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, 56 insertions, 0 deletions

diff --git a/Documentation/fb/cmap_xfbdev.rst b/Documentation/fb/cmap_xfbdev.rst
new file mode 100644
index 000000000..5db5e9787
--- /dev/null
+++ b/Documentation/fb/cmap_xfbdev.rst
@@ -0,0 +1,56 @@
+==========================
+Understanding fbdev's cmap
+==========================
+
+These notes explain how X's dix layer uses fbdev's cmap structures.
+
+-  example of relevant structures in fbdev as used for a 3-bit grayscale cmap::
+
+    struct fb_var_screeninfo {
+	    .bits_per_pixel = 8,
+	    .grayscale      = 1,
+	    .red =          { 4, 3, 0 },
+	    .green =        { 0, 0, 0 },
+	    .blue =         { 0, 0, 0 },
+    }
+    struct fb_fix_screeninfo {
+	    .visual =       FB_VISUAL_STATIC_PSEUDOCOLOR,
+    }
+    for (i = 0; i < 8; i++)
+	info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16;
+    memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8);
+    memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8);
+
+-  X11 apps do something like the following when trying to use grayscale::
+
+    for (i=0; i < 8; i++) {
+	char colorspec[64];
+	memset(colorspec,0,64);
+	sprintf(colorspec, "rgb:%x/%x/%x", i*36,i*36,i*36);
+	if (!XParseColor(outputDisplay, testColormap, colorspec, &wantedColor))
+		printf("Can't get color %s\n",colorspec);
+	XAllocColor(outputDisplay, testColormap, &wantedColor);
+	grays[i] = wantedColor;
+    }
+
+There's also named equivalents like gray1..x provided you have an rgb.txt.
+
+Somewhere in X's callchain, this results in a call to X code that handles the
+colormap. For example, Xfbdev hits the following:
+
+xc-011010/programs/Xserver/dix/colormap.c::
+
+  FindBestPixel(pentFirst, size, prgb, channel)
+
+  dr = (long) pent->co.local.red - prgb->red;
+  dg = (long) pent->co.local.green - prgb->green;
+  db = (long) pent->co.local.blue - prgb->blue;
+  sq = dr * dr;
+  UnsignedToBigNum (sq, &sum);
+  BigNumAdd (&sum, &temp, &sum);
+
+co.local.red are entries that were brought in through FBIOGETCMAP which come
+directly from the info->cmap.red that was listed above. The prgb is the rgb
+that the app wants to match to. The above code is doing what looks like a least
+squares matching function. That's why the cmap entries can't be set to the left
+hand side boundaries of a color range.