Bootable 64-bit RPi3/Pi4 Gentoo image (OpenRC/Xfce/VC4) Pt 2

[n1ese]

I'm working with a fresh install of 1.5.4-Lite. If I do a genup before customizing the system, it wants to install everything from the full image especially X11 and xfce. Is there a way around this? Thanks.

I screwed up, I was coping some things from /etc/portage and happened to include RPI-64bit-meta by mistake. Sorry.

[sqeeezy]

Hi, trying to get my gentoo vm on my mac to play distcc with my rpi4, as described in

https://github.com/sakaki-/gentoo-on-rp ... tcc-Client

and linked pages, I see the tildes (~) in the echo commands below. Doesn’t that mean the unstable branch? Is it two typos?
As we are using a stable-branch compiler on the PC, but the RPi image uses the 'testing' (aka ~arm64) branch by default, begin by downgrading your gcc compiler (and binutils, for consistency) on the RPi to the 'stable' version (this version is also available on the binhost, so the following process shouldn't take long). Issue:
pi64 ~ # echo "sys-devel/binutils -~arm64" >> /etc/portage/package.accept_keywords/binutils
pi64 ~ # echo "sys-devel/gcc -~arm64" >> /etc/portage/package.accept_keywords/gcc

[NeddySeagoon]

sqeeezy,

Code: Select all

-~arm64

~arm64 as you say, means testing but if you have ~arm64 in make.conf anyway, everything is testing,

Code: Select all

-~arm64

with the -ve sign, means not testing.

Its a way to keep stable packages on an otherwise testing system.

Its not used much as most users add a few testing packages to an otherwise stable install.
This does the opposite.

[sqeeezy]

Neddy,

thanks for that. I stupidly misinterpreted the minus sign as the dash sign used for arguments. ^^

What’s the difference between, in this case, writing arm64, i.e. stable,
as opposed to -~arm64, i.e. not testing?

Regards,

sqeeezy

[n1ese]

I'm blind, and possibly stupid as usual. Where is the configuration for the graphical login manager on the full image? I want to switch to a different WM. Is it as simple as making my own .xinitrc in my home directory. I'm trying to maximize RAM usage on my 2GB PI and, in testing on a lite image, CTWM is working best for my needs.

Edit: I don't want to stay with lite or remove xfce or core from full just yet. I have applications that are picky with alsa and portaudio and having problems figuring out how to configure alsa/portaudio to work with them on my lite with CTWM image. The apps in question are working fine with the full image.

Thanks as usual.

[NeddySeagoon]

sqeeezy,

With allowing ~arm64 everywhere putting arm64 in keywords does nothing as ~arm64 implies arm64 too.
Look at your

Code: Select all

emerge --info

-~arm64 disables ~arm64 for that one package.

Consider this contrived example. Don't do this at home.

Suppose you wanted to try a package that was not keyworded for arm64 at all but was stable on say x66
Adding

Code: Select all

<package>/<atom> x86

would allow portage to consider the stable x86 version for install on your Pi.

Like I say, Don't do this at home ... its a contrived example and you get to keep the all the pieces.

[chris_andrew]

Thanks to [profile=322314]Sakaki[/profile] for rekindling my interest in Gentoo....it's been a long time.

As I begin to learn the 'Gentoo Way', I found i'd make a quick note of how I managed to install a 'Masked' package, as this seems to be a commonly asked question, so I thought i'd try to document my experience, for others to find. It seems like a lot of people ask why they can't install masked packages, and I found myself in this situation, when I wanted to install an amateur radio package called JS8CALL, which had HAMLIB as a dependency.

I followed Sakaki's example 'shed', here , but some how messed it up. I wrote to him, and he was kind enough to reply to my individual request. This was one of the reasons i'm posting, here.

Essentially, my understanding is that in this context, MASKING means that the required package hasn't been made available for this arch, but it is available under another arch, such as i686. This theoretically means that the package should be able to be emerged for RasPi.

Hopefully, format permitting, the following will give you an indication of what I did (as root):

Code: Select all

pi64 ~ # echo "media-libs/hamlib * ~*" >> /etc/portage/package.accept_keywords/hamlib
pi64 ~ # eix --verbose media-libs/hamlib
* media-libs/hamlib
     Available versions:  (*)3.3^t
     IUSE (all versions): doc perl python tcl PYTHON_SINGLE_TARGET="python2_7 python3_6 python3_7"
     Best versions/slot:  (*)3.3^t
     Homepage:            https://www.hamlib.org
     Description:         Ham radio backend rig control libraries
     License:             LGPL-2 GPL-2

pi64 ~ # emerge --verbose media-libs/hamlib

Local copy of remote index is up-to-date and will be used.

Local copy of remote index is up-to-date and will be used.

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild  N    *] media-libs/hamlib-3.3::gentoo  USE="-doc -perl -python -tcl" PYTHON_SINGLE_TARGET="python3_6 -python2_7 -python3_7" 2,141 KiB

Total: 1 package (1 new), Size of downloads: 2,141 KiB

!!! The following binary packages have been ignored due to changed dependencies:

     dev-libs/libxml2-2.9.9-r3::gentoo

NOTE: The --binpkg-changed-deps=n option will prevent emerge
      from ignoring these binary packages if possible.
      Using --binpkg-changed-deps=y will silence this warning.

>>> Verifying ebuild manifests
>>> Emerging (1 of 1) media-libs/hamlib-3.3::gentoo
>>> Installing (1 of 1) media-libs/hamlib-3.3::gentoo
>>> Recording media-libs/hamlib in "world" favorites file...
>>> Jobs: 1 of 1 complete                           Load avg: 1.98, 5.19, 7.12
>>> Auto-cleaning packages...
>>> No outdated packages were found on your system.

 * Regenerating GNU info directory index...
 * Processed 114 info files.
pi64 ~ # 

As i'm new to all this, I checked that the package was installed:

Code: Select all

pi64 ~ # emerge --search media-libs/hamlib

Local copy of remote index is up-to-date and will be used.
  
[ Results for search key : media-libs/hamlib ]
Searching...

*  media-libs/hamlib
      Latest version available: 3.3
      Latest version installed: 3.3
      Size of files: 2,141 KiB
      Homepage:      https://www.hamlib.org
      Description:   Ham radio backend rig control libraries
      License:       LGPL-2 GPL-2

[ Applications found : 1 ]

Now to try the actual objective, which is to install JS8CALL....



... oh well, one victory at a time:

Code: Select all

pi64 ~ # emerge js8call

Local copy of remote index is up-to-date and will be used.
Calculating dependencies... done!
[binary     U  ] dev-libs/icu-66.1 [65.1-r1]
[binary  N     ] media-libs/portaudio-19.06.00-r2  USE="alsa cxx -debug -doc -jack -oss -static-libs" 
[binary  N     ] virtual/fortran-0  USE="openmp" 
[ebuild  N     ] sci-libs/fftw-3.3.8-r1  USE="fortran openmp threads (-altivec) -doc (-mpi) (-neon) -quad -static-libs -test (-zbus)" 
[binary     U  ] dev-qt/qtcore-5.14.2 [5.14.1-r1] USE="-old-kernel%" 
[ebuild  N     ] dev-qt/qtserialport-5.14.2  USE="-debug -test" 
[ebuild  N    *] media-radio/js8call-2.1.1  USE="-doc" 

!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

dev-qt/qtcore:5

  (dev-qt/qtcore-5.14.1-r1:5/5.14::gentoo, installed) USE="icu -debug -systemd -test" pulled in by
    ~dev-qt/qtcore-5.14.1 required by (dev-qt/qtconcurrent-5.14.1:5/5.14::gentoo, installed) USE="-debug -test"
    ^              ^^^^^^                                                                                                                                                                                   
    (and 15 more with the same problem)

  (dev-qt/qtcore-5.14.2:5/5.14.2::gentoo, binary scheduled for merge) USE="icu -debug -old-kernel -systemd -test" pulled in by
    ~dev-qt/qtcore-5.14.2 required by (dev-qt/qtserialport-5.14.2:5/5.14::gentoo, ebuild scheduled for merge) USE="-debug -test"
    ^              ^^^^^^                                                                                                                                                                                                    

dev-libs/icu:0

  (dev-libs/icu-66.1:0/66.1::gentoo, binary scheduled for merge) USE="-debug -doc -examples -static-libs" pulled in by
    dev-libs/icu:0/66.1= required by (dev-qt/qtcore-5.14.2:5/5.14.2::gentoo, binary scheduled for merge) USE="icu -debug -old-kernel -systemd -test"
                ^^^^^^^^                                                                                                                                                                                                                                                               

  (dev-libs/icu-65.1-r1:0/65.1::gentoo, installed) USE="-debug -doc -examples -static-libs" pulled in by
    dev-libs/icu:0/65.1= required by (dev-db/sqlite-3.30.1:3/3::gentoo, installed) USE="icu readline secure-delete -debug -doc -static-libs -tcl -test -tools"
                ^^^^^^^^                                                                                                                                                                                                                                                                                                                                                     
    dev-libs/icu:0/65.1= required by (media-libs/libvisio-0.1.7:0/0::gentoo, installed) USE="-doc -static-libs -test -tools"
                ^^^^^^^^                                                                                                                                                                                                                    
    dev-libs/icu:0/65.1= required by (dev-qt/qtwebkit-5.212.0_pre20190629:5/5.212::gentoo, installed) USE="X gstreamer hyphen opengl printsupport -geolocation -gles2 (-jit) -multimedia -nsplugin -orientation -qml -webp"
                ^^^^^^^^                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
    >=dev-libs/icu-64.1:0/65.1= required by (www-client/firefox-73.0.1:0/0::gentoo, installed) USE="bindist hwaccel lto pulseaudio screenshot startup-notification system-av1 system-harfbuzz system-icu system-jpeg system-libevent system-libvpx system-sqlite system-webp -clang -custom-cflags -custom-optimization -debug (-eme-free) -geckodriver -gmp-autoupdate (-hardened) -jack (-pgo) (-selinux) -test -wayland -wifi" CPU_FLAGS_ARM="(-neon)" L10N="en-GB -ach -af -an -ar -ast -az -be -bg -bn -br -bs -ca -cak -cs -cy -da -de -dsb -el -en-CA -eo -es-AR -es-CL -es-ES -es-MX -et -eu -fa -ff -fi -fr -fy -ga -gd -gl -gn -gu -he -hi -hr -hsb -hu -hy -ia -id -is -it -ja -ka -kab -kk -km -kn -ko -lij -lt -lv -mk -mr -ms -my -nb -nl -nn -oc -pa -pl -pt-BR -pt-PT -rm -ro -ru -si -sk -sl -son -sq -sr -sv -ta -te -th -tr -uk -ur -uz -vi -xh -zh-CN -zh-TW"
                       ^^^^^^^^                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
    (and 18 more with the same problems)

NOTE: Use the '--verbose-conflicts' option to display parents omitted above

!!! The slot conflict(s) shown above involve package(s) which may need to
!!! be rebuilt in order to solve the conflict(s). However, the following
!!! package(s) cannot be rebuilt for the reason(s) shown:

  (dev-qt/qtwebkit-5.212.0_pre20190629:5/5.212::gentoo, installed): ebuild is masked or unavailable
  (www-client/firefox-73.0.1:0/0::gentoo, installed): ebuild is masked or unavailable
  (dev-db/sqlite-3.30.1:3/3::gentoo, installed): ebuild is masked or unavailable


It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously.  If such a conflict exists in
the dependencies of two different packages, then those packages can
not be installed simultaneously.

For more information, see MASKED PACKAGES section in the emerge man
page or refer to the Gentoo Handbook.


The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by media-radio/js8call-2.1.1::gentoo
# required by js8call (argument)
>=sci-libs/fftw-3.3.8-r1 threads

Use --autounmask-write to write changes to config files (honoring
CONFIG_PROTECT). Carefully examine the list of proposed changes,
paying special attention to mask or keyword changes that may expose
experimental or unstable packages.

 * In order to avoid wasting time, backtracking has terminated early
 * due to the above autounmask change(s). The --autounmask-backtrack=y
 * option can be used to force further backtracking, but there is no
 * guarantee that it will produce a solution.
pi64 ~ # 

[/code]

Hmmmm!


Chris
2E0FRU

[NeddySeagoon]

chris_andrew,

You cannot update Qt piecemeal.

It looks like you have a @world update in the offing.
Do that first. I suspect it will update a lot of Qt.

Come back to

Code: Select all

emerge js8call

[chris_andrew]

chris_andrew,

You cannot update Qt piecemeal.

It looks like you have a @world update in the offing.
Do that first. I suspect it will update a lot of Qt.

Come back to

Code: Select all

emerge js8call

Hi, Neddy.

Thanks for your reply. I haven't got as far as working out an action plan, so no plans to install Qt, or anything else, yet

When I completed the install, I recall doing an emerge --sync, and also manually invoking genup. I think 2009 was the last time I looked at Gentoo; with this in mind, could you say how you recognized that I need to do a World update. I'm sure you're correct, I just want to understand what to watch out for, and what to do.

I followed Sakaki's install instructions, but I may need to check the Gentoo docs, for a 'What to do after the installation'

Many thanks,

Chris.

[NeddySeagoon]

chris_andrew,

Code: Select all

[binary     U  ] dev-qt/qtcore-5.14.2 [5.14.1-r1] USE="-old-kernel%" 

Its very rare to only have one Qt package installed. It might be correct but its rare.
That like wants to update date just one Qt package, so do the whole installed set.

Further, you have

Code: Select all

www-client/firefox-73.0.1:0/0::gentoo, installed

but that's no longer in portage, so that needs to be updated too.

[safeness]

Huge thanks to Sakaki for making this great image! I've been out of the Gentoo scene for several years now but it's nice to be back. Currently building thunderbird (5hrs in with a 4G 4B) just for the fun of it!

I had a question about making some changes and I wasn't quite sure where to look.

While trying to do a 'emerge --sync' it appears that it's just verifying that I'm in sync with Sakaki's overlay. I absolutely want to keep all the raspberry pi specific stuff from the overlay in tact, but I'd like a more recent portage sync from Gentoo proper if possible.

How would I go about doing this?

Thanks again everyone!

[chris_andrew]

While trying to do a 'emerge --sync' it appears that it's just verifying that I'm in sync with Sakaki's overlay. I absolutely want to keep all the raspberry pi specific stuff from the overlay in tact, but I'd like a more recent portage sync from Gentoo proper if possible.

How would I go about doing this?

Thanks again everyone!

Hmm, maybe this is why I have outdated packages, even though I have done an emerge --sync and a manual genup.

Chris

[safeness]

Update: Thunderbird finished after about 7.5 hours. Firefox crashed due to lack of RAM

Trying it again with -j4 -l3.

Edit: It failed all the way down to MAKEOPTS="-j2". I'm over it.

[Sakaki]

All,

I've just pushed a fairly major set of package updates to the binhost, and corresponding changes have been released into the custom profile and (gated) Portage tree.

(safeness, chris_andrew: the gated copy of the main Gentoo tree used by the image has been on hold while these changes have been worked through - with this update, you should find you have a much more current copy, 22 Apr timestamp).

Your machine should pick all this up on its normal weekly genup cycle. Please be aware of one possible issue: following upstream, this update will transition you from consolekit to elogind. As a result, once the update completes, you'll need to power-cycle your system to get full session functionality working again.

If you want to pre-empt the weekly update, to flush this change out sooner, then simply run (as root, or using sudo):

Code: Select all

pi64 ~ # emaint sync --repo genpi64
pi64 ~ # emaint sync --repo sakaki-tools
pi64 ~ # emerge -v1u app-portage/genup
pi64 ~ # /etc/cron.weekly/fixup
pi64 ~ # genup

and reboot once done (you get the prompt "All done! Your system is now up-to-date."). The update will take a while (for most users, it will involve a full --emptytree emerge, albeit using binary packages for almost all atoms), so best to leave it running overnight if possible.

Apologies for the inconvenience ><

[Sakaki]

I'm blind, and possibly stupid as usual. Where is the configuration for the graphical login manager on the full image? I want to switch to a different WM. Is it as simple as making my own .xinitrc in my home directory. I'm trying to maximize RAM usage on my 2GB PI and, in testing on a lite image, CTWM is working best for my needs.

Edit: I don't want to stay with lite or remove xfce or core from full just yet. I have applications that are picky with alsa and portaudio and having problems figuring out how to configure alsa/portaudio to work with them on my lite with CTWM image. The apps in question are working fine with the full image.

Thanks as usual.

Basically, ensure you have no work in any open graphical apps, then issue:

Code: Select all

demouser@pi64 ~ $ sudo rc-service xdm stop

This will quit your Xfce4 session and stop the X11 server. Once you issue the above command, the screen will go black, and you will need to press [Ctrl][Alt][F1] to get a virtual console. There, you can log in as e.g. demouser (default password, raspberrypi64). Then you start your own window manager via e.g. a simple ~/.xinitrc / startx as usual.

For example, these notes show how to run twm starting from the 'lite' version of the image - but you could do something similar from the virtual console prompt. When finished, exit your graphical environment back to the console prompt, and enter:

Code: Select all

demouser@pi64 ~ $ sudo rc-service xdm start

and Xfce4 should start back up again.

As a more complex example, see e.g. this post for an example of running (as a blast from the past ^-^) CDE, starting from an Xfce4 session (and ending back up there again afterwards).

hth, sakaki

[chris_andrew]

All,

I've just pushed a fairly major set of package updates to the binhost, and corresponding changes have been released into the custom profile and (gated) Portage tree.

(safeness, chris_andrew: the gated copy of the main Gentoo tree used by the image has been on hold while these changes have been worked through - with this update, you should find you have a much more current copy, 22 Apr timestamp).

Your machine should pick all this up on its normal weekly genup cycle. Please be aware of one possible issue: following upstream, this update will transition you from consolekit to elogind. As a result, once the update completes, you'll need to power-cycle your system to get full session functionality working again.

If you want to pre-empt the weekly update, to flush this change out sooner, then simply run (as root, or using sudo):

Code: Select all

pi64 ~ # emaint sync --repo genpi64
pi64 ~ # emaint sync --repo sakaki-tools
pi64 ~ # emerge -v1u app-portage/genup
pi64 ~ # /etc/cron.weekly/fixup
pi64 ~ # genup

and reboot once done (you get the prompt "All done! Your system is now up-to-date."). The update will take a while (for most users, it will involve a full --emptytree emerge, albeit using binary packages for almost all atoms), so best to leave it running overnight if possible.

Apologies for the inconvenience ><

Hi, Sakaki.

Many thanks for this work that you have done. I'm just doing the updates now, and am manually invoking the 'genup'. This seems to be working OK, but I did notice a possible error with 'layman'; I've copied the output, below:

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pi64 ~ # genup
* Checking Portage configuration, please wait...
* Gentoo System Updater v1.0.27
* Updating Portage tree and syncing the eix cache
* (this may take some time)...
 * Syncing all portage overlays
/usr/bin/eix-sync: line 396: layman: command not found
 * layman -S failed
 * Running emerge --sync
>>> Syncing repository 'gentoo' into '/var/db/repos/gentoo'...
 * Using keys from /usr/share/openpgp-keys/gentoo-release.asc
 * Refreshing keys via WKD ...                                           [ ok ]
>>> Starting rsync with rsync://[2a01:90:200:10::1a]/gentoo-portage...
>>> Checking server timestamp ...
Welcome to starling.gentoo.org / rsync.gentoo.org

Hope this helps.

Chris.

[Sakaki]

chris_andrew,

thanks! The message actually comes from eix-sync, which genup calls, and is due to layman not being installed on the base image. It's benign, as the repo sync process still works absolutely fine.

[chris_andrew]

Thank you. With regard to elogin, I saw this in the news items, and followed the prescribed steps.

Thanks for all your good work.

Chris.

[chris_andrew]

If you want to pre-empt the weekly update, to flush this change out sooner, then simply run (as root, or using sudo):

Code: Select all

pi64 ~ # emaint sync --repo genpi64
pi64 ~ # emaint sync --repo sakaki-tools
pi64 ~ # emerge -v1u app-portage/genup
pi64 ~ # /etc/cron.weekly/fixup
pi64 ~ # genup

Hi, all.

As per my previous post, I'm now running genup. Things seem to be moving forward in an orderly fashion, but I have noticed some warnings about insufficient space to upgrade Firefox (>=9 gigs) and some of the other big apps. My sdcard is 32gb. At least these warnings are graceful, and the update hasn't just suddenly exited (yet).

Due to Sakaki's good work, I'm now becoming more invested in Gentoo. I have a spare powered HDD that I could attach via USB, but I'm also considering buying a large sdcard, ie, >=128gb.

My question is, is there a way to copy all the configuration and updates from my current sdcard, to a new one? This would save a lot of already invested processor time.

Alternatively, if I continued to use the existing sdcard, what would be the best way to utilize the external HDD? Looks like Portage needs a large amount of space, but moving /var onto a new device could get messy?

Any thoughts, I'll let the updates continue, with the current setup, for now

Cheers,

Chris.

[safeness]

Cool, thanks for the updates, Sakaki!

[safeness]

Ok, so I lied before when I said I was 'over with' trying to compile firefox.

I used MAKEOPTS="-j1" and increased swap to 4G (might be unnecessary) and it finally built in almost 12 hours with a 2GHz overclock.

[NeddySeagoon]

chris_andrew,

To move to another SD card, partition and format the new SD card.
Now copy the filesystems on the old SD card to their correspondng locations on the new SD card.

If you only dave a single card reader, you need make an intermediate copy on your PC.
rsync is good as it can resume a partial copy.
cp -a works too but there is no resume.

You can cange the size of the filesystems or add extra partitions during the copy if you like.

To use an external HDD effectivery put all of portages bits and pieces on the HDD.
Thats the repos, the build space, distfiles and packages. The Pi will still run without the HDD but not install or update.
You can move /usr/src off the SD card. Thats another build time only thing.
Its possible to have several swap spaces. Make a swap on the HDD with the highest priority and one on the SD card with a lower priority.
The HDD swap will be used first.

SD cards generally, do not have very good wear levelling (if any), so not building on the SD card is a good thing for the SD life.

The Pi3 can boot from USB, no SD card required at all.
Thats under test in the Pi4.

[chris_andrew]

chris_andrew,

To move to another SD card, partition and format the new SD card.
Now copy the filesystems on the old SD card to their correspondng locations on the new SD card.

If you only dave a single card reader, you need make an intermediate copy on your PC.
rsync is good as it can resume a partial copy.
cp -a works too but there is no resume.

You can cange the size of the filesystems or add extra partitions during the copy if you like.

To use an external HDD effectivery put all of portages bits and pieces on the HDD.
Thats the repos, the build space, distfiles and packages. The Pi will still run without the HDD but not install or update.
You can move /usr/src off the SD card. Thats another build time only thing.
Its possible to have several swap spaces. Make a swap on the HDD with the highest priority and one on the SD card with a lower priority.
The HDD swap will be used first.

SD cards generally, do not have very good wear levelling (if any), so not building on the SD card is a good thing for the SD life.

The Pi3 can boot from USB, no SD card required at all.
Thats under test in the Pi4.

Many thanks, NeddySeagoon. That's really good to know.

The Pi3 can boot from USB, no SD card required at all.

Now this is very interesting. I guess the question is how to use Sakaki's great image, to do a sdcard'less install, or maybe, literally just have the /boot on the sdcard. This would mean I, and others, could give Portage and /swap the room that it needs, on the Pi3B. In the mean time, I may try a vanilla Gentoo install and see how this might work.

Cheers,

Chris.

[popetto]

Hi all, I'll ask you a very, very stupid question. Is it possible to install gentoo alongside raspbian on the same sd card? I know that it is not recommended, but I've got a 128 GB micro sd card and I'd like to split that space. Is it possible? Is there a guide? Thank you in advance

[NeddySeagoon]

popetto,

Yes, its possible.

The normal raspberry Pi install does not have a boot menu but lets come back to that.

You would have a filesystem tree for Raspbian and a filesystem tree for Gentoo and separate filesystem trees for any other distro you wanted.
/boot and <swap> should be shared. So far, that the same as you would do to multiboot a PC.

Now you need a boot loader to choose among distros at boot time.
You can have several config.txt and cmdline.txt files but that's a bit clunky. You don't want to edit /boot to switch distros.

I think grub2 runs on arm/arm64. That will provide a boot menu.
u-boot runs on the Pi and I think it can do menus.

One wart. For 64 bit capable Pis, the bitness is set as a part of the CPU initialisation and cant be changed by the CPU.
However a Pi in 64 bit mode must have a 64 bit kernel. It can have a 32 bit userland. There is no multilib on arm64, so its not possible to have 32 bit and 64 bit libraries side by side. You can cheat a little. Statically linked 32 bit arm binaries will run on a 64 bit Pi.

The upshot of all this is that having chosen the bitness for your boot loader you are stuck with it.

Well, there is the horrible hack approach too.
Boot the Pi into <random distro> run a scrip that modifies /boot to boot <other distro> on the next boot.
That would let you have both 32 bit and 64 bit kernels.