drm/amd/display: Try RGB before YCbCr 4:4:4 in stream validationgrafted

amdgpu_dm_create_validate_stream_for_sink() walks encoding_order[] and
uses the first encoding that validates. YCbCr 4:4:4 is listed before
RGB, so an HDMI sink that advertises 4:4:4 gets YCbCr 4:4:4 whenever the
"color format" property is left at AUTO, even though RGB fits the same
link.

That contradicts the documented AUTO behaviour for HDMI in enum
drm_connector_color_format (RGB, falling back to YCbCr 4:2:0 only when
the bandwidth is not available or the mode is 4:2:0-only), which the
amdgpu implementation of the property also describes. It also leaves
the "Broadcast RGB" property without effect on such sinks, since the
quantization range it selects only applies to RGB output.

Try RGB first. The mask still holds every encoding the sink supports,
so a mode that cannot carry RGB falls back exactly as before.

For reference, v7.2 picked RGB here unless YCbCr 4:4:4 was forced
through debugfs, while earlier kernels picked YCbCr 4:4:4 for any HDMI
sink that advertised it.

Fixes: 0b0ff65d3ca1 ("drm/amd/display: Refactor stream validation")
Suggested-by: Adolfo Rodrigues <adolfotregosa@gmail.com>
Assisted-by: Claude Code:claude-fable-5-1
Signed-off-by: Adrian Betschart <adrian.betschart@cinemaone.ch>
Reviewed-by: Fangzhi Zuo <jerry.zuo@amd.com>
Tested-by: Adolfo Rodrigues <adolfotregosa@gmail.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 7de432bc753133764dc40d37f9388da56de251e7)

1 files changed, 47 insertions, 0 deletions

diff --git a/Documentation/dev-tools/testing-devices.rst b/Documentation/dev-tools/testing-devices.rst
new file mode 100644
index 000000000..ab26adb99
--- /dev/null
+++ b/Documentation/dev-tools/testing-devices.rst
@@ -0,0 +1,47 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. Copyright (c) 2024 Collabora Ltd
+
+=============================
+Device testing with kselftest
+=============================
+
+
+There are a few different kselftests available for testing devices generically,
+with some overlap in coverage and different requirements. This document aims to
+give an overview of each one.
+
+Note: Paths in this document are relative to the kselftest folder
+(``tools/testing/selftests``).
+
+Device oriented kselftests:
+
+* Devicetree (``dt``)
+
+  * **Coverage**: Probe status for devices described in Devicetree
+  * **Requirements**: None
+
+* Error logs (``devices/error_logs``)
+
+  * **Coverage**: Error (or more critical) log messages presence coming from any
+    device
+  * **Requirements**: None
+
+* Discoverable bus (``devices/probe``)
+
+  * **Coverage**: Presence and probe status of USB or PCI devices that have been
+    described in the reference file
+  * **Requirements**: Manually describe the devices that should be tested in a
+    YAML reference file (see ``devices/probe/boards/google,spherion.yaml`` for
+    an example)
+
+* Exist (``devices/exist``)
+
+  * **Coverage**: Presence of all devices
+  * **Requirements**: Generate the reference (see ``devices/exist/README.rst``
+    for details) on a known-good kernel
+
+Therefore, the suggestion is to enable the error log and devicetree tests on all
+(DT-based) platforms, since they don't have any requirements. Then to greatly
+improve coverage, generate the reference for each platform and enable the exist
+test. The discoverable bus test can be used to verify the probe status of
+specific USB or PCI devices, but is probably not worth it for most cases.