obj_t self, display_byte_type_t byte_type, display_chip_select_behavior_t chip_select, uint8_t *data, uint32_t data_length);
+void common_hal_displayio_fourwire_send(mp_obj_t self, display_byte_type_t byte_type,
+    display_chip_select_behavior_t chip_select, const uint8_t *data, uint32_t data_length);
 
 void common_hal_displayio_fourwire_end_transaction(mp_obj_t self);
 
diff --git a/shared-bindings/displayio/I2CDisplay.h b/shared-bindings/displayio/I2CDisplay.h
index bae53c491..37520202e 100644
--- a/shared-bindings/displayio/I2CDisplay.h
+++ b/shared-bindings/displayio/I2CDisplay.h
@@ -44,7 +44,8 @@ bool common_hal_displayio_i2cdisplay_bus_free(mp_obj_t self);
 
 bool common_hal_displayio_i2cdisplay_begin_transaction(mp_obj_t self);
 
-void common_hal_displayio_i2cdisplay_send(mp_obj_t self, display_byte_type_t byte_type, display_chip_select_behavior_t chip_select, uint8_t *data, uint32_t data_length);
+void common_hal_displayio_i2cdisplay_send(mp_obj_t self, display_byte_type_t byte_type,
+    display_chip_select_behavior_t chip_select, const uint8_t *data, uint32_t data_length);
 
 void common_hal_displayio_i2cdisplay_end_transaction(mp_obj_t self);
 
diff --git a/shared-bindings/displayio/ParallelBus.h b/shared-bindings/displayio/ParallelBus.h
index be2aef7d4..1e74e3a0a 100644
--- a/shared-bindings/displayio/ParallelBus.h
+++ b/shared-bindings/displayio/ParallelBus.h
@@ -46,7 +46,8 @@ bool common_hal_displayio_parallelbus_bus_free(mp_obj_t self);
 
 bool common_hal_displayio_parallelbus_begin_transaction(mp_obj_t self);
 
-void common_hal_displayio_parallelbus_send(mp_obj_t self, display_byte_type_t byte_type, display_chip_select_behavior_t chip_select, uint8_t *data, uint32_t data_length);
+void common_hal_displayio_parallelbus_send(mp_obj_t self, display_byte_type_t byte_type,
+    display_chip_select_behavior_t chip_select, const uint8_t *data, uint32_t data_length);
 
 void common_hal_displayio_parallelbus_end_transaction(mp_obj_t self);
 
diff --git a/shared-bindings/displayio/__init__.h b/shared-bindings/displayio/__init__.h
index a7748d029..4fc666c59 100644
--- a/shared-bindings/displayio/__init__.h
+++ b/shared-bindings/displayio/__init__.h
@@ -42,7 +42,8 @@ typedef enum {
 typedef bool (*display_bus_bus_reset)(mp_obj_t bus);
 typedef bool (*display_bus_bus_free)(mp_obj_t bus);
 typedef bool (*display_bus_begin_transaction)(mp_obj_t bus);
-typedef void (*display_bus_send)(mp_obj_t bus, display_byte_type_t byte_type, display_chip_select_behavior_t chip_select, uint8_t *data, uint32_t data_length);
+typedef void (*display_bus_send)(mp_obj_t bus, display_byte_type_t byte_type,
+    display_chip_select_behavior_t chip_select, const uint8_t *data, uint32_t data_length);
 typedef void (*display_bus_end_transaction)(mp_obj_t bus);
 
 void common_hal_displayio_release_displays(void);