drm/vc4: tests: pv-muxing: Document test scenario

We've had a couple of tests that weren't really obvious, nor did they
document what they were supposed to test. Document that to make it
hopefully more obvious.
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Link: https://lore.kernel.org/r/20230728-kms-kunit-actions-rework-v3-11-952565ccccfe@kernel.orgSigned-off-by: Maxime Ripard <mripard@kernel.org>

drivers/gpu/drm/vc4/tests/vc4_test_pv_muxing.c

@@ -845,6 +845,13 @@ static void drm_test_vc5_pv_muxing_bugs_subsequent_crtc_enable(struct kunit *tes
 	KUNIT_EXPECT_NE(test, hdmi0_channel, hdmi1_channel);
 }
 
+/*
+ * This test makes sure that we never change the FIFO of an active HVS
+ * channel if we disable a FIFO with a lower index.
+ *
+ * Doing so would result in a FIFO stall and would disrupt an output
+ * supposed to be unaffected by the commit.
+ */
 static void drm_test_vc5_pv_muxing_bugs_stable_fifo(struct kunit *test)
 {
 	struct drm_modeset_acquire_ctx *ctx;
@@ -924,6 +931,21 @@ static void drm_test_vc5_pv_muxing_bugs_stable_fifo(struct kunit *test)
 	}
 }
 
+/*
+ * Test that if we affect a single output, only the CRTC state of that
+ * output will be pulled in the global atomic state.
+ *
+ * This is relevant for two things:
+ *
+ *   - If we don't have that state at all, we are unlikely to affect the
+ *     FIFO muxing. This is somewhat redundant with
+ *     drm_test_vc5_pv_muxing_bugs_stable_fifo()
+ *
+ *   - KMS waits for page flips to occur on all the CRTC found in the
+ *     CRTC state. Since the CRTC is unaffected, we would over-wait, but
+ *     most importantly run into corner cases like waiting on an
+ *     inactive CRTC that never completes.
+ */
 static void
 drm_test_vc5_pv_muxing_bugs_subsequent_crtc_enable_too_many_crtc_state(struct kunit *test)
 {