Public bug reported:
[ Impact ]
Users connecting a Lenovo USB-C to VGA Adapter (or similar USB-C DP adapters
with
a FIXED_VS LTTPR retimer) get no display output. The AMD display driver fails DP
link training and logs:
amdgpu 0000:c4:00.0: [drm] enabling link 2 failed: 15
The fix is a single-line change:
- if (memcmp("\x0,\x0,\x0", &link->dpcd_caps.lttpr_caps.lttpr_ieee_oui[0], 3)
== 0)
+ if (memcmp("\x00\x00\x00", &link->dpcd_caps.lttpr_caps.lttpr_ieee_oui[0],
3) == 0)
The root cause is a typo in a C string literal used to check whether a FIXED_VS
LTTPR retimer has no IEEE OUI. The literal "\x0,\x0,\x0" was intended to be
three
zero bytes but actually evaluates to five bytes {0x00, 0x2C, 0x00, 0x2C, 0x00}
(0x2C = ASCII comma) because commas are not valid hex digits and terminate the
\x
escape early. The memcmp() against the 3-byte LTTPR OUI field therefore never
matches, so the vendor-specific link rate toggle workaround required by these
retimers is never applied, and link training fails.
[ Test Plan ]
1. Connect a Lenovo USB-C to VGA Adapter to a system with an AMD GPU.
2. Without patch: confirm no display output and observe the following in dmesg:
amdgpu ...: [drm] enabling link N failed: 15
3. Install a new kernel with patch: confirm display output appears and the
"enabling link failed: 15"
message is absent from dmesg.
[ Where problems could occur ]
The change is confined to the FIXED_VS LTTPR link training path
(dp_perform_fixed_vs_pe_training_sequence). It only affects systems where:
- An AMD GPU with USB-C DP output is present, AND
- A DP retimer (LTTPR) with no IEEE OUI is detected on the link.
The risk of regression is very low — the fix corrects a comparison that was
always
false before, restoring the originally intended behaviour. Systems without a
FIXED_VS LTTPR retimer are completely unaffected.
[ Other Info ]
Upstream commit: 531fe6e0fee85a1bdb5b8223a706fff654ed0a61 (v7.0-rc1)
Stable backport: 91e6b6a7fe57d48c392d353ee50162a525d9e80c (v6.19.6)
Reported against: linux-oem-6.17 6.17.0-1025.25
** Affects: linux (Ubuntu)
Importance: Undecided
Assignee: Bin Li (binli)
Status: In Progress
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https://bugs.launchpad.net/bugs/2155619
Title:
[SRU] Correct FIXED_VS Link Rate Toggle Condition
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