本文参考以下博文：

Linux内核4.14版本——drm框架分析(5)——plane分析

特此致谢！

1. 简介

一个plane代表一个image layer（硬件图层），最终的image由一个或者多个plane(s)组成。plane和 Framebuffer 一样是内存地址。plane主要包括以下3种类型：

• DRM_PLANE_TYPE_PRIMARY：主要图层，通常用于仅支持RGB格式的简单图层
• DRM_PLANE_TYPE_OVERLAY：叠加图层，通常用于YUV格式的视频图层
• DRM_PLANE_TYPE_CURSOR：光标图层，一般用于pc系统，用于显示鼠标

2. 核心结构

在Linux内核的DRM中，plane对应的核心结构体为：struct drm_plane。该结构体在include/drm/drm_plane.h中定义，代码如下（Linux内核版本：6.1）：

/**
 * struct drm_plane - central DRM plane control structure
 *
 * Planes represent the scanout hardware of a display block. They receive their
 * input data from a &drm_framebuffer and feed it to a &drm_crtc. Planes control
 * the color conversion, see `Plane Composition Properties`_ for more details,
 * and are also involved in the color conversion of input pixels, see `Color
 * Management Properties`_ for details on that.
 */
struct drm_plane {
	/** @dev: DRM device this plane belongs to */
	struct drm_device *dev;

	/**
	 * @head:
	 *
	 * List of all planes on @dev, linked from &drm_mode_config.plane_list.
	 * Invariant over the lifetime of @dev and therefore does not need
	 * locking.
	 */
	struct list_head head;

	/** @name: human readable name, can be overwritten by the driver */
	char *name;

	/**
	 * @mutex:
	 *
	 * Protects modeset plane state, together with the &drm_crtc.mutex of
	 * CRTC this plane is linked to (when active, getting activated or
	 * getting disabled).
	 *
	 * For atomic drivers specifically this protects @state.
	 */
	struct drm_modeset_lock mutex;

	/** @base: base mode object */
	struct drm_mode_object base;

	/**
	 * @possible_crtcs: pipes this plane can be bound to constructed from
	 * drm_crtc_mask()
	 */
	uint32_t possible_crtcs;
	/** @format_types: array of formats supported by this plane */
	uint32_t *format_types;
	/** @format_count: Size of the array pointed at by @format_types. */
	unsigned int format_count;
	/**
	 * @format_default: driver hasn't supplied supported formats for the
	 * plane. Used by the non-atomic driver compatibility wrapper only.
	 */
	bool format_default;

	/** @modifiers: array of modifiers supported by this plane */
	uint64_t *modifiers;
	/** @modifier_count: Size of the array pointed at by @modifier_count. */
	unsigned int modifier_count;

	/**
	 * @crtc:
	 *
	 * Currently bound CRTC, only meaningful for non-atomic drivers. For
	 * atomic drivers this is forced to be NULL, atomic drivers should
	 * instead check &drm_plane_state.crtc.
	 */
	struct drm_crtc *crtc;

	/**
	 * @fb:
	 *
	 * Currently bound framebuffer, only meaningful for non-atomic drivers.
	 * For atomic drivers this is forced to be NULL, atomic drivers should
	 * instead check &drm_plane_state.fb.
	 */
	struct drm_framebuffer *fb;

	/**
	 * @old_fb:
	 *
	 * Temporary tracking of the old fb while a modeset is ongoing. Only
	 * used by non-atomic drivers, forced to be NULL for atomic drivers.
	 */
	struct drm_framebuffer *old_fb;

	/** @funcs: plane control functions */
	const struct drm_plane_funcs *funcs;

	/** @properties: property tracking for this plane */
	struct drm_object_properties properties;

	/** @type: Type of plane, see &enum drm_plane_type for details. */
	enum drm_plane_type type;

	/**
	 * @index: Position inside the mode_config.list, can be used as an array
	 * index. It is invariant over the lifetime of the plane.
	 */
	unsigned index;

	/** @helper_private: mid-layer private data */
	const struct drm_plane_helper_funcs *helper_private;

	/**
	 * @state:
	 *
	 * Current atomic state for this plane.
	 *
	 * This is protected by @mutex. Note that nonblocking atomic commits
	 * access the current plane state without taking locks. Either by going
	 * through the &struct drm_atomic_state pointers, see
	 * for_each_oldnew_plane_in_state(), for_each_old_plane_in_state() and
	 * for_each_new_plane_in_state(). Or through careful ordering of atomic
	 * commit operations as implemented in the atomic helpers, see
	 * &struct drm_crtc_commit.
	 */
	struct drm_plane_state *state;

	/**
	 * @alpha_property:
	 * Optional alpha property for this plane. See
	 * drm_plane_create_alpha_property().
	 */
	struct drm_property *alpha_property;
	/**
	 * @zpos_property:
	 * Optional zpos property for this plane. See
	 * drm_plane_create_zpos_property().
	 */
	struct drm_property *zpos_property;
	/**
	 * @rotation_property:
	 * Optional rotation property for this plane. See
	 * drm_plane_create_rotation_property().
	 */
	struct drm_property *rotation_property;
	/**
	 * @blend_mode_property:
	 * Optional "pixel blend mode" enum property for this plane.
	 * Blend mode property represents the alpha blending equation selection,
	 * describing how the pixels from the current plane are composited with
	 * the background.
	 */
	struct drm_property *blend_mode_property;

	/**
	 * @color_encoding_property:
	 *
	 * Optional "COLOR_ENCODING" enum property for specifying
	 * color encoding for non RGB formats.
	 * See drm_plane_create_color_properties().
	 */
	struct drm_property *color_encoding_property;
	/**
	 * @color_range_property:
	 *
	 * Optional "COLOR_RANGE" enum property for specifying
	 * color range for non RGB formats.
	 * See drm_plane_create_color_properties().
	 */
	struct drm_property *color_range_property;

	/**
	 * @scaling_filter_property: property to apply a particular filter while
	 * scaling.
	 */
	struct drm_property *scaling_filter_property;
};

3. drm_plane结构释义

（0）总述

DRM全解析 —— plane详解（1）

struct drm_plane —— 核心的DRM plane控制结构。

planes表示显示块的扫描输出硬件。它们从drm_framebuffer中接收输入数据，并将其提供给drm_crtc。planes控制颜色转换（有关详细信息，请参见“平面合成属性”），并且还参与输入像素的颜色转换（请参见“颜色管理属性”以了解详细信息）。

作用如下图示：

（1）struct drm_device *dev

    /** @dev: DRM device this plane belongs to */
	struct drm_device *dev;

此plane所属的DRM设备。

（2）struct list_head head

    /**
	 * @head:
	 *
	 * List of all planes on @dev, linked from &drm_mode_config.plane_list.
	 * Invariant over the lifetime of @dev and therefore does not need
	 * locking.
	 */
	struct list_head head;

@dev上所有平面的列表，链接自&drm_mode_config.plane_List。在@dev的生命周期内保持不变，因此不需要锁定。

（3）char *name

    /** @name: human readable name, can be overwritten by the driver */
	char *name;

人类可读的名称（名字），可以被驱动程序覆盖。

下一篇文章继续深入释义drm_plane结构中其余成员。