Adding NOTE about building 32-bit kernel

[aallan]

If you are attempting to compile a 32-bit kernel on the 32-bit distribution of Raspberry Pi OS you should be aware that while the userland in this distribution is 32-bit, the kernel is a 64-bit kernel. You should therefore follow the instructions for cross-compiling the kernel.

Closes #3019 when merged.

[pelwell]

That doesn't quite match my comment on #3019:

Following the cross-compilation instructions for the 64-bit and 32-bit kernel removes the need for guesswork.
However, on the case of building 32-bit kernel on top of a 32-bit OS with a 64-bit kernel, the only necessary change is ARCH=arm

In other words, building the 64-bit kernel on the 32-bit OS is more of a cross-compilation exercise because it requires the installation of the cross-compiler (which is what gcc-aarch64-linux-gnu is on the 32-bit OS). Building the 32-bit kernel on the 64-bit kernel + 32-bit OS combo just needs the ARCH=arm hint, although it's likely that users wanting to build the 32-bit kernel will already be selecting it with arm_64bit=0.

[mtaunton]

@aallan Re your changes in commit [0e99ba4].

Sorry, I hate to be awkward, but I still find the wording problematic.

To build a 32-bit kernel on 32-bit Raspberry Pi OS, it says to "set ARCH=arm", but doesn't explain how / where you do that. I assume it has to be given at least as an extra argument when making the default configuration, namely as:

make ARCH=arm bcm2711_defconfig

Is that correct / sufficient? Or does ARCH=arm need to be added also to the make command that builds the kernel itself?

Further, mentioning the possibility that "arm_64bit=0 will have been set" is misleading. If the user is indeed building a "32-bit kernel on the 32-bit distribution of Raspberry Pi OS — which has a 32-bit userland, and 64-bit kernel", then arm_64bit=1 cannot have been set, since the kernel by default is explicitly a "64-bit kernel".

[lurch]

"the 32-bit distribution of Raspberry Pi OS — which has a 32-bit userland, and 64-bit kernel" - that's only the case on Pi 4B, Pi 400, CM4 and CM4S ? https://www.raspberrypi.com/documentation/computers/config_txt.html#arm_64bit

[pelwell]

Yes, unless the user has explicitly set arm_64bit=1 on BCM2710-based device.

[aallan]

@lurch Can you nip in and fix that up?

[aallan]

Sorry, I hate to be awkward, but I still find the wording problematic.

Suggestions for revised wording are welcome.

[mtaunton]

OK, will give it a go (but not immediately, maybe tomorrow).

[mtaunton]

In the meantime, could someone (@pelwell?) answer my previous question, namely:

To build a 32-bit kernel on 32-bit Raspberry Pi OS, it says to "set ARCH=arm", but doesn't explain how / where you do that. I assume it has to be given at least as an extra argument when making the default configuration, namely as:

make ARCH=arm bcm2711_defconfig

Is that correct / sufficient? Or does ARCH=arm need to be added (also) to the make command that builds the kernel itself?

[pelwell]

You can pass it to each make call or make it available to all subsequent make invocations with... export ARCH=arm.

[mtaunton]

So do both "make" calls need ARCH to be set, ie the setting of various values in .config by "make bcm2711_defconfig" is not sufficient?

[pelwell]

You can pass it to each make call

So yes, both/all of them.