Years ago Applied Micro (APM) released XGene processor. It went to APM BlackBird, APM Mustang, HPe M400 and several other systems. For some time there was no other AArch64 cpu available on market so those machines got popular as distribution builders, developer machines etc…
Then APM got aquired by someone, CPU part got bought by someone else and any support just vanished. Their developers moved to work on XGene2/XGene3 cpus (APM Merlin etc systems). And people woke up with not-supported hardware.
For some time it was not an issue - Linux boots, system works. Some companies got rid of their XGene systems by sending them to Linaro lab, some moved them to ‘internal use only, no external support’ queue etc.
Each mainline kernel release was “let us check what is broken on XGene this time” time. No serial console output again? Ok, we have that ugly patch for it (got cleaned and upstreamed). Now we have kernel 4.16 and guess what? Yes, it broke. Turned out that 4.15 was already faulty (we skipped it at Linaro).
Red Hat bugzilla has a Fedora bug for it. Turns out that firmware has wrong ACPI tables. Nothing new, right? We already know that it lacks PPTT for example (but it is quite new thing for processors topology). This time bug is present in DSDT one.
Sounds familiar? If you had x86 laptop about 10 years ago then it could. DSDT stands for Differentiated System Description Table. It is a major ACPI table used to describe what peripherals the machine has. And serial ports are described wrong there so kernel ignores them.
One of solutions is bundling fixed DSDT to kernel/initrd but that would require adding support for it into Debian and probably not get merged as no one needs that nowadays (unless they have XGene1).
So far I decided to stay on 4.14 for my development cartridges. It works and allows me to continue my Nova work. Do not plan to move to other platform as at Linaro we have probably over hundred XGene1 systems (M400 and Mustangs) which will stay there for development (hard to replace 4.3U case with 45 cartridges by something else).