DDC communication fails more often than succeeds, and setting features never succeeds.

[kennyrkun]

Hello, I am trying to control the brightness on a Planar PCT2435 monitor connected to a Raspberry Pi 4. The monitor should support DDC, per page 16 of the user manual.

Occasionally, ddcutil detect reports Display 1 with the correct information about the monitor and VCP Version: 2.1. Typically, this command only works one time and using it again after will report "Invalid display", as well as "VCP version: Detection failed". These does not appear to be exclusive to one another, as sometimes it will report "Invalid display" with a correct VCP version, and other times it will report "Display 1" with DDC communication failed.

I will attach the output of ddcutil interrogate, because I'm really not sure what's going on :( I believe this issue could be related to #288, "ddcutil sometimes work, sometimes detection error". I created a separate issue because we are using different hardware.

ddcutil interrogate.txt

[rockowitz]

First, thank you for creating a separate issue, and also for including the interrogate output.

You are using a very old release of ddcutil, 0.9.2 from November 2018. In particular, it lacks the --sleep-multiplier option added in ddcutil 0.9.8.

Your symptoms (sometimes things work, sometimes they don't) suggest a marginal DDC implementation in the monitor. The one way that ddcutil can sometimes address this problem is by increasing the amount of time from that specified for waiting between sending a DDC request packet and reading the reply packet. The way to do that is the *--sleep-multiplier option, e.g. --sleep-multiplier 2.0.

[kennyrkun]

Okay, the package is outdated. I have installed
ddcutil using apt, and I'm not super familiar with apt. Is there a newer version of the package that I can upgrade to, or do I need to compile and install from source? If so, I can compile but I do not know how to install the package manually.

edit: 2022/03/19 22:33 gmt
I updated my Pi to Raspbain Bullseye, and am running version 0.9.9 of ddcutil, and having the same problem. I ran ddcutil setvcp 10 50 and it worked, but I couldn't get any commands to work again after that. Most of the time I'm still getting Display not found as the result for setvcp. I've also been getting this error with every run of ddcutil: Open failed for /dev/usb/hiddev2: errno=EACCES(13): Permission denied, though, I believe this is an issue with an internal configuration of mine somewhere rather than an issue with ddcutil, and I am not running with sudo.

I have attached another run of ddcutil interrogate.

dductil_interrogate.txt

[rockowitz]

Have you tried using option --sleep-multiplier 2.0?

[kennyrkun]

It appears the issue persists even when using --sleep-multiplier 2.0. Occasionally the command will work, but usually it says Display not found..

ddcutil interrogate.txt

[rockowitz]

I'm afraid I don't have any further suggestions. The statistics in the interrogate output (which you can see if you use the --stats option on a command) show large numbers of data errors, and even those operations that ultimately succeed often require a large number of retries. In addition to --sleep-multiplier, you could also try raising the --maxtries values, but given the extent of data errors I doubt that would resolve the problem. FYI, the option --ddc will report the individual data errors.