drm/{i915,xe}: driver agnostic drm to display pointer chase

The display driver needs to get from the struct drm_device pointer to
the struct intel_display pointer. Currently, this depends on knowledge
of the struct drm_i915_private and struct xe_device definitions, but
we'd like to hide those definitions from display.

Require the struct drm_device and struct intel_display * members within
struct drm_i915_private and struct xe_device to be placed next to each
other, to be able to figure out the display pointer without knowledge of
the structures.

Use a generic dummy device structure to define the relative offsets of
the drm and display members, and add static assertions to ensure this
holds for both i915 and xe. Use the dummy structure to do the pointer
chase from struct drm_device * to struct intel_display *.

This requires moving the display member in struct xe_device after the
drm member.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Suggested-by: Simona Vetter <simona.vetter@ffwll.ch>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250926111032.1188876-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

1 files changed, 42 insertions, 0 deletions

diff --git a/include/drm/intel/display_member.h b/include/drm/intel/display_member.h
new file mode 100644
index 000000000000..0319ea560b60
--- /dev/null
+++ b/include/drm/intel/display_member.h
@@ -0,0 +1,42 @@
+/* SPDX-License-Identifier: MIT */
+/* Copyright © 2025 Intel Corporation */
+
+#ifndef __DRM_INTEL_DISPLAY_H__
+#define __DRM_INTEL_DISPLAY_H__
+
+#include <linux/build_bug.h>
+#include <linux/stddef.h>
+#include <linux/stringify.h>
+
+#include <drm/drm_device.h>
+
+struct intel_display;
+
+/*
+ * A dummy device struct to define the relative offsets of drm and display
+ * members. With the members identically placed in struct drm_i915_private and
+ * struct xe_device, this allows figuring out the struct intel_display pointer
+ * without the definition of either driver specific structure.
+ */
+struct __intel_generic_device {
+	struct drm_device drm;
+	struct intel_display *display;
+};
+
+/**
+ * INTEL_DISPLAY_MEMBER_STATIC_ASSERT() - ensure correct placing of drm and display members
+ * @type: The struct to check
+ * @drm_member: Name of the struct drm_device member
+ * @display_member: Name of the struct intel_display * member.
+ *
+ * Use this static assert macro to ensure the struct drm_i915_private and struct
+ * xe_device struct drm_device and struct intel_display * members are at the
+ * same relative offsets.
+ */
+#define INTEL_DISPLAY_MEMBER_STATIC_ASSERT(type, drm_member, display_member) \
+	static_assert( \
+		offsetof(struct __intel_generic_device, display) - offsetof(struct __intel_generic_device, drm) == \
+		offsetof(type, display_member) - offsetof(type, drm_member), \
+		__stringify(type) " " __stringify(drm_member) " and " __stringify(display_member) " members at invalid offsets")
+
+#endif