[What I recommend] Use aptitude to install libdrm-dev by automatically downgrading your installed dependencies
I figured it out, and this worked: use aptitude to downgrade dependencies.
Just the commands:
sudo aptitude install libdrm-dev
# then choose these options:
n # do NOT leave things as-is
y # DO downgrade dependencies
y # Do continue with the install
After running sudo aptitude install libdrm-dev, choose no to its proposal to Keep the following packages at their current version, and choose yes to its proposal where it says it will Remove the following packages, Install the following packages, and Downgrade the following packages. Then choose yes again where it says: 0 packages upgraded, 2 newly installed, 13 downgraded, 5 to remove and 0 not upgraded. Need to get 33.4 MB of archives. After unpacking 140 MB will be freed. Do you want to continue? [Y/n/?]. That's it! It downgrades (slightly) some packages, and installs just fine now!
Details:
Here is the full command and output (I'll explain it below). I've marked where I entered n, y, and y, with some <====== markers:
$ sudo aptitude install libdrm-dev
The following NEW packages will be installed:
libdrm-dev{b}
0 packages upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 126 kB of archives. After unpacking 658 kB will be used.
The following packages have unmet dependencies:
libdrm-dev : Depends: libdrm2 (= 2.4.101-2) but 2.4.107-8ubuntu1~20.04.2 is installed
Depends: libdrm-intel1 (= 2.4.101-2) but 2.4.107-8ubuntu1~20.04.2 is installed
Depends: libdrm-radeon1 (= 2.4.101-2) but 2.4.107-8ubuntu1~20.04.2 is installed
Depends: libdrm-nouveau2 (= 2.4.101-2) but 2.4.107-8ubuntu1~20.04.2 is installed
Depends: libdrm-amdgpu1 (= 2.4.101-2) but 2.4.107-8ubuntu1~20.04.2 is installed
The following actions will resolve these dependencies:
Keep the following packages at their current version:
libdrm-dev [Not Installed]
Accept this solution? [Y/n/q/?] n <=================
The following actions will resolve these dependencies:
Remove the following packages:
mesa-va-drivers [21.2.6-0ubuntu0.1~20.04.2 (now)]
va-driver-all [2.7.0-2 (focal, now)]
Install the following packages:
libllvm9 [1:9.0.1-12 (focal)]
Downgrade the following packages:
libdrm-amdgpu1 [2.4.107-8ubuntu1~20.04.2 (now) -> 2.4.101-2 (focal)]
libdrm-intel1 [2.4.107-8ubuntu1~20.04.2 (now) -> 2.4.101-2 (focal)]
libdrm-nouveau2 [2.4.107-8ubuntu1~20.04.2 (now) -> 2.4.101-2 (focal)]
libdrm-radeon1 [2.4.107-8ubuntu1~20.04.2 (now) -> 2.4.101-2 (focal)]
libdrm2 [2.4.107-8ubuntu1~20.04.2 (now) -> 2.4.101-2 (focal)]
libegl-mesa0 [21.2.6-0ubuntu0.1~20.04.2 (now) -> 20.0.4-2ubuntu1 (focal)]
libgbm1 [21.2.6-0ubuntu0.1~20.04.2 (now) -> 20.0.4-2ubuntu1 (focal)]
libgl1-mesa-dri [21.2.6-0ubuntu0.1~20.04.2 (now) -> 20.0.4-2ubuntu1 (focal)]
libglapi-mesa [21.2.6-0ubuntu0.1~20.04.2 (now) -> 20.0.4-2ubuntu1 (focal)]
libglx-mesa0 [21.2.6-0ubuntu0.1~20.04.2 (now) -> 20.0.4-2ubuntu1 (focal)]
libosmesa6 [21.2.6-0ubuntu0.1~20.04.2 (now) -> 20.0.4-2ubuntu1 (focal)]
mesa-vdpau-drivers [21.2.6-0ubuntu0.1~20.04.2 (now) -> 20.0.4-2ubuntu1 (focal)]
mesa-vulkan-drivers [21.2.6-0ubuntu0.1~20.04.2 (now) -> 20.0.4-2ubuntu1 (focal)]
Accept this solution? [Y/n/q/?] y <=================
The following packages will be DOWNGRADED:
libdrm-amdgpu1 libdrm-intel1 libdrm-nouveau2 libdrm-radeon1 libdrm2 libegl-mesa0 libgbm1 libgl1-mesa-dri libglapi-mesa libglx-mesa0 libosmesa6 mesa-vdpau-drivers mesa-vulkan-drivers
The following NEW packages will be installed:
libdrm-dev libllvm9{a}
The following packages will be REMOVED:
i965-va-driver{u} intel-media-va-driver{u} libigdgmm11{u} mesa-va-drivers{a} va-driver-all{a}
0 packages upgraded, 2 newly installed, 13 downgraded, 5 to remove and 0 not upgraded.
Need to get 33.4 MB of archives. After unpacking 140 MB will be freed.
Do you want to continue? [Y/n/?] y <=================
Notice the unmet dependencies. As you can see, it says that it needs libdrm2 version 2.4.101-2, but I have version 2.4.107-8ubuntu1~20.04.2 installed, etc. My version is, oddly enough, too new for it.
The following packages have unmet dependencies:
libdrm-dev : Depends: libdrm2 (= 2.4.101-2) but 2.4.107-8ubuntu1~20.04.2 is installed
Depends: libdrm-intel1 (= 2.4.101-2) but 2.4.107-8ubuntu1~20.04.2 is installed
Depends: libdrm-radeon1 (= 2.4.101-2) but 2.4.107-8ubuntu1~20.04.2 is installed
Depends: libdrm-nouveau2 (= 2.4.101-2) but 2.4.107-8ubuntu1~20.04.2 is installed
Depends: libdrm-amdgpu1 (= 2.4.101-2) but 2.4.107-8ubuntu1~20.04.2 is installed
So, aptitude first proposes a solution to:
Keep the following packages at their current version:
1) libdrm-dev [Not Installed]
I press n to reject that solution. It then proposes a better solution to downgrade the dependencies and perform the install. I choose y for that option. Then I must choose y again later.
Once done, you can see that the regular sudo apt install libdrm-dev works just fine now, too, showing that libdrm-dev is already successfully installed!:
$ sudo apt install libdrm-dev
[sudo] password for gabriel:
Reading package lists... Done
Building dependency tree
Reading state information... Done
libdrm-dev is already the newest version (2.4.101-2).
0 upgraded, 0 newly installed, 0 to remove and 5 not upgraded.
Done!
And now I can go back to Installing displaylink-driver on Ubuntu 20.04, so I can use my new dual 4k 60 Hz USB-A adapter! See my answer here: USB to HDMI adapter that works with Ubuntu.
Alternative: how to build and install the latest libdrm from source
This doesn't answer my question exactly, but here's how to build libdrm from source:
The source code is here: https://gitlab.freedesktop.org/mesa/drm
Get the latest tagged release here: https://gitlab.freedesktop.org/mesa/drm/-/tags
Download and extract it.
See the "Compiling" instructions in the readme: https://gitlab.freedesktop.org/mesa/drm
Run:
meson builddir/ # prep a folder
time sudo ninja -C builddir/ install # build and install
It only took 20 seconds to build & install. It installs into /usr/local/include/libdrm.
