displayio: the display does not survive a VM restart on SAMD21

[deshipu]

CircuitPython version

Adafruit CircuitPython 7.3.0-alpha.0-22-g102ee716a-dirty on 2022-04-12; PewPew LCD with samd21e18

Code/REPL

import board
import terminalio
import displayio
from adafruit_display_text import label
from adafruit_st7789 import ST7789
import busio

# Release any resources currently in use for the displays
displayio.release_displays()

spi = busio.SPI(clock=board.SCK, MOSI=board.MOSI)
tft_cs = board.D5
tft_dc = board.D9

display_bus = displayio.FourWire(
    spi, command=tft_dc, chip_select=tft_cs, reset=board.D6
)

display = ST7789(display_bus, width=240, height=240, rowstart=80)

# Make the display context
splash = displayio.Group()
display.show(splash)

color_bitmap = displayio.Bitmap(24, 24, 1)
color_palette = displayio.Palette(1)
color_palette[0] = 0x00FF00  # Bright Green

bg_sprite = displayio.TileGrid(color_bitmap, pixel_shader=color_palette, x=0, y=0)
splash.append(bg_sprite)

# Draw a smaller inner rectangle
inner_bitmap = displayio.Bitmap(20, 20, 1)
inner_palette = displayio.Palette(1)
inner_palette[0] = 0xAA0088  # Purple
inner_sprite = displayio.TileGrid(inner_bitmap, pixel_shader=inner_palette, x=20, y=20)
splash.append(inner_sprite)

# Draw a label
text_group = displayio.Group(scale=2, x=50, y=120)
text = "Hello World!"
text_area = label.Label(terminalio.FONT, text=text, color=0xFFFF00)
text_group.append(text_area)  # Subgroup for text scaling
splash.append(text_group)

Behavior

The display keeps displaying two squares and text even after the code finishes running. It should instead display the REPL with the "Code done running." message.

Same happens when the code is interrupted with ctrl+c — instead of REPL, the display shows the last thing the program was displaying.

Description

The reverse happens if I copy-paste the display initialization code in the REPL — the display works until ctrl+c is pressed.

Additional information

If the display is initialized in board.c as part of the startup, then it initializes properly, but never displays anything.

[deshipu]

With more testing, I noticed that sometimes, randomly, I can get a line or two of the traceback, before the display freezes, so there is some kind of race condition in there perhaps?

[deshipu]

A much easier reproducer: the HalloWing M0 Express display no longer works with recent versions of CircuitPython.

[deshipu]

The last version that works is adafruit-circuitpython-hallowing_m0_express-en_US-7.0.0-alpha.6.uf2, the first version that is broken is adafruit-circuitpython-hallowing_m0_express-en_US-7.0.0-beta.0.uf2.

[deshipu]

Further testing leads me to believe that it's commit f0859ac954e71e00e298843dc7f9f9c09a9bf570 that broke this.

[deshipu]

I have tried to experiment with this, but I really don't know what is happening. Perhaps the clock that the SPI peripheral uses gets shut down? The code itself doesn't refer to anything directly related to displayio. I give up.

[dhalbert]

Further testing leads me to believe that it's commit f0859ac that broke this.

Confirmed via bisect.