[Bug 1784156] Re: [regression][amdgpu] analog output stops working on R9 380/Tonga Pro after upgrading to Ubuntu 18.04

I'm running an AMD Radeon R9 380 (Tonga, GCN 1.2) on Ubuntu 24.04-based
system with kernel 6.17.0-14-generic.

I'm experiencing HDMI resume instability when DC is enabled. After
suspend-to-RAM (deep), the system wakes but the screen returns either
blank or with corrupted output. Input devices respond (cursor moves),
but the session is unusable and requires forced reboot.

The issue is completely resolved by adding:

amdgpu.dc=0

to the kernel parameters.

This reminds me of historical Tonga/DC regressions such as bug #1784156,
where disabling DC restored proper output detection and signal behavior.

Given that Tonga (GCN 1.2) is a legacy architecture, it may not be
receiving the same level of DC resume-path validation as newer RDNA
GPUs. This might indicate a DC-specific regression affecting older GCN
display pipelines during resume.

I can provide additional logs (dmesg before/after suspend, drm debug
logs, etc.) if needed.

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Title:
[regression][amdgpu] analog output stops working on R9 380/Tonga Pro
after upgrading to Ubuntu 18.04

Status in linux package in Ubuntu:
Triaged
Status in linux source package in Bionic:
Triaged
Status in linux source package in Cosmic:
Won't Fix

Bug description:
After upgrading to Ubuntu 18.04 (and thus from Linux 4.13 to Linux
4.15) the Monitor connected via VGA (through DVI-I) shows a 'No
Signal' message after amdgpu takes over from efifb and turns off.

The monitor worked fine in Ubuntu 17.10 and Linux 4.13.

When looking at the output of xrandr, the analog monitor is now
detected as DVI-D instead of DVI-I and thus probably won't receive an
analog signal anymore.

After some investigation, it turns out the monitor will light up when
adding the `amdgpu.dc=0` kernel parameter.

I've tried Linux 4.18-rc6 from the mainline ppa and the issue persists
unless I add the kernel parameter to disable DC.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: linux-image-generic 4.15.0.29.31
ProcVersionSignature: Ubuntu 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
AudioDevicesInUse:
USER PID ACCESS COMMAND
/dev/snd/pcmC0D0p: benpicco 2151 F...m pulseaudio
/dev/snd/controlC0: benpicco 2151 F.... pulseaudio
/dev/snd/controlC1: benpicco 2151 F.... pulseaudio
CurrentDesktop: MATE
Date: Sat Jul 28 17:55:35 2018
EcryptfsInUse: Yes
HibernationDevice: RESUME=UUID=ef400a46-b237-4272-b4a7-f866e92d00a0
InstallationDate: Installed on 2016-08-06 (720 days ago)
InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
IwConfig:
lo no wireless extensions.

eno1 no wireless extensions.
MachineType: System manufacturer System Product Name
ProcFB: 0 amdgpudrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-29-generic root=UUID=8e6b0d6a-eb87-44cf-8fc3-b13bd97b71d2 ro amdgpu.dc=0
RelatedPackageVersions:
linux-restricted-modules-4.15.0-29-generic N/A
linux-backports-modules-4.15.0-29-generic N/A
linux-firmware 1.173.1
RfKill:
0: hci0: Bluetooth
Soft blocked: no
Hard blocked: no
SourcePackage: linux
UpgradeStatus: Upgraded to bionic on 2018-07-28 (0 days ago)
dmi.bios.date: 11/09/2012
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 3603
dmi.board.asset.tag: To be filled by O.E.M.
dmi.board.name: P8Z68-V
dmi.board.vendor: ASUSTeK Computer INC.
dmi.board.version: Rev 1.xx
dmi.chassis.asset.tag: Asset-1234567890
dmi.chassis.type: 3
dmi.chassis.vendor: Chassis Manufacture
dmi.chassis.version: Chassis Version
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr3603:bd11/09/2012:svnSystemmanufacturer:pnSystemProductName:pvrSystemVersion:rvnASUSTeKComputerINC.:rnP8Z68-V:rvrRev1.xx:cvnChassisManufacture:ct3:cvrChassisVersion:
dmi.product.family: To be filled by O.E.M.
dmi.product.name: System Product Name
dmi.product.version: System Version
dmi.sys.vendor: System manufacturer

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