Boot animation and login screen use different display scales

[Daniel van Vugt]

Boot animation and login screen use different display scales, on some (not many) machines.

For example on a Microsoft Surface Laptop 4, Plymouth seems to use scale 2 for the boot screen while GNOME selects scale 1 for the login screen. This means the Ubuntu logo shrinks.

I've tried a few laptops and so far it's only the Surface Laptop 4 that has the problem: 13.5" 2256x1504, 201 PPI

[Daniel van Vugt (vanvugt)]

See also bug 1872594.

[Daniel van Vugt (vanvugt)]

It looks like plymouth's threshold for scale 200% is 192 DPI. And mutter's threshold is 135 DPI. Since my laptop with the bug is 201 DPI (or 198.6 according to the code) then it should be scale 200% for both mutter and plymouth. But a fudge factor was introduced in mutter 44.0 (https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2880) which means it doesn't actually honour 135 as the target DPI despite the source code saying it does.

[Daniel van Vugt (vanvugt)]

Turns out we don't need to change mutter at all. I just failed to understand its complicated scale selection rules. I've proposed a simplification to mutter anyway:

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3616

but it's no longer required to fix this bug.

[Daniel van Vugt (vanvugt)]

Fix proposed: https://gitlab.freedesktop.org/plymouth/plymouth/-/merge_requests/307

[Daniel van Vugt (vanvugt)]

Here's the fix for Noble.

[Daniel van Vugt (vanvugt)]

Also everything is back in git again:
https://salsa.debian.org/ubuntu-dev-team/plymouth/-/commits/ubuntu/latest

[Daniel van Vugt (vanvugt)]

It looks like this will need fixing again in future :(

While working on bug 1970069 I've found simpledrm causes Plymouth to choose a different default scale again.

[Daniel van Vugt (vanvugt)]

I was wrong. You only get the wrong scale in Plymouth if you blacklist your native DRM driver as I had for testing. Otherwise Plymouth will wait for a preferred driver instead of using SimpleDRM immediately.

So bug 2054769 is still fixed by the patch in comment #5.

[Daniel van Vugt (vanvugt)]

And now there's a new cause of this bug. If I simplify the framebuffer drivers installed in initrd then it causes i915 to take 10 seconds to start, whereas Plymouth stopped waiting for it at the 8 second mark and so fell back to using SimpleDRM.

[Daniel van Vugt (vanvugt)]

Let's ignore comment #9 because it only affects my desktop with a 27" 4K monitor. The original laptop this bug was about still works with the latest proposed fix (which will be rolled into bug 1970069).

[Launchpad Janitor (janitor)]

This bug was fixed in the package plymouth - 24.004.60-1ubuntu6

---------------
plymouth (24.004.60-1ubuntu6) noble; urgency=medium

  * Cherry-pick upstream fixes for consistent default scale selection
    that matches what Mutter chooses on the login screen (LP: #2054769)
  * plymouth.hook: Stop automatically re-installing DRM kernel modules in
    initrd. initramfs-tools already does this for us in hooks/framebuffer and
    should be the authority for which drivers to include.
  * Install a /usr/share/initramfs-tools/conf-hooks.d/plymouth with
    FRAMEBUFFER=y to make it clear to initramfs-tools that we always want
    framebuffer support when Plymouth is installed. (LP: #1970069)

 -- Daniel van Vugt <email address hidden> Thu, 21 Mar 2024 14:46:56 +0100