# DF-4911 — `dm_dp_aux_transfer` discards `dc_link_aux_transfer()` return value on both AUX *write* paths, reporting failed DPCD/I2C writes as full success

File: `sys/dev/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c:105-117,133-148`
Severity: Low | CWE-252 | confidence: certain (defect) / likely (impact)

## Build / run

The bug is in the kernel's AUX transfer callback; it is exercised whenever the
amdgpu DM driver performs DPCD/AUX writes (link training, MST sideband
messaging) against real DisplayPort hardware. There is no direct unprivileged
trigger: the raw AUX character device (`/dev/drm_dp_auxN`) is compiled out on
DragonFly (`CONFIG_DRM_DP_AUX_CHARDEV` is not set in
`sys/dev/drm/kconfig.h`), so writes are kernel-initiated.

```
./build.sh   # cc -o aux_chardev_probe aux_chardev_probe.c
./run.sh     # on AMD hardware with CONFIG_DRM_DP_AUX_CHARDEV: write to a DPCD
             # address while the sink NACKs; drm dp core logs success anyway.
```

## Expected result (on AMD DP hardware with failing/NACKing sink)

`dc_link_aux_transfer` returns `-1` on AUX_CHANNEL_OPERATION_FAILED_TIMEOUT /
INVALID_REPLY (`sys/dev/drm/amd/display/dc/core/dc_link_ddc.c:669-683`), but
`dm_dp_aux_transfer` executes:

```c
	case DP_AUX_NATIVE_WRITE:
		...
		dc_link_aux_transfer(...);          /* return value dropped */
		result = msg->size;                 /* unconditional success */
```

so `drm_dp_dpcd_write()` callers (`sys/dev/drm/drm_dp_helper.c:334+`) see
`msg->size` (>0 == success) for a write that never reached the sink. The MST
sideband engine then advances message state / believes payload allocations
were programmed while the branch device never received them -> topology
desync, wedged links, black screens. A hostile DP dongle can NACK every write
deliberately, turning this into a persistent display-DoS amplifier and
confusing the kernel's model of programmed payload tables.

## Actual result on this audit guest

No AMD display device, no /dev/dri, aux chardev compiled out — runtime
unreachable; status honestly `untested`. Defect itself is certain from
source.