[PATCH i-g-t v5 0/4] kms_dp_linktrain_fallback: Add UHBR-to-HBR fallback subtest

[Sowmiya S <sowmiya.s@intel.com>]

DP 2.x links use 128b/132b channel encoding for UHBR rates
(UHBR10/13.5/20) and 8b/10b for legacy HBR3 and below. The transition
between the two encodings goes through a port-slice reset on the
source side. The existing dp-fallback subtest only covers 8b/10b-to-8b/10b
fallback; this series adds a dedicated subtest for the UHBR->HBR direction.

Adds two library helpers to lib/i915/i915_dp:
  - i915_dp_is_uhbr_rate(): mirrors the kernel's drm_dp_is_uhbr_rate()
  - i915_dp_get_next_lower_rate(): parses the force_link_rate debugfs list
    with proper strtok_r/strtol/errno handling

const char * signature for i915_dp_set_link_params() so string literals
can be passed directly without a writable-array workaround.

Fixes a pre-existing bug where the file-static traversed_mst_output_count
was never reset between subtests, causing all MST outputs to be silently
skipped as "already visited" in a full-binary run.

Adds the uhbr-to-hbr-fallback subtest. The subtest pins the link at the
highest sustainable UHBR rate (stepping down if the cable cannot sustain
the sink's max), forces repeated LT_FAILURE_REDUCED_CAPS failures, and
asserts the rate drops below UHBR10. MST topologies are supported since
all siblings share the physical link. The existing run_lt_fallback_test()
is extended with force_uhbr parameter to avoid ~45 lines of duplication.

v2: Resets MST traversal state per test run
v3:
  - Drop "exercising the port slice reset path" from SUBTEST description
  - Revert unrelated cosmetic hunk in setup_mst_outputs()
  - Single UHBR capability gate using i915_dp_get_max_link_rate()
  - remove duplicate gate using i915_dp_get_max_supported_rate()
  - Fix link rate log units (10 kbit/s, not kbps)
  - Add igt_reset_connectors() to pin-loop early-return path
  - Replace UHBR10_LINK_RATE define with i915_dp_is_uhbr_rate() helper
  - Replace open-coded strtok() parsing with i915_dp_get_next_lower_rate()
  - Hard-assert "fallback not reached" instead of returning false
  - Remove dead traversed_mst_output_count reset in run_dsc path
  - Merge run_uhbr_to_hbr_fallback_test() into run_lt_fallback_test()
  - Use "auto" string literal directly (const char * param fix)
  - Move reset before UHBR capability gate read in run_lt_fallback_test()
  - Document force_uhbr parameter and "clear the pin" reset behaviour
v4:
  - name the UHBR10 threshold and make i915_dp_is_uhbr_rate() a
    static inline in the header to avoid a cross-TU call
  - parse intel_dp_allowed_link_configs instead of the source-only
    i915_dp_force_link_rate list in i915_dp_get_next_lower_rate()
  - avoid the implicit long to int narrowing on the return value 
v5: commit display reset before fallback 

Sowmiya S (4):
  lib/i915/i915_dp: Add UHBR helpers and const-correct set_link_params
  tests/intel/kms_dp_linktrain_fallback: Reset traversed MST output
    count
  tests/intel/kms_dp_linktrain_fallback: Add UHBR-to-HBR fallback
    subtest
  tests/intel/kms_dp_linktrain_fallback: commit display reset before
    fallback

 lib/i915/i915_dp.c                      |  58 ++++++-
 lib/i915/i915_dp.h                      |  24 ++-
 tests/intel/kms_dp_linktrain_fallback.c | 202 +++++++++++++++++++++---
 3 files changed, 260 insertions(+), 24 deletions(-)

-- 
2.43.0