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.

13 files changed, 152 insertions, 17 deletions

diff --git a/ports/esp32s2/boards/adafruit_magtag_2.9_grayscale/board.c b/ports/esp32s2/boards/adafruit_magtag_2.9_grayscale/board.c
index 9f708874b..ecd44e423 100644
--- a/ports/esp32s2/boards/adafruit_magtag_2.9_grayscale/board.c
+++ b/ports/esp32s2/boards/adafruit_magtag_2.9_grayscale/board.c
@@ -26,7 +26,83 @@
 
 #include "boards/board.h"
 #include "mpconfigboard.h"
+#include "shared-bindings/busio/SPI.h"
+#include "shared-bindings/displayio/FourWire.h"
 #include "shared-bindings/microcontroller/Pin.h"
+#include "shared-module/displayio/__init__.h"
+#include "supervisor/shared/board.h"
+
+#define DELAY 0x80
+
+// This is an ILO373 control chip. The display is a 2.9" grayscale EInk.
+
+const uint8_t display_start_sequence[] = {
+    0x01, 5, 0x03, 0x00, 0x2b, 0x2b, 0x13, // power setting
+    0x06, 3, 0x17, 0x17, 0x17, // booster soft start
+    0x04, DELAY, 200, // power on and wait 200 ms
+    0x00, 1, 0x7f, // panel setting
+    0x50, 1, 0x97, // CDI setting
+    0x30, 1, 0x3c, // PLL set to 50 Hx (M = 7, N = 4)
+    0x61, 3, 0x80, 0x01, 0x28, // Resolution
+    0x82, DELAY | 1, 0x12, 50, // VCM DC and delay 50ms
+
+    // Look up tables for voltage sequence for pixel transition
+    // Common voltage
+    0x20, 0x2a,
+    0x00, 0x0a, 0x00, 0x00, 0x00, 0x01,
+    0x60, 0x14, 0x14, 0x00, 0x00, 0x01,
+    0x00, 0x14, 0x00, 0x00, 0x00, 0x01,
+    0x00, 0x13, 0x0a, 0x01, 0x00, 0x01,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+
+    // White to white
+    0x21, 0x2a,
+    0x40, 0x0a, 0x00, 0x00, 0x00, 0x01,
+    0x90, 0x14, 0x14, 0x00, 0x00, 0x01,
+    0x10, 0x14, 0x0a, 0x00, 0x00, 0x01,
+    0xa0, 0x13, 0x01, 0x00, 0x00, 0x01,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+
+    // Black to white
+    0x22, 0x2a,
+    0x40, 0x0a, 0x00, 0x00, 0x00, 0x01,
+    0x90, 0x14, 0x14, 0x00, 0x00, 0x01,
+    0x00, 0x14, 0x0a, 0x00, 0x00, 0x01,
+    0x99, 0x0c, 0x01, 0x03, 0x04, 0x01,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+
+    // White to black
+    0x23, 0x2a,
+    0x40, 0x0a, 0x00, 0x00, 0x00, 0x01,
+    0x90, 0x14, 0x14, 0x00, 0x00, 0x01,
+    0x00, 0x14, 0x0a, 0x00, 0x00, 0x01,
+    0x99, 0x0b, 0x04, 0x04, 0x01, 0x01,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+
+    // Black to black
+    0x24, 0x2a,
+    0x80, 0x0a, 0x00, 0x00, 0x00, 0x01,
+    0x90, 0x14, 0x14, 0x00, 0x00, 0x01,
+    0x20, 0x14, 0x0a, 0x00, 0x00, 0x01,
+    0x50, 0x13, 0x01, 0x00, 0x00, 0x01,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00
+};
+
+const uint8_t display_stop_sequence[] = {
+    0x50, 0x01, 0x17,  // CDI Setting
+    0x82, 0x01, 0x00,  // VCM DC to -0.1V
+    0x02, 0x00  // Power off
+};
 
 void board_init(void) {
     // USB
@@ -36,6 +112,52 @@ void board_init(void) {
     // Debug UART
     common_hal_never_reset_pin(&pin_GPIO43);
     common_hal_never_reset_pin(&pin_GPIO44);
+
+    busio_spi_obj_t* spi = &displays[0].fourwire_bus.inline_bus;
+    common_hal_busio_spi_construct(spi, &pin_GPIO36, &pin_GPIO35, NULL);
+    common_hal_busio_spi_never_reset(spi);
+
+    displayio_fourwire_obj_t* bus = &displays[0].fourwire_bus;
+    bus->base.type = &displayio_fourwire_type;
+    common_hal_displayio_fourwire_construct(bus,
+        spi,
+        &pin_GPIO7, // EPD_DC Command or data
+        &pin_GPIO8, // EPD_CS Chip select
+        &pin_GPIO6, // EPD_RST Reset
+        4000000, // Baudrate
+        0, // Polarity
+        0); // Phase
+
+    displayio_epaperdisplay_obj_t* display = &displays[0].epaper_display;
+    display->base.type = &displayio_epaperdisplay_type;
+    common_hal_displayio_epaperdisplay_construct(
+        display,
+        bus,
+        display_start_sequence, sizeof(display_start_sequence),
+        display_stop_sequence, sizeof(display_stop_sequence),
+        296,  // width
+        128,  // height
+        160,  // ram_width
+        296,  // ram_height
+        0,  // colstart
+        0,  // rowstart
+        270,  // rotation
+        NO_COMMAND,  // set_column_window_command
+        NO_COMMAND,  // set_row_window_command
+        NO_COMMAND,  // set_current_column_command
+        NO_COMMAND,  // set_current_row_command
+        0x10,  // write_black_ram_command
+        false,  // black_bits_inverted
+        0x13,  // write_color_ram_command
+        false,  // color_bits_inverted
+        0x000000,  // highlight_color
+        0x12,  // refresh_display_command
+        1.0,  // refresh_time
+        &pin_GPIO5,  // busy_pin
+        false,  // busy_state
+        5.0, // seconds_per_frame
+        false,  // always_toggle_chip_select
+        true);  // grayscale
 }
 
 bool board_requests_safe_mode(void) {
diff --git a/ports/esp32s2/boards/adafruit_magtag_2.9_grayscale/pins.c b/ports/esp32s2/boards/adafruit_magtag_2.9_grayscale/pins.c
index 65bc3fb53..40c9e91e4 100644
--- a/ports/esp32s2/boards/adafruit_magtag_2.9_grayscale/pins.c
+++ b/ports/esp32s2/boards/adafruit_magtag_2.9_grayscale/pins.c
@@ -1,5 +1,7 @@
 #include "shared-bindings/board/__init__.h"
 
+#include "shared-module/displayio/__init__.h"
+
 STATIC const mp_rom_map_elem_t board_global_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_D10), MP_ROM_PTR(&pin_GPIO10) },
     { MP_ROM_QSTR(MP_QSTR_AD1), MP_ROM_PTR(&pin_GPIO18) },
@@ -37,5 +39,7 @@ STATIC const mp_rom_map_elem_t board_global_dict_table[] = {
 
     { MP_ROM_QSTR(MP_QSTR_I2C), MP_ROM_PTR(&board_i2c_obj) },
     { MP_ROM_QSTR(MP_QSTR_SPI), MP_ROM_PTR(&board_spi_obj) },
+
+    { MP_ROM_QSTR(MP_QSTR_DISPLAY), MP_ROM_PTR(&displays[0].epaper_display)}
 };
 MP_DEFINE_CONST_DICT(board_module_globals, board_global_dict_table);
diff --git a/ports/esp32s2/common-hal/displayio/ParallelBus.c b/ports/esp32s2/common-hal/displayio/ParallelBus.c
index 314b72ff7..d0c98f361 100644
--- a/ports/esp32s2/common-hal/displayio/ParallelBus.c
+++ b/ports/esp32s2/common-hal/displayio/ParallelBus.c
@@ -57,7 +57,8 @@ bool common_hal_displayio_parallelbus_begin_transaction(mp_obj_t obj) {
     return false;
 }
 
-void common_hal_displayio_parallelbus_send(mp_obj_t obj, 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 obj, display_byte_type_t byte_type,
+        display_chip_select_behavior_t chip_select, const uint8_t *data, uint32_t data_length) {
 
 }
 
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);
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];