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From: Laurent Bigonville <bigon@debian.org>
Newsgroups: linux.debian.bugs.dist,linux.debian.kernel
Subject: Bug#897572: linux-image-4.16.0-1-amd64 breaks plymouth LUKS prompt
Date: Sun, 06 May 2018 17:30:01 +0200
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X-Original-Cc: 897572@bugs.debian.org, Ben Hutchings <ben@decadent.org.uk>
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On Sat, 05 May 2018 20:01:45 +0100 Ben Hutchings <ben@decadent.org.uk> 
wrote:
 > On Fri, 2018-05-04 at 12:20 +1200, Ben Caradoc-Davies wrote:
 > > On 04/05/18 11:52, Ben Caradoc-Davies wrote:
 > > > - Pressing *any* key repeatedly is enough to eventually wake up the
 > > > plymouth LUKS screen. For example, pressing Backspace many times.
 > >
 > > Even a modifier key is sufficient. Without input, the screen remains
 > > blank indefinitely (with just a blinking cursor for "quiet" boot).
 > > Pressing right Alt 11-18 times (varies from test to test) causes the
 > > plymouth LUKS passphrase screen to appear.
 > >
 > > I have attached a photo of the screen for a boot with "quiet" removed
 > > from and "plymouth.debug" added to the kernel command line.
 >
 > I wonder if this is related to the recent RNG changes. It seems that
 > many programs have started using blocking RNG functions like
 > getentropy(), and now that the kernel is more conservative in its
 > initial entropy estimation they can block for a long time. Keyboard or
 > mouse input adds entropy.
 >
 > At a guess, plymouth is starting the X server and the X server wants
 > random bits for MIT-MAGIC-COOKIE authentication.

Hello Ben,

plymouth doesn't uses Xorg, it uses libdrm and KMS directly.