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, 15 insertions, 11 deletions

diff --git a/shared-module/displayio/ColorConverter.c b/shared-module/displayio/ColorConverter.c
index 03ec99ceb..80558d037 100644
--- a/shared-module/displayio/ColorConverter.c
+++ b/shared-module/displayio/ColorConverter.c
@@ -55,7 +55,7 @@ uint8_t displayio_colorconverter_compute_luma(uint32_t color_rgb888) {
     uint32_t r8 = (color_rgb888 >> 16);
     uint32_t g8 = (color_rgb888 >> 8) & 0xff;
     uint32_t b8 = color_rgb888 & 0xff;
-    return (r8 * 19) / 255 + (g8 * 182) / 255 + (b8 + 54) / 255;
+    return (r8 * 19 + g8 * 182 + b8 * 54) / 255;
 }
 
 uint8_t displayio_colorconverter_compute_chroma(uint32_t color_rgb888) {
diff --git a/shared-module/displayio/EPaperDisplay.c b/shared-module/displayio/EPaperDisplay.c
index 514b99a13..acca92ac1 100644
--- a/shared-module/displayio/EPaperDisplay.c
+++ b/shared-module/displayio/EPaperDisplay.c
@@ -43,7 +43,8 @@
 #include <string.h>
 
 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,
@@ -133,14 +134,15 @@ STATIC void wait_for_busy(displayio_epaperdisplay_obj_t* self) {
     }
 }
 
-STATIC void send_command_sequence(displayio_epaperdisplay_obj_t* self, bool should_wait_for_busy, uint8_t* sequence, uint32_t sequence_len) {
+STATIC void send_command_sequence(displayio_epaperdisplay_obj_t* self,
+        bool should_wait_for_busy, const uint8_t* sequence, uint32_t sequence_len) {
     uint32_t i = 0;
     while (i < sequence_len) {
-        uint8_t *cmd = sequence + i;
+        const uint8_t *cmd = sequence + i;
         uint8_t data_size = *(cmd + 1);
         bool delay = (data_size & DELAY) != 0;
         data_size &= ~DELAY;
-        uint8_t *data = cmd + 2;
+        const uint8_t *data = cmd + 2;
         displayio_display_core_begin_transaction(&self->core);
         self->core.send(self->core.bus, DISPLAY_COMMAND, self->chip_select, cmd, 1);
         self->core.send(self->core.bus, DISPLAY_DATA, self->chip_select, data, data_size);
@@ -375,8 +377,8 @@ void release_epaperdisplay(displayio_epaperdisplay_obj_t* self) {
 
 void displayio_epaperdisplay_collect_ptrs(displayio_epaperdisplay_obj_t* self) {
     displayio_display_core_collect_ptrs(&self->core);
-    gc_collect_ptr(self->start_sequence);
-    gc_collect_ptr(self->stop_sequence);
+    gc_collect_ptr((void *) self->start_sequence);
+    gc_collect_ptr((void *) self->stop_sequence);
 }
 
 bool maybe_refresh_epaperdisplay(void) {
diff --git a/shared-module/displayio/EPaperDisplay.h b/shared-module/displayio/EPaperDisplay.h
index 4103fe5fc..d0668ff44 100644
--- a/shared-module/displayio/EPaperDisplay.h
+++ b/shared-module/displayio/EPaperDisplay.h
@@ -38,9 +38,9 @@ typedef struct {
     displayio_display_core_t core;
     digitalio_digitalinout_obj_t busy;
     uint32_t milliseconds_per_frame;
-    uint8_t* start_sequence;
+    const uint8_t* start_sequence;
     uint32_t start_sequence_len;
-    uint8_t* stop_sequence;
+    const uint8_t* stop_sequence;
     uint32_t stop_sequence_len;
     uint16_t refresh_time;
     uint16_t set_column_window_command;
diff --git a/shared-module/displayio/FourWire.c b/shared-module/displayio/FourWire.c
index 8c56d7ab6..06f8a84e3 100644
--- a/shared-module/displayio/FourWire.c
+++ b/shared-module/displayio/FourWire.c
@@ -113,7 +113,8 @@ bool common_hal_displayio_fourwire_begin_transaction(mp_obj_t obj) {
     return true;
 }
 
-void common_hal_displayio_fourwire_send(mp_obj_t obj, display_byte_type_t data_type, display_chip_select_behavior_t chip_select, uint8_t *data, uint32_t data_length) {
+void common_hal_displayio_fourwire_send(mp_obj_t obj, display_byte_type_t data_type,
+        display_chip_select_behavior_t chip_select, const uint8_t *data, uint32_t data_length) {
     displayio_fourwire_obj_t* self = MP_OBJ_TO_PTR(obj);
     common_hal_digitalio_digitalinout_set_value(&self->command, data_type == DISPLAY_DATA);
     if (chip_select == CHIP_SELECT_TOGGLE_EVERY_BYTE) {
diff --git a/shared-module/displayio/I2CDisplay.c b/shared-module/displayio/I2CDisplay.c
index 5bd03dcd6..cc811e83d 100644
--- a/shared-module/displayio/I2CDisplay.c
+++ b/shared-module/displayio/I2CDisplay.c
@@ -103,7 +103,8 @@ bool common_hal_displayio_i2cdisplay_begin_transaction(mp_obj_t obj) {
     return common_hal_busio_i2c_try_lock(self->bus);
 }
 
-void common_hal_displayio_i2cdisplay_send(mp_obj_t obj, display_byte_type_t data_type, display_chip_select_behavior_t chip_select, uint8_t *data, uint32_t data_length) {
+void common_hal_displayio_i2cdisplay_send(mp_obj_t obj, display_byte_type_t data_type,
+        display_chip_select_behavior_t chip_select, const uint8_t *data, uint32_t data_length) {
     displayio_i2cdisplay_obj_t* self = MP_OBJ_TO_PTR(obj);
     if (data_type == DISPLAY_COMMAND) {
         uint8_t command_bytes[2 * data_length];