Hello Werner,
The way we decided to deal with the GPL-obligations is:
Thank you for your detailed explanation of how you handle your GPL responsibilities. What you describe makes perfect sense to me.
You have the advantage on us of having an output device on your system. And, since you run Linux your user base must be well aware of GPL.
Still, the more I look at this, the more it sounds like we would meet our obligations if we provide an About tab in our control panel application, and from there present the license and provide a means to fetch the source code from our hardware, thus tying the source tightly with the firmware. Every user must have that application installed to be able to run our hardware at all no matter how they got the hardware. So, every user who ever had the device and had any ability to use it would have access to the About tab.
Assuming this setup works for our responsibility, this also seems to free our customers from having to do anything to comply with GPL, though clearly we have to make them aware that our system contains GPL licensed code.
I do have one question: You clearly have no issue for your compliance. But, do you tell your OEM customers that they need to provide the GPL license to their customers as part of their product packaging, or do you consider the availability of all the required information built into your system (in the screen you describe) sufficient to meet your OEM's responsibilities as well as your own as long as they do not add any new GPL licensed code that you have not handled?
Ian