[PATCH AUTOSEL 7.0-6.19] drm/amd/display: Fix number of opp
From: Sasha Levin
Date: Mon Apr 20 2026 - 10:31:47 EST
From: Austin Zheng <Austin.Zheng@xxxxxxx>
[ Upstream commit 2c5f15ee2c760514c5be0f02cf9c9f1ff68b9ac8 ]
[Why/How]
Patch number of opp based on IP caps
Reviewed-by: Dillon Varone <dillon.varone@xxxxxxx>
Signed-off-by: Austin Zheng <Austin.Zheng@xxxxxxx>
Signed-off-by: Chuanyu Tseng <chuanyu.tseng@xxxxxxx>
Tested-by: Dan Wheeler <daniel.wheeler@xxxxxxx>
Signed-off-by: Alex Deucher <alexander.deucher@xxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
LLM Generated explanations, may be completely bogus:
Now I have all the information needed for a comprehensive analysis. Let
me compile the full report.
---
## PHASE 1: COMMIT MESSAGE FORENSICS
**Step 1.1: Subject Line**
- Subsystem: `drm/amd/display`
- Action verb: "Fix"
- Summary: Fix the number of OPP (Output Pixel Processors) patching from
IP caps.
Record: [drm/amd/display] [fix] [Ensures max_num_opp is patched from IP
capabilities]
**Step 1.2: Tags**
- Reviewed-by: Dillon Varone (AMD display team member)
- Signed-off-by: Austin Zheng (author), Chuanyu Tseng (series submitter)
- Tested-by: Dan Wheeler (AMD's display QA)
- Signed-off-by: Alex Deucher (AMD display maintainer)
- No Fixes: tag (expected for autosel candidates)
- No Cc: stable (expected)
Record: Reviewed, tested, and signed off by the AMD display team. No
syzbot or external reports.
**Step 1.3: Body Text**
- "[Why/How] Patch number of opp based on IP caps"
- Terse message typical of AMD display DML patches. Indicates the OPP
count should come from IP capabilities (hardware-specific) rather than
remaining at the compile-time default.
Record: Bug is that `max_num_opp` was not being patched from hardware IP
caps, leaving it at a static default regardless of actual hardware.
**Step 1.4: Hidden Bug Fix Detection**
This is explicitly labeled "Fix" and adds a missing field assignment
that was omitted when OPP validation was introduced.
Record: This is a direct bug fix for a missing field patching, not a
hidden fix.
## PHASE 2: DIFF ANALYSIS
**Step 2.1: Inventory**
- 1 file changed: `dml2_core_dcn4.c`
- +1 line added
- Function modified: `patch_ip_params_with_ip_caps()`
- Scope: Single-file, surgical 1-line fix
**Step 2.2: Code Flow Change**
The single line added:
```c
ip_params->max_num_opp = ip_caps->otg_count;
```
BEFORE: `patch_ip_params_with_ip_caps()` copies all IP capability fields
to IP params EXCEPT `max_num_opp`. The `max_num_opp` remains at the
compile-time default from `core_dcn4_ip_caps_base` (hardcoded to 4).
AFTER: `max_num_opp` is correctly patched from `ip_caps->otg_count`,
matching the actual hardware's OTG count.
**Step 2.3: Bug Mechanism**
This is a **logic/correctness fix** - an omission bug. Commit
`610cf76e9453b` ("Add opp count validation to dml2.1") added OPP count
validation checks in `dml2_core_dcn4_calcs.c` that read
`mode_lib->ip.max_num_opp`, but the function that patches IP params from
IP caps (`patch_ip_params_with_ip_caps`) was not updated to copy
`max_num_opp`. The validation uses a stale default value instead of the
actual hardware capability.
The validation code at lines 8588 checks:
```c
if (mode_lib->ms.TotalNumberOfActiveOPP > (unsigned
int)mode_lib->ip.max_num_opp)
mode_lib->ms.support.TotalAvailablePipesSupport = false;
```
If `max_num_opp` is wrong, display modes may be incorrectly accepted or
rejected.
**Step 2.4: Fix Quality**
- Obviously correct: follows the exact same pattern as ALL other fields
in the function
- Minimal/surgical: 1 line
- Regression risk: effectively zero - it only adds missing
initialization
- No red flags
## PHASE 3: GIT HISTORY INVESTIGATION
**Step 3.1: Blame**
The `patch_ip_params_with_ip_caps` function was introduced by commit
`70839da636050` (Aurabindo Pillai, 2024-04-19, "Add new DCN401
sources"). The function was created without a `max_num_opp` line because
at that time there was no `max_num_opp` field or OPP validation.
**Step 3.2: Fixes Target**
The commit that introduced the bug is `610cf76e9453b` ("Add opp count
validation to dml2.1", by Dmytro Laktyushkin, v6.19). That commit:
- Added `max_num_opp = 4` to `core_dcn4_ip_caps_base` static struct
- Added `max_num_opp` field to `dml2_core_ip_params`
- Added OPP validation in `dml2_core_dcn4_calcs.c`
- BUT did NOT add `max_num_opp` patching to
`patch_ip_params_with_ip_caps()`
Record: The bug was introduced in v6.19. It exists in v6.19 and v7.0.
**Step 3.3: File History**
Only one commit in the 7.0 tree modified this specific file (the rename
from dml2/ to dml2_0/). The original code has had many "reintegration"
commits prior to v7.0.
**Step 3.4: Author**
Austin Zheng is a regular AMD display team contributor. Other commits
include DML-related fixes and data type corrections.
**Step 3.5: Dependencies**
The fix depends on commit `610cf76e9453b` ("Add opp count validation")
being present. Verified:
- v6.19: Has this prerequisite (confirmed via `git show`)
- v6.18 and older: Do NOT have this prerequisite
- v6.12 LTS: Does NOT have this prerequisite
## PHASE 4: MAILING LIST RESEARCH
Found the original submission: "[PATCH v2 0/9] DC Patches March 10,
2026" on amd-gfx mailing list. The fix was patch 7 of 9 in a v2 series
submitted by Chuanyu Tseng. The series was merged via the normal AMD
display patch flow. It was NOT part of drm-fixes-7.0 (the -fixes pull
only had different urgent fixes).
No NAKs or objections found. No explicit stable nomination.
## PHASE 5: CODE SEMANTIC ANALYSIS
**Key function:** `patch_ip_params_with_ip_caps()` - called from
`core_dcn4_initialize()` during DML2 core initialization.
**Impact path:** `dml21_populate_dml_init_params()` ->
`core_dcn4_initialize()` -> `patch_ip_params_with_ip_caps()`. This runs
during display mode validation for every display configuration change on
DCN4+ hardware.
**Consumer of `max_num_opp`:** Used in `CalculateODMMode()` and the main
mode support validation loop in `dml2_core_dcn4_calcs.c` (lines 8421,
8442, 8588) to validate that active OPP count doesn't exceed hardware
capability.
## PHASE 6: STABLE TREE ANALYSIS
**Bug existence by tree:**
- v7.0: BUG EXISTS (verified - `max_num_opp` in struct at line 31,
validation in calcs, but missing patching)
- v6.19: BUG EXISTS (verified - same state as 7.0, file at dml2_0 path)
- v6.18: Bug does NOT exist (no `max_num_opp` field or validation)
- v6.12 LTS: Bug does NOT exist
- v6.6 LTS, v6.1 LTS: Bug does NOT exist
**Backport complexity:** For 7.0.y: should apply cleanly. For 6.19.y:
path may need adjustment (file is at `dml2_0/` in v6.19 already, so it
may apply cleanly).
## PHASE 7: SUBSYSTEM AND MAINTAINER CONTEXT
- Subsystem: drm/amd/display - DML2 (Display Mode Library) for DCN4+
- Criticality: IMPORTANT - affects AMD GPU display output for newer
hardware
- The DML2.1 code is actively developed with frequent "reintegration"
commits
## PHASE 8: IMPACT AND RISK ASSESSMENT
**Who is affected:** Users of AMD DCN4+ GPUs where the actual OPP/OTG
count differs from the compile-time default of 4 (e.g., harvested
silicon, different SKUs).
**Trigger conditions:** Any display mode validation on affected
hardware. This runs during every display configuration change (mode set,
multi-monitor setup, etc.).
**Failure mode:** Incorrect DML mode validation:
- If real OPP count < 4: modes could be accepted that the hardware can't
support (display corruption or failure)
- If real OPP count > 4: modes could be incorrectly rejected (user can't
use supported display configurations)
- Severity: MEDIUM-HIGH for affected hardware
**Risk-Benefit:**
- Benefit: Correct mode validation on all DCN4 hardware variants
- Risk: VERY LOW - 1 line, follows established pattern, no behavioral
change for hardware where count == 4
## PHASE 9: FINAL SYNTHESIS
**Evidence FOR backporting:**
- Fixes a real bug (missing field initialization) with concrete
consequences
- Trivially correct (1 line, follows exact pattern of all other fields)
- Reviewed and tested by AMD display team
- Affects display functionality on AMD DCN4 hardware
- Zero regression risk for hardware where OPP count == 4 (most current
hardware)
**Evidence AGAINST backporting:**
- Only affects hardware with non-default OPP counts (subset of DCN4
ASICs)
- Commit message is terse with minimal detail
- Not submitted via -fixes flow (went through normal DC patches)
- Only applicable to trees containing v6.19+ (610cf76e9453b)
**Stable rules checklist:**
1. Obviously correct? YES - exact pattern match with all other fields
2. Fixes real bug? YES - incorrect DML mode validation
3. Important issue? YES for affected hardware (display functionality)
4. Small and contained? YES - 1 line
5. No new features? YES
6. Applies to stable? YES for 7.0.y and 6.19.y
---
## Verification
- [Phase 1] Parsed tags: Reviewed-by Dillon Varone, Tested-by Dan
Wheeler, author Austin Zheng, maintainer Alex Deucher
- [Phase 2] Diff analysis: 1 line added to
`patch_ip_params_with_ip_caps()`, adds missing `max_num_opp` field
patching
- [Phase 3] git blame: `patch_ip_params_with_ip_caps()` introduced in
70839da636050 (v6.12 era) without `max_num_opp`
- [Phase 3] git show 610cf76e9453b: confirmed this commit added opp
validation but missed updating `patch_ip_params_with_ip_caps()` - this
is the root cause
- [Phase 3] git show v6.19 file: confirmed `max_num_opp = 4` in struct
AND validation in calcs, BUT missing patching - bug exists in v6.19
- [Phase 3] git show v6.18 file: confirmed no `max_num_opp` at all - bug
does NOT exist pre-6.19
- [Phase 3] git show v6.12 file: confirmed no `max_num_opp` at all
- [Phase 4] Found original submission: "[PATCH v2 0/9] DC Patches March
10, 2026" on amd-gfx mailing list
- [Phase 4] No NAKs or concerns found in mailing list
- [Phase 5] `max_num_opp` is consumed at lines 8421, 8442, 8588 of
dml2_core_dcn4_calcs.c for display mode validation
- [Phase 5] `patch_ip_params_with_ip_caps()` called from
`core_dcn4_initialize()`, which runs during every DML initialization
- [Phase 6] Bug exists in v6.19 and v7.0 only - confirmed by checking
file contents at each tagged version
- [Phase 8] Failure mode: incorrect mode validation leading to display
issues, severity MEDIUM-HIGH for affected hardware
- UNVERIFIED: Exact set of AMD ASICs where OPP count differs from
default 4 could not be determined (dcn42 bounding box header not found
in tree)
The fix is a trivially correct 1-line addition that patches a missing
field from the hardware IP capabilities, fixing incorrect DML mode
validation on AMD DCN4 hardware where the OPP count may differ from the
compile-time default.
**YES**
.../amd/display/dc/dml2_0/dml21/src/dml2_core/dml2_core_dcn4.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/drivers/gpu/drm/amd/display/dc/dml2_0/dml21/src/dml2_core/dml2_core_dcn4.c b/drivers/gpu/drm/amd/display/dc/dml2_0/dml21/src/dml2_core/dml2_core_dcn4.c
index eba948e187c11..93d479083acde 100644
--- a/drivers/gpu/drm/amd/display/dc/dml2_0/dml21/src/dml2_core/dml2_core_dcn4.c
+++ b/drivers/gpu/drm/amd/display/dc/dml2_0/dml21/src/dml2_core/dml2_core_dcn4.c
@@ -107,6 +107,7 @@ static void patch_ip_params_with_ip_caps(struct dml2_core_ip_params *ip_params,
{
ip_params->max_num_dpp = ip_caps->pipe_count;
ip_params->max_num_otg = ip_caps->otg_count;
+ ip_params->max_num_opp = ip_caps->otg_count;
ip_params->num_dsc = ip_caps->num_dsc;
ip_params->max_num_dp2p0_streams = ip_caps->max_num_dp2p0_streams;
ip_params->max_num_dp2p0_outputs = ip_caps->max_num_dp2p0_outputs;
--
2.53.0