How to cross-build a debian package for Raspberry Pi OS 64 bit

Question

I have a working debian package that I'd like to backport to the current version of Raspberry Pi OS 64 bit (not 32-bit Raspbian).

Confusingly, while Debian itself seems to be robust about enabling cross-builds in their own package, there seems to be much less official documentation about how raspberry Pi OS (64 bit) packages are built¹.

Since I'm relatively certain this should be possible, I ask:

How to take a debian .dsc / debian rules, and build, on an x86_64, a 64 bit Raspberry Pi OS 64-bit compatible image

• without using QEMU to actually build the image on arm64, without access to an actual RPi,
• using an existing debian package that is known to work on sid on aarch64, and should be backportable,
• making sure it's actually built against the correct set of Raspbian dependencies.

I'm guess this is a rather standard thing, I just don't know how to do it. I'm happy with using containers and similar technology, as I can easily integrate that with CI.

I do not plan to use an Arm64 VM, as the software in question takes about an hour to build and test, on an x86_64 server, natively.

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^{¹I've talked to plugwash of Raspbian fame, and as earlier versions of this question showed: there's significant confusion about the heredity of Raspbian OS 64 bit: It's not Raspbian nor based on it. But people including Wikipedia and the RPi Foundation themselves conflate Raspberry Pi OS and Raspbian ("Raspberry Pi OS, formerly Raspbian"), which is 32 bit only.}

Answer 1

First, I would take a look here: https://github.com/Truelite/qt5custom for inspiration. I checked and those scritps work. However, you might have problems going completely „qemuless”; e.g. in case of QT some libraries needed to be added to host machine sysroot and qemu was simply the easiest way to add them properly: it seems to me that multiarch Debian has some deficiencies in the field of cross-compilation and the simplest way to overcome them is to pretend it’s the native one.

Answer 2

RaspberryPi documentation here has explained how to build the x64 kernel from the source. What you want is in a way exactly like that. Notice this line on the Kernel building page:

sudo apt install crossbuild-essential-arm64

This command on your Linux host machine installs a compiler that runs on an AMD64 machine but produces a binary that runs on an ARM machine.

And this line tells the compiler to actually build the source for that architecture:

make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- Image modules dtbs

Image modules dtbs are specific to your project. they may differ.

As for your Debian package, there is no way that you can transform an AMD64 package into an ARM one. Your package for the RaspberryPi if doesn't exist in an official or some third-party repository, must be built from the source.

Find the source code of your package and build it very similarly to RaspberryOS.

If your Package has dependencies it gets a little more complicated. First, install the dependency on your RaspberryPi. Then you should set up a sysroot on your host machine which is basically a mirror image of the preinstalled packages on RaspberryPi. Then for compiling your package you should give the sysroot address to cross compiler so that it can find dependencies.

There is another way too, you can put the source code of your package on your RaspberryPi and build it locally which can take a very long time based on the source code. Just to have a sense, Qt source code without WebEngine module took 48h for me. But Qt is big.

In conclusion, if your package binary is not on any repository you must compile it from the source.

Cross-compilation of different projects and executables are very similar to each other. To have a clear understanding of the process it can be beneficial to look for some other projects that were ported to RaspberryPi OS. Things like Qt, TagLib for android, and ...