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, 106 insertions, 0 deletions

diff --git a/Documentation/trace/ring-buffer-map.rst b/Documentation/trace/ring-buffer-map.rst
new file mode 100644
index 000000000..8e296bcc0
--- /dev/null
+++ b/Documentation/trace/ring-buffer-map.rst
@@ -0,0 +1,106 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================================
+Tracefs ring-buffer memory mapping
+==================================
+
+:Author: Vincent Donnefort <vdonnefort@google.com>
+
+Overview
+========
+Tracefs ring-buffer memory map provides an efficient method to stream data
+as no memory copy is necessary. The application mapping the ring-buffer becomes
+then a consumer for that ring-buffer, in a similar fashion to trace_pipe.
+
+Memory mapping setup
+====================
+The mapping works with a mmap() of the trace_pipe_raw interface.
+
+The first system page of the mapping contains ring-buffer statistics and
+description. It is referred to as the meta-page. One of the most important
+fields of the meta-page is the reader. It contains the sub-buffer ID which can
+be safely read by the mapper (see ring-buffer-design.rst).
+
+The meta-page is followed by all the sub-buffers, ordered by ascending ID. It is
+therefore effortless to know where the reader starts in the mapping:
+
+.. code-block:: c
+
+        reader_id = meta->reader->id;
+        reader_offset = meta->meta_page_size + reader_id * meta->subbuf_size;
+
+When the application is done with the current reader, it can get a new one using
+the trace_pipe_raw ioctl() TRACE_MMAP_IOCTL_GET_READER. This ioctl also updates
+the meta-page fields.
+
+Limitations
+===========
+When a mapping is in place on a Tracefs ring-buffer, it is not possible to
+either resize it (either by increasing the entire size of the ring-buffer or
+each subbuf). It is also not possible to use snapshot and causes splice to copy
+the ring buffer data instead of using the copyless swap from the ring buffer.
+
+Concurrent readers (either another application mapping that ring-buffer or the
+kernel with trace_pipe) are allowed but not recommended. They will compete for
+the ring-buffer and the output is unpredictable, just like concurrent readers on
+trace_pipe would be.
+
+Example
+=======
+
+.. code-block:: c
+
+        #include <fcntl.h>
+        #include <stdio.h>
+        #include <stdlib.h>
+        #include <unistd.h>
+
+        #include <linux/trace_mmap.h>
+
+        #include <sys/mman.h>
+        #include <sys/ioctl.h>
+
+        #define TRACE_PIPE_RAW "/sys/kernel/tracing/per_cpu/cpu0/trace_pipe_raw"
+
+        int main(void)
+        {
+                int page_size = getpagesize(), fd, reader_id;
+                unsigned long meta_len, data_len;
+                struct trace_buffer_meta *meta;
+                void *map, *reader, *data;
+
+                fd = open(TRACE_PIPE_RAW, O_RDONLY | O_NONBLOCK);
+                if (fd < 0)
+                        exit(EXIT_FAILURE);
+
+                map = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
+                if (map == MAP_FAILED)
+                        exit(EXIT_FAILURE);
+
+                meta = (struct trace_buffer_meta *)map;
+                meta_len = meta->meta_page_size;
+
+                printf("entries:        %llu\n", meta->entries);
+                printf("overrun:        %llu\n", meta->overrun);
+                printf("read:           %llu\n", meta->read);
+                printf("nr_subbufs:     %u\n", meta->nr_subbufs);
+
+                data_len = meta->subbuf_size * meta->nr_subbufs;
+                data = mmap(NULL, data_len, PROT_READ, MAP_SHARED, fd, meta_len);
+                if (data == MAP_FAILED)
+                        exit(EXIT_FAILURE);
+
+                if (ioctl(fd, TRACE_MMAP_IOCTL_GET_READER) < 0)
+                        exit(EXIT_FAILURE);
+
+                reader_id = meta->reader.id;
+                reader = data + meta->subbuf_size * reader_id;
+
+                printf("Current reader address: %p\n", reader);
+
+                munmap(data, data_len);
+                munmap(meta, meta_len);
+                close (fd);
+
+                return 0;
+        }