PIO UART unexpected in_waiting and read results

[anecdata]

CircuitPython version and board name

Adafruit CircuitPython 9.2.4 on 2025-01-29; Pimoroni Pico Plus 2 W with rp2350b
Adafruit CircuitPython 9.2.4 on 2025-01-29; Raspberry Pi Pico W with rp2040

Code/REPL

import time
import random
import board
import busio
import adafruit_pio_uart

time.sleep(3)  # wait for serial

# hardware setup: GP0 connected to GP27; GP1 connected to GP26
uart_busio = busio.UART(board.GP0, board.GP1)
uart_pio = adafruit_pio_uart.UART(board.GP26, board.GP27)

for buflen in (1, 2, 3, 7, 8, 9 ,10, 15, 16, 17, 18,):
    buffer = bytearray()
    for _ in range(0, buflen):
        buffer.append(random.choice(b'abcdefghijklmnopqrstuvwxyz'))
    print(f"\n{buflen=} {buffer=}")

    # PIO to busio
    uart_busio.reset_input_buffer()
    print(f"{uart_pio.write(buffer)=}")
    print(f"{uart_busio.in_waiting=}")
    print(f"{uart_busio.read(buflen)=}")

    # busio to PIO
    uart_pio.reset_input_buffer()
    print(f"{uart_busio.write(buffer)=}")
    time.sleep(0.25)  # w/o some wait, .in_waiting = 0
    print(f"{uart_pio.in_waiting=}")
    print(f"{uart_pio.read(buflen)=}")

Behavior

Code above compares busio UART behavior to PIO UART behavior (using the adafruit_pio_uart library).

busio correctly shows in_waiting bytes matching the number of bytes sent. read correctly reads the number of bytes sent.

For PIO, in_waiting maxxes out at 8, and read maxxes out at 9 bytes regardless of how many bytes (>9) are written to the UART.

Description

No response

Additional information

As noted in the code comments above, .in_waiting for the PIO will return 0 unless there is some delay before checking it. This extra blocking could be a problem for efficient user code, especially asyncio.