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/driver-api/driver-model/design-patterns.rst b/Documentation/driver-api/driver-model/design-patterns.rst
new file mode 100644
index 000000000..965b2b93b
--- /dev/null
+++ b/Documentation/driver-api/driver-model/design-patterns.rst
@@ -0,0 +1,116 @@
+=============================
+Device Driver Design Patterns
+=============================
+
+This document describes a few common design patterns found in device drivers.
+It is likely that subsystem maintainers will ask driver developers to
+conform to these design patterns.
+
+1. State Container
+2. container_of()
+
+
+1. State Container
+~~~~~~~~~~~~~~~~~~
+
+While the kernel contains a few device drivers that assume that they will
+only be probed() once on a certain system (singletons), it is custom to assume
+that the device the driver binds to will appear in several instances. This
+means that the probe() function and all callbacks need to be reentrant.
+
+The most common way to achieve this is to use the state container design
+pattern. It usually has this form::
+
+  struct foo {
+      spinlock_t lock; /* Example member */
+      (...)
+  };
+
+  static int foo_probe(...)
+  {
+      struct foo *foo;
+
+      foo = devm_kzalloc(dev, sizeof(*foo), GFP_KERNEL);
+      if (!foo)
+          return -ENOMEM;
+      spin_lock_init(&foo->lock);
+      (...)
+  }
+
+This will create an instance of struct foo in memory every time probe() is
+called. This is our state container for this instance of the device driver.
+Of course it is then necessary to always pass this instance of the
+state around to all functions that need access to the state and its members.
+
+For example, if the driver is registering an interrupt handler, you would
+pass around a pointer to struct foo like this::
+
+  static irqreturn_t foo_handler(int irq, void *arg)
+  {
+      struct foo *foo = arg;
+      (...)
+  }
+
+  static int foo_probe(...)
+  {
+      struct foo *foo;
+
+      (...)
+      ret = request_irq(irq, foo_handler, 0, "foo", foo);
+  }
+
+This way you always get a pointer back to the correct instance of foo in
+your interrupt handler.
+
+
+2. container_of()
+~~~~~~~~~~~~~~~~~
+
+Continuing on the above example we add an offloaded work::
+
+  struct foo {
+      spinlock_t lock;
+      struct workqueue_struct *wq;
+      struct work_struct offload;
+      (...)
+  };
+
+  static void foo_work(struct work_struct *work)
+  {
+      struct foo *foo = container_of(work, struct foo, offload);
+
+      (...)
+  }
+
+  static irqreturn_t foo_handler(int irq, void *arg)
+  {
+      struct foo *foo = arg;
+
+      queue_work(foo->wq, &foo->offload);
+      (...)
+  }
+
+  static int foo_probe(...)
+  {
+      struct foo *foo;
+
+      foo->wq = create_singlethread_workqueue("foo-wq");
+      INIT_WORK(&foo->offload, foo_work);
+      (...)
+  }
+
+The design pattern is the same for an hrtimer or something similar that will
+return a single argument which is a pointer to a struct member in the
+callback.
+
+container_of() is a macro defined in <linux/container_of.h>
+
+What container_of() does is to obtain a pointer to the containing struct from
+a pointer to a member by a simple subtraction using the offsetof() macro from
+standard C, which allows something similar to object oriented behaviours.
+Notice that the contained member must not be a pointer, but an actual member
+for this to work.
+
+We can see here that we avoid having global pointers to our struct foo *
+instance this way, while still keeping the number of parameters passed to the
+work function to a single pointer.