The Generalist's Curse

It’s time for another blog post. I’ve been working on a side project over the past couple of weeks. It goes by the preliminary name kodinix and is yet another incarnation of the kodi media center software on a small single board computer (SBC). It is similar to LibreELEC, but adding a crucial bit of functionality: the ability to open a browser as fallback for accessing web content that is not covered by a kodi plugin. More details will follow shortly (the usual promise :wink:).

In this context I had to debug some issues with the graphics stack that drove me nuts for a few days. Working with embedded linux systems both professionally and for fun I often touch a wide variety of software components. Each and every one of them is a complex beast by itself. There’s people and teams out there for whom the development and maintenance for any single of these projects is a full-time job spanning years to decades, they are without doubt experts in their domain. Stitching together those components into a full system, selecting, packaging and confgiuring them to work well together is a task that leaves me with a glimpse into a wide variety of components and technologies, but for very few of these I have deeper knowledge or insight. I’d thus consider myself a generalist. And that sometimes makes it hard to track down issues.

The generalist’s curse is that the toes are far out in unknown waters when tracking down an issue. Each of the many ponds around us could hold the key for solving it. But getting acquainted to one pond, aquiring the needed throughsight just to find out that the problem lays somewhere else takes a lot of energy and time. And this can be frustrating as hell.

For this project I’m using a Pine64 H64 (model B) board. It’s a nicely specced board and should be able to suffice for video streaming. But I couldn’t even get it to load th kodi GUI. Well no, that’s not true. It did. At first. But than it didn’t anymore. So I reverted to the previous NixOS configuration…


… it still didn’t work. Down the rabbit hole I went.

To let you parttake in this I’ll elaborate on the relevant bits of the tech stack a little. We’ve got:

  • the Pine64 H64 board
    • ARM SoC
    • ARM Mali T720 GPU
    • Display connected via HDMI
  • mainline 64-bit ARM (aarch64) Linux kernel (v 5.10)
  • a 32-bit ARM (armv7l-hf) userland
  • Mesa3D graphics libs with panfrost GPU driver
  • cage wayland compositor
  • kodi media center software using wayland backend
  • kodi browser launcher plugin
  • firefox web browser

“Why mix architectures?” you might ask. Well, I’m stubborn and don’t want to run a 32bit kernel on hardware that can do better. Besides, I don’t really know if building a 32bit Linux kernel for this SoC and board would even properly work since some relevant device-tree bits live in arm64 subdirs of the kernel source tree. The 32bit userland is needed because I want widevine to work, a proprietary library needed to play DRM1 protected restricted media. Google doesn’t really release this library for ARM architectures, but their chromebooks ship a 32-bit ARM variant. There’s archane knowledge and utilities in the kodi community to extract the library from chromebook recovery images and use it inside kodi.

The problems manifested after small changes and a reboot. The graphics crashed whenever the cage compositor was started. The TTY1 console was displayed fine though. Reverting to the previous configuration didn’t bring the system back into a working state. This got me off track, since NixOS manages the system configuration in a declarative manner and thus should prevent exactly these runtime inconsistencies. But I hadn’t done any deeper inspection of the graphics or the general system before this error occured, so I was also lacking the reference to compare against.

I first blamed mesa and the panfrost GPU driver for this. The panfrost driver is still quite young. Mesa as a system library linking to hardware specific backends is complex and not 100% pure even in NixOS. Enough clues to start digging here. I spent some time recompiling mesa, ensuring it provided the panfrost driver and left out unrelated drivers. In the meantime I read up on mesa’s docs and the development state of the panfrost driver. By the time everything was ready for testing I had almost forgotten what the next planned step was. The long compile times of system components make it hard to stay focused on the assumption that I’m currently evaluating. I find hand-written notes quite helpful to keep some structure with this, whilest digital notekeeping add even more context switches and thus make things even worse.

I figured out it wasn’t panfrosts fault. Starting the system with a software rendering driver yielded the same result. Mesa’s documentation listed some environment variables that increased verbosity. The wall of text hinted at a problem with buffer allocation. After some research I tried increasing the memory region reserved for the kernels ContiniousMemoryAllocator (CMA) by adding a cma= argument to the kernel cmdline. It didn’t work and I moved on. In hindsight this was the correct approach probably just carried out wrong.

I trimmed down my test setup a bit, experienced the same issue with test programs like kmscube and started inspection of why I couldn’t get any core dumps (probably due to systemd-coredumpd and the kernel being incompatible due to the architecture mismatch). Many time consuming dead ends. At some intermediary point I had it mostly working again by hard coding the ExtendedDisplayIdentificationData (EDID) to use a lower display resolution. The graphics didn’t crash immediately anymore, the crash was delayed to some random point later in time. This and the fact that the crashlogs - where present - still mentioned buffer allocation errors made me revisit the CMA a few days later. Despite having tried it before. This time it worked.

I cursed. And I was happy to finally have a solution to the problem. I’ve learned some more about the linux graphics stack and especially mesa in the meantime, but I still feel like the next similar poroblem will probably be way above my head again. This is the curse of the generalist…

Feel free to contact me via mail or mastodon if you’ve got any notes on this post or start a public discussion via this blogs github issue tracker. You can also subscribe to the RSS feed to stay tuned!

  1. Speaking of DigitalRightsManagement aka DigitalRestrictionsManagement here. The linux kernel graphics stack also includes a component named DRM that can lead to a little confusion. 

Written on December 29, 2021