[PATCH 19/82] drm/amd/display: Test MST stream feature read failure

From: Alex Hung <[email protected]>

[WHAT]
Add a KUnit test for dm_helpers_mst_enable_stream_features() that fails
the DP_DOWNSPREAD_CTRL read.

The helper must bail out without writing the register back, which covers
the early return on a failed DPCD read.

Assisted-by: Copilot:Claude-Opus-5
Reviewed-by: Bhawanpreet Lakha <[email protected]>
Signed-off-by: Alex Hung <[email protected]>
Signed-off-by: Ivan Lipski <[email protected]>
---
 .../amdgpu_dm/tests/amdgpu_dm_helpers_test.c  | 33 +++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/tests/amdgpu_dm_helpers_test.c b/drivers/gpu/drm/amd/display/amdgpu_dm/tests/amdgpu_dm_helpers_test.c
index bc8b607b6c83b..6b4f6993ace94 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/tests/amdgpu_dm_helpers_test.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/tests/amdgpu_dm_helpers_test.c
@@ -2661,6 +2661,38 @@ static void dm_test_mst_enable_stream_features_writes_downspread(struct kunit *t
 	KUNIT_EXPECT_EQ(test, fixture->dpcd_write_value, (u8)BIT(7));
 }
 
+/**
+ * dm_test_mst_enable_stream_features_read_fail - Test early return on DPCD read failure
+ * @test: The KUnit test context
+ */
+static void dm_test_mst_enable_stream_features_read_fail(struct kunit *test)
+{
+	struct dm_test_synaptics_aux *fixture;
+	struct amdgpu_dm_connector *aconnector;
+	struct dc_stream_state *stream;
+	struct amdgpu_device *adev;
+	struct dc_link *link;
+
+	adev = dm_kunit_alloc_adev(test);
+	fixture = dm_test_alloc_synaptics_aux_with_dev(test, &adev->ddev);
+	aconnector = dm_kunit_alloc_connector(test, adev, NULL);
+	link = dm_kunit_alloc_link(test);
+	stream = kunit_kzalloc(test, sizeof(*stream), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, stream);
+
+	fixture->fail_address = DP_DOWNSPREAD_CTRL;
+	dm_test_current_aux_recorder = fixture;
+	aconnector->dm_dp_aux.aux.drm_dev = &adev->ddev;
+	aconnector->dm_dp_aux.aux.transfer = dm_test_current_aux_transfer;
+	drm_dp_aux_init(&aconnector->dm_dp_aux.aux);
+	link->priv = aconnector;
+	stream->link = link;
+
+	dm_helpers_mst_enable_stream_features(stream);
+
+	KUNIT_EXPECT_EQ(test, fixture->downspread_writes, 0U);
+}
+
 /* Tests for dm_helpers_enable_periodic_detection() */
 
 struct dm_test_idle_work {
@@ -4013,6 +4045,7 @@ static struct kunit_case amdgpu_dm_helpers_test_cases[] = {
 	/* dm_helpers_mst_enable_stream_features */
 	KUNIT_CASE(dm_test_mst_enable_stream_features_aux_disabled),
 	KUNIT_CASE(dm_test_mst_enable_stream_features_writes_downspread),
+	KUNIT_CASE(dm_test_mst_enable_stream_features_read_fail),
 	/* dm_helpers_enable_periodic_detection */
 	KUNIT_CASE(dm_test_enable_periodic_detection_no_workqueue),
 	KUNIT_CASE(dm_test_enable_periodic_detection_updates_enable),
-- 
2.43.0