I went through a rocky update on a Latitude E6510 with an old nvidia adapter. I had to kiss the proprietary driver goodbye and install the nouveau driver. It did not work until I manually un-blacklisted nouveau and removed the nvidia X11 config files.
After that, the PC booted to a black screen with just a mouse pointer. No SDDM greeting.
Here is what I collected related to sddm from journalctl
Dec 31 14:55:00 latitude audit[1]: SERVICE_START pid=1 uid=0 auid=4294967295 ses=4294967295 msg='unit=sddm comm="systemd" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
Dec 31 14:55:01 latitude sddm[555]: Initializing...
Dec 31 14:55:01 latitude sddm[555]: Starting...
Dec 31 14:55:01 latitude sddm[555]: Logind interface found
Dec 31 14:55:01 latitude sddm[555]: Adding new display on vt 1 ...
Dec 31 14:55:01 latitude sddm[555]: Loading theme configuration from ""
Dec 31 14:55:01 latitude sddm[555]: Display server starting...
Dec 31 14:55:01 latitude sddm[555]: Adding cookie to "/var/run/sddm/{f5165ae9-9d28-40ac-b41b-a29dc45fa727}"
Dec 31 14:55:01 latitude sddm[555]: Running: /usr/bin/X -dpi 110 -auth /var/run/sddm/{f5165ae9-9d28-40ac-b41b-a29dc45fa727} -background none -noreset -displayfd 17 -seat seat0 vt1
Dec 31 14:55:03 latitude sddm[555]: Setting default cursor
Dec 31 14:55:03 latitude sddm[555]: Running display setup script "/usr/share/sddm/scripts/Xsetup"
Dec 31 14:55:03 latitude sddm[555]: Display server started.
Dec 31 14:55:03 latitude sddm[555]: Socket server starting...
Dec 31 14:55:03 latitude sddm[555]: Socket server started.
Dec 31 14:55:03 latitude sddm[555]: Loading theme configuration from "/usr/share/sddm/themes/breath2/theme.conf"
Dec 31 14:55:03 latitude sddm[555]: Greeter starting...
Dec 31 14:55:03 latitude sddm-helper[969]: [PAM] Starting...
Dec 31 14:55:03 latitude audit[969]: USER_AUTH pid=969 uid=0 auid=4294967295 ses=4294967295 msg='op=PAM:authentication grantors=pam_permit acct="sddm" exe="/usr/lib/sddm/sddm-helper" hostname=? addr=? terminal=? res=success'
Dec 31 14:55:03 latitude audit[969]: USER_ACCT pid=969 uid=0 auid=4294967295 ses=4294967295 msg='op=PAM:accounting grantors=pam_permit acct="sddm" exe="/usr/lib/sddm/sddm-helper" hostname=? addr=? terminal=? res=success'
Dec 31 14:55:03 latitude sddm-helper[969]: [PAM] Authenticating...
Dec 31 14:55:03 latitude sddm-helper[969]: [PAM] returning.
Dec 31 14:55:03 latitude audit[969]: CRED_ACQ pid=969 uid=0 auid=4294967295 ses=4294967295 msg='op=PAM:setcred grantors=pam_permit acct="sddm" exe="/usr/lib/sddm/sddm-helper" hostname=? addr=? terminal=? res=success'
Dec 31 14:55:03 latitude sddm-helper[969]: pam_unix(sddm-greeter:session): session opened for user sddm(uid=973) by (uid=0)
Dec 31 14:55:03 latitude systemd-logind[372]: New session c1 of user sddm.
Dec 31 14:55:03 latitude audit[971]: USER_ACCT pid=971 uid=0 auid=4294967295 ses=4294967295 msg='op=PAM:accounting grantors=pam_access,pam_unix,pam_permit,pam_time acct="sddm" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
Dec 31 14:55:03 latitude audit[971]: CRED_ACQ pid=971 uid=0 auid=4294967295 ses=4294967295 msg='op=PAM:setcred grantors=? acct="sddm" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=failed'
Dec 31 14:55:03 latitude systemd[971]: pam_warn(systemd-user:setcred): function=[pam_sm_setcred] flags=0x8002 service=[systemd-user] terminal=[] user=[sddm] ruser=[<unknown>] rhost=[<unknown>]
Dec 31 14:55:03 latitude systemd[971]: pam_unix(systemd-user:session): session opened for user sddm(uid=973) by (uid=0)
Dec 31 14:55:03 latitude audit[971]: USER_START pid=971 uid=0 auid=973 ses=1 msg='op=PAM:session_open grantors=pam_loginuid,pam_loginuid,pam_keyinit,pam_limits,pam_unix,pam_permit,pam_mail,pam_systemd,pam_env acct="sddm" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'
Dec 31 14:55:03 latitude systemd[1]: Started Session c1 of user sddm.
Dec 31 14:55:03 latitude audit[969]: USER_START pid=969 uid=0 auid=4294967295 ses=4294967295 msg='op=PAM:session_open grantors=pam_unix,pam_systemd acct="sddm" exe="/usr/lib/sddm/sddm-helper" hostname=? addr=? terminal=:0 res=success'
Dec 31 14:55:03 latitude sddm[555]: Greeter session started successfully
Dec 31 14:55:03 latitude sddm-greeter[982]: High-DPI autoscaling not Enabled
Dec 31 14:55:04 latitude sddm-greeter[982]: Reading from "/usr/share/xsessions/plasma.desktop"
Dec 31 14:55:04 latitude sddm-greeter[982]: Loading theme configuration from "/usr/share/sddm/themes/breath2/theme.conf"
Dec 31 14:55:04 latitude sddm-greeter[982]: Connected to the daemon.
Dec 31 14:55:04 latitude sddm-greeter[982]: QFont::fromString: Invalid description '(empty)'
Dec 31 14:55:04 latitude sddm[555]: Message received from greeter: Connect
Dec 31 14:55:04 latitude sddm-greeter[982]: QGLXContext: Failed to create dummy context
Dec 31 14:55:04 latitude sddm-greeter[982]: Loading file:///usr/share/sddm/themes/breath2/Main.qml...
Dec 31 14:55:04 latitude sddm-greeter[982]: QObject: Cannot create children for a parent that is in a different thread.
Dec 31 14:55:04 latitude sddm-greeter[982]: QObject: Cannot create children for a parent that is in a different thread.
Dec 31 14:55:04 latitude sddm-greeter[982]: QObject: Cannot create children for a parent that is in a different thread.
Dec 31 14:55:04 latitude sddm-greeter[982]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Dec 31 14:55:05 latitude dbus-daemon[366]: [system] Activating via systemd: service name='org.freedesktop.UDisks2' unit='udisks2.service' requested by ':1.17' (uid=973 pid=982 comm="/usr/bin/sddm-greeter --socket /tmp/sddm-:0-NnAHIl")
Dec 31 14:55:05 latitude dbus-daemon[366]: [system] Activating via systemd: service name='org.freedesktop.UPower' unit='upower.service' requested by ':1.17' (uid=973 pid=982 comm="/usr/bin/sddm-greeter --socket /tmp/sddm-:0-NnAHIl")
Dec 31 14:55:05 latitude sddm-greeter[982]: failed to acquire GL context to resolve capabilities, using defaults..
Dec 31 14:55:05 latitude sddm-greeter[982]: file:///usr/share/sddm/themes/breath2/Main.qml:408:5: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
Dec 31 14:55:05 latitude sddm-greeter[982]: <input>:406:376: Could not add child element to parent element because the types are incorrect.
Dec 31 14:55:05 latitude sddm-greeter[982]: <input>:407:130: Could not add child element to parent element because the types are incorrect.
Dec 31 14:55:05 latitude sddm-greeter[982]: <input>:408:130: Could not add child element to parent element because the types are incorrect.
Dec 31 14:55:05 latitude sddm-greeter[982]: <input>:408:393: Could not add child element to parent element because the types are incorrect.
Dec 31 14:55:05 latitude sddm-greeter[982]: <input>:409:130: Could not add child element to parent element because the types are incorrect.
Dec 31 14:55:05 latitude sddm-greeter[982]: <input>:410:129: Could not add child element to parent element because the types are incorrect.
Dec 31 14:55:05 latitude sddm-greeter[982]: <input>:411:129: Could not add child element to parent element because the types are incorrect.
Dec 31 14:55:05 latitude sddm-greeter[982]: <input>:412:129: Could not add child element to parent element because the types are incorrect.
Dec 31 14:55:05 latitude sddm-greeter[982]: <input>:413:129: Could not add child element to parent element because the types are incorrect.
Dec 31 14:55:05 latitude sddm-greeter[982]: <input>:413:379: Could not add child element to parent element because the types are incorrect.
Dec 31 14:55:05 latitude sddm-greeter[982]: <input>:413:631: Could not add child element to parent element because the types are incorrect.
Dec 31 14:55:05 latitude sddm-greeter[982]: file:///usr/lib/qt/qml/QtQuick/Controls/Styles/Plasma/ToolButtonStyle.qml:196:13: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
Dec 31 14:55:05 latitude sddm-greeter[982]: file:///usr/lib/qt/qml/QtQuick/Controls/Styles/Plasma/ToolButtonStyle.qml:196:13: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
Dec 31 14:55:05 latitude sddm-greeter[982]: file:///usr/lib/qt/qml/QtQuick/Controls/Styles/Plasma/ToolButtonStyle.qml:196:13: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
Dec 31 14:55:05 latitude sddm-greeter[982]: file:///usr/share/sddm/themes/breath2/Login.qml:84:9: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
Dec 31 14:55:06 latitude sddm-greeter[982]: Adding view for "eDP-1" QRect(0,0 1920x1080)
Dec 31 14:55:06 latitude sddm-greeter[982]: Failed to create OpenGL context for format QSurfaceFormat(version 2.0, options QFlags<QSurfaceFormat::FormatOption>(ResetNotification), depthBufferSize 24, redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize -1, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::DoubleBuffer, swapInterval 1, colorSpace QSurfaceFormat::DefaultColorSpace, profile QSurfaceFormat::NoProfile)
Dec 31 14:55:06 latitude audit[982]: ANOM_ABEND auid=4294967295 uid=973 gid=973 ses=4294967295 pid=982 comm="sddm-greeter" exe="/usr/bin/sddm-greeter" sig=6 res=1
Dec 31 14:55:06 latitude kernel: audit: type=1701 audit(1609422906.148:43): auid=4294967295 uid=973 gid=973 ses=4294967295 pid=982 comm="sddm-greeter" exe="/usr/bin/sddm-greeter" sig=6 res=1
Dec 31 14:55:07 latitude systemd-coredump[1024]: Process 982 (sddm-greeter) of user 973 dumped core.
#17 0x000055b4631cfd41 main (sddm-greeter + 0x19d41)
#19 0x000055b4631d007e _start (sddm-greeter + 0x1a07e)
Dec 31 14:55:07 latitude kernel: audit: type=1106 audit(1609422907.431:47): pid=969 uid=0 auid=4294967295 ses=4294967295 msg='op=PAM:session_close grantors=pam_unix,pam_systemd acct="sddm" exe="/usr/lib/sddm/sddm-helper" hostname=? addr=? terminal=:0 res=success'
Dec 31 14:55:07 latitude kernel: audit: type=1104 audit(1609422907.431:48): pid=969 uid=0 auid=4294967295 ses=4294967295 msg='op=PAM:setcred grantors=pam_permit acct="sddm" exe="/usr/lib/sddm/sddm-helper" hostname=? addr=? terminal=:0 res=success'
Dec 31 14:55:07 latitude audit[969]: USER_END pid=969 uid=0 auid=4294967295 ses=4294967295 msg='op=PAM:session_close grantors=pam_unix,pam_systemd acct="sddm" exe="/usr/lib/sddm/sddm-helper" hostname=? addr=? terminal=:0 res=success'
Dec 31 14:55:07 latitude audit[969]: CRED_DISP pid=969 uid=0 auid=4294967295 ses=4294967295 msg='op=PAM:setcred grantors=pam_permit acct="sddm" exe="/usr/lib/sddm/sddm-helper" hostname=? addr=? terminal=:0 res=success'
Dec 31 14:55:07 latitude sddm-helper[969]: [PAM] Closing session
Dec 31 14:55:07 latitude sddm-helper[969]: pam_unix(sddm-greeter:session): session closed for user sddm
Dec 31 14:55:07 latitude sddm-helper[969]: [PAM] Ended.
Dec 31 14:55:07 latitude sddm[555]: Auth: sddm-helper exited with 6
Dec 31 14:55:07 latitude sddm[555]: Greeter stopped.
Dec 31 14:55:17 latitude systemd[972]: pam_warn(systemd-user:setcred): function=[pam_sm_setcred] flags=0x8004 service=[systemd-user] terminal=[] user=[sddm] ruser=[<unknown>] rhost=[<unknown>]
This looks similar to this (but I have not touched the BIOS): SDDM hangs at startup/login (black screen with cursor) - Support / Graphics & Display - Manjaro Linux Forum
I used nouveau on this machine in the past and it worked reasonably well.
Help.