Using Qt in Open Source App that Communicates with Closed Source Hardware

Question

Preface

I'm having trouble determining if I can use the Qt framework for developing a cross platform desktop app.

Situation

The app is open source, and the libraries it uses are also open source. So I can provide the source code for the whole app without issue. However, the app will be communicating with hardware that has closed source firmware. The app will read and write data from the device. Read data will be used to visualize what the hardware is doing. It will also update the hardware by feeding the device an encrypted firmware file, which its boot-loader (also closed source) will encrypt and commit to flash. LGPL and GPL licensed code will in no way be used in the firmware or boot-loader.

Questions

Does the free LGPL version of the Qt framework allow me to keep my firmware as closed source? If so, does the GPL license allow for this use-case as well?

Edit

I read something interesting on a similar question involving the LGPL and GPL licenses:

"As a rule of thumb, the GPL reaches as far as the address space of the licensed code."

If I am reading this correctly, my Qt app should have no licensing issues.

Answer 1

From your description I think the use of LGPL (and probably GPL) code in the app is probably ok.

The GPL FAQ covers this:

To do this validly, you must make sure that the free and nonfree programs communicate at arms length, that they are not combined in a way that would make them effectively a single program.