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Subject: bug#76554: Laptop screen blank after kexec reboot
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Hello,

After rebooting with =E2=80=98reboot -k=E2=80=99, my laptop=E2=80=99s scree=
n remains blank.  I
have to type my LUKS passphrase blindly and then it boots fine, even
displays stuff normally on its external monitor, but its own monitor
remains desperately blank.

That=E2=80=99s a 2016 HP laptop with Intel graphics.

Are there known remedies?

At the Guix Days, someday said they experienced a similar problem.

Ludo=E2=80=99.

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To: Ludovic =?UTF-8?Q?Court=C3=A8s?= <ludo@HIDDEN>
Subject: bug#76554: Acknowledgement (Laptop screen blank after kexec reboot)
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Subject: bug#76554: Laptop screen blank after kexec reboot
Resent-From: Simon Streit <simon@HIDDEN>
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Hello Ludovic,

Ludovic Court=C3=A8s <ludo@HIDDEN> writes:

> After rebooting with =E2=80=98reboot -k=E2=80=99, my laptop=E2=80=99s scr=
een remains blank.  I
> have to type my LUKS passphrase blindly and then it boots fine, even
> displays stuff normally on its external monitor, but its own monitor
> remains desperately blank.

Interesting.  I just tested it and both internal and external monitor
will remain blank until the passphrase is entered.

Reconnecting the external monitor will not help either.  They appear to
be switched off.


Kind regards

--=20
Simon

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Subject: bug#76554: Laptop screen blank after kexec reboot
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 26 Feb 2025 23:10:50 +0100")
References: <874j0ioz74.fsf@HIDDEN> <yguo6yoe4r9.fsf@HIDDEN>
Date: Thu, 27 Feb 2025 17:30:51 +0100
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Hello,

Simon Streit <simon@HIDDEN> skribis:

> Ludovic Court=C3=A8s <ludo@HIDDEN> writes:
>
>> After rebooting with =E2=80=98reboot -k=E2=80=99, my laptop=E2=80=99s sc=
reen remains blank.  I
>> have to type my LUKS passphrase blindly and then it boots fine, even
>> displays stuff normally on its external monitor, but its own monitor
>> remains desperately blank.
>
> Interesting.  I just tested it and both internal and external monitor
> will remain blank until the passphrase is entered.
>
> Reconnecting the external monitor will not help either.  They appear to
> be switched off.

Oh, worse that what I experienced.

Do you happen to know if it works on other distros with the same
hardware?

It seems to me that it=E2=80=99s a Linux issue because there=E2=80=99s very=
 little we
can do as kexec users, but who knows.

Ludo=E2=80=99.

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Ludovic Court=C3=A8s <ludo@HIDDEN> writes:

> Oh, worse than what I experienced.

Still more convenient than typing in the password two times.  I first
thought kexec was not working.  It is an improvement for me already. :)

I just did a reboot with kexec from kexec-tools on an identical machine
running Debian Bookworm with LUKS encryption:

The screen goes on with a blinking prompt.  The external video is on too
and can plug it in and out again.

I just tried it manually with kexec from kexec-tools in Guix.  The
display stayed dark here too and I somehow didn't manage to unlock
the device.  The HDD-Lamp hinted that something was happening.

I looked into dmesg and /var/log/messages.  I see a time gap until the
partition is mounted.  But no hint that a video or state of hardware is
being changed:

--8<---------------cut here---------------start------------->8---
[    2.735156] usb 1-2: New USB device strings: Mfr=3D1, Product=3D2, Seria=
lNumber=3D0
[    2.735161] usb 1-2: Product: Biometric Coprocessor
[    2.735165] usb 1-2: Manufacturer: STMicroelectronics
[    2.874331] Btrfs loaded, zoned=3Dyes, fsverity=3Dyes
[   32.859424] PM: Image not found (code -22)
[   32.874392] BTRFS: device label LABEL devid 1 transid 408044 /dev/dm-0 (=
253:0) scanned by init (1)
[   32.875143] BTRFS info (device dm-0): first mount of filesystem HASHVALUE
[   32.875162] BTRFS info (device dm-0): using crc32c (crc32c-generic) chec=
ksum algorithm
[   32.875179] BTRFS info (device dm-0): using free-space-tree
[   35.910187] shepherd[1]: GNU Shepherd 1.0.2 (Guile 3.0.9, x86_64-unknown=
-linux-gnu)
[   35.910434] shepherd[1]: Starting service root...
[   35.911316] shepherd[1]: Service root started.
[   35.911676] shepherd[1]: Service root running with value #<<process> id:=
 1 command: #f>.
[   35.912303] shepherd[1]: Service root has been started.
[   35.918603] shepherd[1]: starting services...
--8<---------------cut here---------------end--------------->8---

Hope that is getting us a bit further.


Kind regards

--=20
Simon

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From: Ludovic =?UTF-8?Q?Court=C3=A8s?= <ludo@HIDDEN>
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Hi Simon,

(Cc: kernel team.)

Simon Streit <simon@HIDDEN> skribis:

> I just did a reboot with kexec from kexec-tools on an identical machine
> running Debian Bookworm with LUKS encryption:
>
> The screen goes on with a blinking prompt.  The external video is on too
> and can plug it in and out again.
>
> I just tried it manually with kexec from kexec-tools in Guix.  The
> display stayed dark here too and I somehow didn't manage to unlock
> the device.  The HDD-Lamp hinted that something was happening.
>
> I looked into dmesg and /var/log/messages.  I see a time gap until the
> partition is mounted.  But no hint that a video or state of hardware is
> being changed:

Interesting.  Could it be that we=E2=80=99re missing something from the ker=
nel?
The syscall interface that we use in Guix (kexec_file_load(2)) and
Shepherd (reboot(2)) is very high-level and we cannot get more control
on graphics or whatever at that level.

I just checked and systemd invokes the =E2=80=98kexec=E2=80=99 program from=
 kexec-tools.
I don=E2=80=99t think the =E2=80=98kexec=E2=80=99 program is doing anything=
 more in the
=E2=80=98kexec_file_load=E2=80=99 case (it can also use the obsolescent kex=
ec_load(2)
and in fact most of the code is here to handle that case).

Thoughts?

Ludo=E2=80=99.

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 Content preview:  Hi, in my case I can see something on the screen after reboot,
    but very dark. This hints that the problem in a backlight, and it is so:
   #+begin_example cd /sys/class/backlight/intel_backlight/ cat brightness 0 
 
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Hi, in my case I can see something on the screen after reboot, but
very dark. This hints that the problem in a backlight, and it is so:

#+begin_example
cd /sys/class/backlight/intel_backlight/

cat brightness
0

cat max_brightness
4437

cat max_brightness > brightness
#+end_example

The last command (run from root) makes my screen light up again.

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Hi,

Evgeny Pisemsky <mail@HIDDEN> writes:

> Hi, in my case I can see something on the screen after reboot, but
> very dark. This hints that the problem in a backlight, and it is so:

Good catch; I just checked and it=E2=80=99s exactly the same for me.

> #+begin_example
> cd /sys/class/backlight/intel_backlight/
>
> cat brightness
> 0
>
> cat max_brightness
> 4437
>
> cat max_brightness > brightness

Which makes me wonder: should we do this in an activation snippet?

Is this an Intel-specific problem?

Thanks,
Ludo=E2=80=99.

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--=-=-=
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable


> Hi,
>=20
> Evgeny Pisemsky <mail@HIDDEN> writes:
>=20
> > Hi, in my case I can see something on the screen after reboot, but
> > very dark. This hints that the problem in a backlight, and it is so:
>=20
> Good catch; I just checked and it=E2=80=99s exactly the same for me.
>=20
> > #+begin_example
> > cd /sys/class/backlight/intel_backlight/
> >
> > cat brightness
> > 0
> >
> > cat max_brightness
> > 4437
> >
> > cat max_brightness > brightness
>=20
> Which makes me wonder: should we do this in an activation snippet?
>=20
> Is this an Intel-specific problem?
>=20
> Thanks,
> Ludo=E2=80=99.

I can confirm that on my end with an intel_backlight the tty is set on
minimal brightness only on kexec reboot. And I don=E2=80=99t use LUKS.

--=-=-=
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Hi Simon,

Simon Streit <simon@HIDDEN> writes:

> Ludovic Court=C3=A8s <ludo@HIDDEN> writes:
>
>> Oh, worse than what I experienced.
>
> Still more convenient than typing in the password two times.  I first
> thought kexec was not working.  It is an improvement for me already. :)
>
> I just did a reboot with kexec from kexec-tools on an identical machine
> running Debian Bookworm with LUKS encryption:
>
> The screen goes on with a blinking prompt.  The external video is on too
> and can plug it in and out again.
>
> I just tried it manually with kexec from kexec-tools in Guix.  The
> display stayed dark here too and I somehow didn't manage to unlock
> the device.  The HDD-Lamp hinted that something was happening.

So just to make sure I got that right, it works correctly on Debian but
incorrectly (black screen) on Guix System on the *same* hardware?

If correct, that'd probably point to our kernel configuration (or
non-free firmware?  do you you use linux-firmware on Debian and not on
Guix System (through nonguix or the likes)) ?

It'd be interesting to see what happens if you use Linux-libre on your
Debian machine and try the same.  Perhaps attach your Debian's
/proc/config.gz that works so that we can diff with ours for clues.

As another data point: I have the black screen issue when I use 'reboot
--kexec', but in my case it's not the backlight that is too low, the
video signal is completely cut and my external monitor (it's a desktop)
goes into standby.  That's with a modern Ryzen 9 machine and its iGPU
for the video (which sadly requires firmware blobs for 3D acceleration
and hardware video decoding).

--=20
Thanks,
Maxim

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Maxim Cournoyer <maxim.cournoyer@HIDDEN> writes:

> So just to make sure I got that right, it works correctly on Debian
> but incorrectly (black screen) on Guix System on the *same* hardware?

correct.

> If correct, that'd probably point to our kernel configuration (or
> non-free firmware?  do you you use linux-firmware on Debian and not on
> Guix System (through nonguix or the likes)) ?

Just booted a libre kernel and confirm that it is still the case: A
blank screen to enter the password.


Kind regards

-- 
Simon

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Simon Streit <simon@HIDDEN> writes:

> Maxim Cournoyer <maxim.cournoyer@HIDDEN> writes:
>
>> So just to make sure I got that right, it works correctly on Debian
>> but incorrectly (black screen) on Guix System on the *same* hardware?
>
> correct.

Actually we =E2=80=9Cjust=E2=80=9D need to set screen brightness at activat=
ion-time:

  https://issues.guix.gnu.org/76554#5

Ludo=E2=80=99.

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