This issue is getting beyond being irritating. I normally run Mint for weeks at a time without restarting it (usually I restart only because there has been a kernel update). But, now, unless I restart my PC every time before using darktable, and multiple times while processing a batch of images, I will never actually be able to use the expensive GPU I have fitted; More, I experience delays in processing and exporting which add up to more time than it takes to restart the PC. This is not a sensible workflow.
Now I am finding that I can start processing an image in the darkroom with OpenCL enabled but before I have finished a simple exposure adjustment I find that OpenCl is not available. I ran darktable-cltest immediately after the latest experience; the terminal out put is as follows:
Again the issue indicated here is the inability of darktable to find libOpenCL and libOpenCl.so. I have looked for both of these in Synaptic. They are not listed. perhaps somebody who understands why could explain to me where I must look to find these important objects and, possibly explain to me why I didn’t have a problem with their absence in darktable 4.6.0 or 4.4 or 42. and so on.