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, 69 insertions, 0 deletions

diff --git a/Documentation/atomic_bitops.txt b/Documentation/atomic_bitops.txt
new file mode 100644
index 000000000..edea4656c
--- /dev/null
+++ b/Documentation/atomic_bitops.txt
@@ -0,0 +1,69 @@
+=============
+Atomic bitops
+=============
+
+While our bitmap_{}() functions are non-atomic, we have a number of operations
+operating on single bits in a bitmap that are atomic.
+
+
+API
+---
+
+The single bit operations are:
+
+Non-RMW ops:
+
+  test_bit()
+
+RMW atomic operations without return value:
+
+  {set,clear,change}_bit()
+  clear_bit_unlock()
+
+RMW atomic operations with return value:
+
+  test_and_{set,clear,change}_bit()
+  test_and_set_bit_lock()
+
+Barriers:
+
+  smp_mb__{before,after}_atomic()
+
+
+All RMW atomic operations have a '__' prefixed variant which is non-atomic.
+
+
+SEMANTICS
+---------
+
+Non-atomic ops:
+
+In particular __clear_bit_unlock() suffers the same issue as atomic_set(),
+which is why the generic version maps to clear_bit_unlock(), see atomic_t.txt.
+
+
+RMW ops:
+
+The test_and_{}_bit() operations return the original value of the bit.
+
+
+ORDERING
+--------
+
+Like with atomic_t, the rule of thumb is:
+
+ - non-RMW operations are unordered;
+
+ - RMW operations that have no return value are unordered;
+
+ - RMW operations that have a return value are fully ordered.
+
+ - RMW operations that are conditional are fully ordered.
+
+Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics,
+clear_bit_unlock() which has RELEASE semantics and test_bit_acquire which has
+ACQUIRE semantics.
+
+Since a platform only has a single means of achieving atomic operations
+the same barriers as for atomic_t are used, see atomic_t.txt.
+