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, 116 insertions, 0 deletions

diff --git a/Documentation/process/generated-content.rst b/Documentation/process/generated-content.rst
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+============================================
+Kernel Guidelines for Tool-Generated Content
+============================================
+
+Purpose
+=======
+
+Kernel contributors have been using tooling to generate contributions
+for a long time. These tools can increase the volume of contributions.
+At the same time, reviewer and maintainer bandwidth is a scarce
+resource. Understanding which portions of a contribution come from
+humans versus tools is helpful to maintain those resources and keep
+kernel development healthy.
+
+The goal here is to clarify community expectations around tools. This
+lets everyone become more productive while also maintaining high
+degrees of trust between submitters and reviewers.
+
+Out of Scope
+============
+
+These guidelines do not apply to tools that make trivial tweaks to
+preexisting content. Nor do they pertain to tooling that helps with
+menial tasks. Some examples:
+
+ - Spelling and grammar fix ups, like rephrasing to imperative voice
+ - Typing aids like identifier completion, common boilerplate or
+   trivial pattern completion
+ - Purely mechanical transformations like variable renaming
+ - Reformatting, like running Lindent, ``clang-format`` or
+   ``rust-fmt``
+
+Even whenever your tool use is out of scope, you should still always
+consider if it would help reviewing your contribution if the reviewer
+knows about the tool that you used.
+
+In Scope
+========
+
+These guidelines apply when a meaningful amount of content in a kernel
+contribution was not written by a person in the Signed-off-by chain,
+but was instead created by a tool.
+
+Detection of a problem and testing the fix for it is also part of the
+development process; if a tool was used to find a problem addressed by
+a change, that should be noted in the changelog. This not only gives
+credit where it is due, it also helps fellow developers find out about
+these tools.
+
+Some examples:
+ - Any tool-suggested fix such as ``checkpatch.pl --fix``
+ - Coccinelle scripts
+ - A chatbot generated a new function in your patch to sort list entries.
+ - A .c file in the patch was originally generated by a coding
+   assistant but cleaned up by hand.
+ - The changelog was generated by handing the patch to a generative AI
+   tool and asking it to write the changelog.
+ - The changelog was translated from another language.
+
+If in doubt, choose transparency and assume these guidelines apply to
+your contribution.
+
+Guidelines
+==========
+
+First, read the Developer's Certificate of Origin:
+Documentation/process/submitting-patches.rst. Its rules are simple
+and have been in place for a long time. They have covered many
+tool-generated contributions. Ensure that you understand your entire
+submission and are prepared to respond to review comments.
+
+Second, when making a contribution, be transparent about the origin of
+content in cover letters and changelogs. You can be more transparent
+by adding information like this:
+
+ - What tools were used?
+ - The input to the tools you used, like the Coccinelle source script.
+ - If code was largely generated from a single or short set of
+   prompts, include those prompts. For longer sessions, include a
+   summary of the prompts and the nature of resulting assistance.
+ - Which portions of the content were affected by that tool?
+ - How is the submission tested and what tools were used to test the
+   fix?
+
+As with all contributions, individual maintainers have discretion to
+choose how they handle the contribution. For example, they might:
+
+ - Treat it just like any other contribution.
+ - Reject it outright.
+ - Treat the contribution specially, for example, asking for extra
+   testing, reviewing with extra scrutiny, or reviewing at a lower
+   priority than human-generated content.
+ - Ask for some other special steps, like asking the contributor to
+   elaborate on how the tool or model was trained.
+ - Ask the submitter to explain in more detail about the contribution
+   so that the maintainer can be assured that the submitter fully
+   understands how the code works.
+ - Suggest a better prompt instead of suggesting specific code changes.
+
+If tools permit you to generate a contribution automatically, expect
+additional scrutiny in proportion to how much of it was generated.
+
+As with the output of any tooling, the result may be incorrect or
+inappropriate. You are expected to understand and to be able to defend
+everything you submit. If you are unable to do so, then do not submit
+the resulting changes.
+
+If you do so anyway, maintainers are entitled to reject your series
+without detailed review.
+
+References
+==========
+
+For specific guidelines on AI coding assistants, see:
+
+* Documentation/process/coding-assistants.rst