I was amazed to learn that the Linux kernel supports 1,400 distinct 32-bit ARM targets!
That's ... a scary amount, and it's easy to see that automated testing might be a good thing, there.
I think the combinatorial explosion happens at least in part because even though there's a limited number of actual ARM cores, the peripherals which Linux needs to support are often vendor-defined and thus different for each system-on-a-chip (or at least different for each device series from a particular manufacturer). I didn't dig through the sources to verify this, but I've heard of the problem before, ARM doesn't define a standard way for the CPU core to learn about its peripherals at run-time.
Keep in mind that's mainly because the ARM world is not standardized the way the PC world is. In practice the differences between most of these targets will be the very early initialization code and things like the clock hierarchy but beyond that most of the code will be shared across many variants.
Imagine having to use a different kernel every time you upgrade your desktop, that's basically how the ARM world works so far.
That's ... a scary amount, and it's easy to see that automated testing might be a good thing, there.
I think the combinatorial explosion happens at least in part because even though there's a limited number of actual ARM cores, the peripherals which Linux needs to support are often vendor-defined and thus different for each system-on-a-chip (or at least different for each device series from a particular manufacturer). I didn't dig through the sources to verify this, but I've heard of the problem before, ARM doesn't define a standard way for the CPU core to learn about its peripherals at run-time.