Document open problems for the experimental Pi 5 image.

TODO.md

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+# Experimental Pi 5 image
+
+## Linux kernel packages
+
+Until the Pi 5 is supported by Linux mainline and until suitable `linux-image-*`
+packages are available in Debian, it makes sense to ship a specific image for
+the Pi 5 that features `linux-image-*` packages (meta and regular) from
+Raspberry OS.
+
+It makes little to no sense to ship the Debian packages as well, as they are not
+going to be useful, and might even make things complicated:
+
+ - What happens when an update shows up in Debian? Could it be preferred over
+   the Raspberry OS one for the following boot, effectively bricking the PiRogue
+   running on Pi 5?
+ - Additionally that means a slightly bigger image to download initially (~ 100
+   MB), plus useless upgrades later on.
+
+If we decide to get rid of them, we must make sure we're not losing any other
+packages pulled via dependencies, or any kind of integration (e.g. initramfs
+hooks).
+
+
+## Firmware packages
+
+A longstanding issue is that the `raspi-firmware` has been shipping Pi-related
+things from a very long while, mainly bootloader files and integration to make
+sure the Linux kernel image, the associated initramfs, and the DTBs are
+available under `/boot/firmware`. But that also include some wireless firmware
+files. It's been requested to move them to the existing `firmware-brcm80211`
+package but that hasn't happened yet.
+
+Unfortunately, Debian 12 doesn't ship firmware files making it possible to
+support the Pi 5 wireless interface (symptom: no `wlan0` interface). That's why
+we're pulling `firmware-brcm80211` from Raspberry OS in addition to Linux kernel
+packages. Since that package and Debian's `raspi-firmware` package have files in
+common, `dpkg` errors out when trying to install them side by side, that's why
+the Pi 5 image build uses `--force-overwrite`.
+
+Open questions:
+
+ - What happens if and when Debian ships an updated `raspi-firmware` package?
+   Will that trigger a new file conflict? Gut feeling: yes. An update during
+   Debian 12's lifetime (either via the security team or the release team) seems
+   *unlikely*.
+ - What happens if and when Raspberry OS ships an updated `firmware-brcm80211`
+   package? Will that trigger a new file conflict? Gut feeling: maybe not, if
+   the initial overwrite triggered this package's being the registered owner of
+   those files, we might be safe. An update during Debian 12's lifetime seems
+   *possible* to *probable*, as the `firmware-nonfree` source package that
+   builds this binary package might get updated to support newer hardware and/or
+   to fix security and stability problems.
+
+It would be good to test both cases to see what happens. If both break horribly
+and we want to be safe, we could adjust the APT pinning (`pirogue.pref`) to
+avoid upgrades for both packages, for example.