Luminosité bloquée / HP 255 G7 / Xubuntu 20.04

Bonjour,

Je ne parviens pas à finaliser correctement l'installation de Xubuntu 20.04 sur un portable HP 255 G7.

J'ai d'abord rencontré et corrigé le problème de tearing :
https://doc.ubuntu-fr.org/radeon#probleme_de_tearing_decalage_de_l_image_par_bandes_horizontales

A présent c,est le reglage de la luminosité qui ne fonctionne pas. Les touchent fonctionnent, mais le réglage reste fixe.

Voici la carte et les drivers :

lspci -vnn | grep -A12 '\''[030[02]\]' | grep -Ei "vga|3d|display|kernel"

00:01.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Stoney [Radeon R2/R3/R4/R5 Graphics] [1002:98e4] (rev e2) (prog-if 00 [VGA controller])
	Kernel driver in use: amdgpu
	Kernel modules: amdgpu

Le journal d'erreurs :

-- Logs begin at Thu 2020-10-08 21:46:45 CEST, end at Mon 2020-10-12 09:43:29 CEST. --
oct. 12 09:43:07 edith-Notebook kernel: ACPI BIOS Error (bug): Could not resolve symbol [\_SB.WLBU._STA.WLVD], AE_NOT_FOUND (20190816/psargs-330)
oct. 12 09:43:07 edith-Notebook kernel: ACPI Error: Aborting method \_SB.WLBU._STA due to previous error (AE_NOT_FOUND) (20190816/psparse-529)
oct. 12 09:43:07 edith-Notebook kernel: ACPI BIOS Error (bug): Could not resolve symbol [\_SB.WLBU._STA.WLVD], AE_NOT_FOUND (20190816/psargs-330)
oct. 12 09:43:07 edith-Notebook kernel: ACPI Error: Aborting method \_SB.WLBU._STA due to previous error (AE_NOT_FOUND) (20190816/psparse-529)
oct. 12 09:43:07 edith-Notebook kernel: ACPI BIOS Error (bug): Could not resolve symbol [\_SB.WLBU._STA.WLVD], AE_NOT_FOUND (20190816/psargs-330)
oct. 12 09:43:07 edith-Notebook kernel: ACPI Error: Aborting method \_SB.WLBU._STA due to previous error (AE_NOT_FOUND) (20190816/psparse-529)
oct. 12 09:43:07 edith-Notebook kernel: Initramfs unpacking failed: Decoding failed
[oct. 12 09:43:09 edith-Notebook systemd-backlight[533]: Failed to get backlight or LED device 'backlight:acpi_video0': No such device
oct. 12 09:43:09 edith-Notebook systemd[1]: Failed to start Load/Save Screen Backlight Brightness of backlight:acpi_video0.
oct. 12 09:43:10 edith-Notebook kernel: ACPI Error: No handler for Region [RCM0] (000000001fe588cc) [SystemCMOS] (20190816/evregion-127)
oct. 12 09:43:10 edith-Notebook kernel: ACPI Error: Region SystemCMOS (ID=5) has no handler (20190816/exfldio-261)
oct. 12 09:43:10 edith-Notebook kernel: ACPI Error: Aborting method \_SB.WMID.ESDT due to previous error (AE_NOT_EXIST) (20190816/psparse-529)
oct. 12 09:43:10 edith-Notebook kernel: ACPI Error: Aborting method \_SB.PCI0.LPC0.EC0._Q42 due to previous error (AE_NOT_EXIST) (20190816/psparse-529)
oct. 12 09:43:13 edith-Notebook pulseaudio[801]: No UCM verb is valid for hw:0
oct. 12 09:43:13 edith-Notebook pulseaudio[801]: No UCM verb is valid for hw:1
oct. 12 09:43:16 edith-Notebook wpa_supplicant[636]: bgscan simple: Failed to enable signal strength monitoring
~

J'ai tenté de modifier /etc/default/grub en modifiant successivement la ligne (suivi d'un sudo update-grub et reboot, bien sûr) :

#GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" # valeur par défaut : le curseur de luminosité s'affiche, mais pas de changement de valeur possible.
#GRUB_CMDLINE_LINUX_DEFAULT="quiet acpi_backlight=vendor" # premier test - ne change rien
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=linux acpi_backlight=video" # deuxième test : le curseur de luminosité fonctionne, mais le réglage reste inopérant

Dans le deux derniers cas, les lignes suivantes disparaissent du journal d'erreur... mais sans résolution :

[oct. 12 09:43:09 edith-Notebook systemd-backlight[533]: Failed to get backlight or LED device 'backlight:acpi_video0': No such device
oct. 12 09:43:09 edith-Notebook systemd[1]: Failed to start Load/Save Screen Backlight Brightness of backlight:acpi_video0.

Auriez-vous une idée ?
Faut-il passer par le driver radeon plutôt que amdgpu ?
Merci d'avance.