Second successive input source retrieve fails

[rosshardy]

• Monitor manufacturer and model number: ASUS VG27A
• Input source (HDMI, VGA, display port, ect.): DisplayPort / HDMI
• Output device (video card, discrete graphics, ect.): discrete nvidia 3080
• Operating system: Windows 11
• Python version: 3.8.3
• monitorcontrol version (monitorcontrol --version): 3.1.0

Steps to Reproduce

The following python code errors:

from monitorcontrol import get_monitors

try:
    with get_monitors()[1] as monitor: 
        inputsource = monitor.get_input_source()
        print(inputsource)

        inputsourceagain = monitor.get_input_source()
        print(inputsourceagain)
except Exception as e:
    print(f"An error occurred: {e}")

Output:

InputSource.HDMI2
An error occurred: failed to get VCP feature: An error occurred while transmitting data to the device on the I2C bus.

[rosshardy]

I think this is due to a second query too soon after the first. Adding a time.sleep(1) is a workaround, but it would be nice if maybe the library could handle this or provide better insight.

[newAM]

There are rate limits defined in the DDC-CI specification, on Linux these are adjustable:

monitorcontrol/monitorcontrol/vcp/vcp_linux.py

Lines 35 to 37 in 6674ac3

	# timeouts
	GET_VCP_TIMEOUT = 0.04 # at least 40ms per the DDCCI specification
	CMD_RATE = 0.05 # at least 50ms between messages

Those values should be enough as-is, but sometimes monitors need a little bit more time. Sadly in Windows these values are fixed; Microsoft is in the right here, but it does make workarounds like this necessary for some monitors :(