Linux内核4.14版本——drm框架分析(14)——Atomic KMS 架构(struct drm_atomic_state)

风雨兼程80232023-09-13 23:25

目录

[1. drm_atomic_state_alloc创建drm_atomic_state](#1. drm_atomic_state_alloc创建drm_atomic_state)

[1.1 drm_atomic_state_init](#1.1 drm_atomic_state_init)

[2. 各个drm object对应的state](#2. 各个drm object对应的state)

[2.1 drm_atomic_get_crtc_state](#2.1 drm_atomic_get_crtc_state)

[2.2 drm_atomic_get_plane_state](#2.2 drm_atomic_get_plane_state)

[2.3 drm_atomic_get_connector_state](#2.3 drm_atomic_get_connector_state)

[2.4 struct _drm{object}_state](#2.4 struct _drm{object}_state)

我们从前面两篇文章可以看到，无论是drm_mode_setcrtc和drm_mode_setplane最终都会调用drm_atomic_commit，其参数为struct drm_atomic_state。我们来分析一下。

Atomic Mode Setting（后续简称A-KMS）。该架构会弥补之前API的不足，由于原先的API不支持同时更新整个DRM显示pipeline的状态，因此KMS过程中会出现一些中间状态，容易造成开发者不希望看见的结果，影响用户体验。同时，原先的KMS接口也不支持回滚，需要应用程序自己记录原先的配置状态，Atomic Mode Setting也解决了这个问题。

**Atomic commit 表示：本次 commit 操作，要么成功，要么保持原来的状态不变。即如果中途操作失败了，那些已经生效的配置需要恢复成之前的状态，就像没发生过 commit 操作似的。**而Commit，则是因为本次操作可能会修改到多个参数，等修改好这些参数后，再一次性发起操作请求，有点类似与填表后"提交"材料的意思

Atomic Mode Setting接口在用户态看来，是将原先各个KMS object的状态由隐式的通过API更新，变成了显式的对象属性。用户态程序可以通过通用的属性操作接口读写KMS object上的属性，更改不会立即生效，而是缓存起来。当应用程序更新完其所有想要更新的属性时，可以通过Commit操作告知要求KMS层真正的更新硬件的状态。此时驱动程序需要验证应用程序要求进行的修改是否合法，在合法的情况下，可以一次性完成整个显示状态的修改。A-KMS也实现了只用于检查新状态是否合法的接口。

KMS框架提供了一套helper函数以帮助驱动程序作者实现原先的Legacy KMS接口，本质上，就是原先的legacy相关的接口都通过A-KMS兼容层实现的helper函数实现，实质上就是使用带有drm_atomic_helper前缀的helper函数实现原有的legacy接口。

用户态相关接口：

cpp 复制代码 
typedef struct _drmModeAtomicReq drmModeAtomicReq, *drmModeAtomicReqPtr;
 
extern drmModeAtomicReqPtr drmModeAtomicAlloc(void);
extern drmModeAtomicReqPtr drmModeAtomicDuplicate(drmModeAtomicReqPtr req);
extern int drmModeAtomicMerge(drmModeAtomicReqPtr base,
                              drmModeAtomicReqPtr augment);
extern void drmModeAtomicFree(drmModeAtomicReqPtr req);
extern int drmModeAtomicGetCursor(drmModeAtomicReqPtr req);
extern void drmModeAtomicSetCursor(drmModeAtomicReqPtr req, int cursor);
extern int drmModeAtomicAddProperty(drmModeAtomicReqPtr req,
                                    uint32_t object_id,
                                    uint32_t property_id,
                                    uint64_t value);
extern int drmModeAtomicCommit(int fd,
                               drmModeAtomicReqPtr req,
                               uint32_t flags,
                               void *user_data);

用户态接口的本质就是扩展原有的属性接口，允许用户描述一个状态集合，然后通过drmModeAtomicCommit函数进行commit操做。从libdrm的代码中可以看到，commit的操作最后实质上调用了DRM_IOCTL_MODE_ATOMIC的ioctl，这就是Native接口唯一的入口。从内核代码中可以看到，该ioctl的处理函数为drm_mode_atomic_ioctl。以drm_mode_atomic_ioctl为线索，可以发现许多相关的实现。A-KMS的核心是整个显示控制器的状态集合，由一个独立的状态对象表示。一个DRM显示pipeline的整体状态由struct drm_atomic_state表示。

cpp 复制代码 
​
struct drm_atomic_state {
	struct kref ref;
 
	struct drm_device *dev;
	bool allow_modeset : 1;
	bool legacy_cursor_update : 1;
	bool async_update : 1;
	struct __drm_planes_state *planes;
	struct __drm_crtcs_state *crtcs;
	int num_connector;
	struct __drm_connnectors_state *connectors;
	int num_private_objs;
	struct __drm_private_objs_state *private_objs;
 
	struct drm_modeset_acquire_ctx *acquire_ctx;
 
    struct work_struct commit_work;
};

drm_atomic_state和其他组件state的继承关系如下图：

1. drm_atomic_state_alloc创建drm_atomic_state

cpp 复制代码 
struct drm_atomic_state *
drm_atomic_state_alloc(struct drm_device *dev)
{
	struct drm_mode_config *config = &dev->mode_config;

	if (!config->funcs->atomic_state_alloc) {
		struct drm_atomic_state *state;

		state = kzalloc(sizeof(*state), GFP_KERNEL);
		if (!state)
			return NULL;
		if (drm_atomic_state_init(dev, state) < 0) {
			kfree(state);
			return NULL;
		}
		return state;
	}

	return config->funcs->atomic_state_alloc(dev);
}

函数中可以看到，drm_mode_config_funcs中提供了名为atomic_state_alloc的hook，允许我们自己实现state对象的创建。在默认情况下，函数会调用简单分配内存，然后使用drm_atomic_state_init进行初始化。

1.1 drm_atomic_state_init

cpp 复制代码 
int
drm_atomic_state_init(struct drm_device *dev, struct drm_atomic_state *state)
{
	kref_init(&state->ref);

	/* TODO legacy paths should maybe do a better job about
	 * setting this appropriately?
	 */
	state->allow_modeset = true;

	state->crtcs = kcalloc(dev->mode_config.num_crtc,
			       sizeof(*state->crtcs), GFP_KERNEL);
	if (!state->crtcs)
		goto fail;
	state->planes = kcalloc(dev->mode_config.num_total_plane,
				sizeof(*state->planes), GFP_KERNEL);
	if (!state->planes)
		goto fail;

	state->dev = dev;

	DRM_DEBUG_ATOMIC("Allocated atomic state %p\n", state);

	return 0;
fail:
	drm_atomic_state_default_release(state);
	return -ENOMEM;
}

初始化函数仅仅是简单分配分配drm_atomic_state中几个指针指向的内存区域。

2. 各个drm object对应的state

对于各个drm object对应的state，其创建操作由其对应的drm_{object}funcs->atomic_duplicate_state实现，在驱动程序没有扩展drm_atomic_state的情况下，这个回调函数一般填写为drm_atomic_helper{object}_duplicate_state。

2.1 drm_atomic_get_crtc_state

cpp 复制代码 
struct drm_crtc_state *
drm_atomic_get_crtc_state(struct drm_atomic_state *state,
			  struct drm_crtc *crtc)
{
	int ret, index = drm_crtc_index(crtc);
	struct drm_crtc_state *crtc_state;

	WARN_ON(!state->acquire_ctx);

	crtc_state = drm_atomic_get_existing_crtc_state(state, crtc);
	if (crtc_state)
		return crtc_state;

	ret = drm_modeset_lock(&crtc->mutex, state->acquire_ctx);
	if (ret)
		return ERR_PTR(ret);

	crtc_state = crtc->funcs->atomic_duplicate_state(crtc);
	if (!crtc_state)
		return ERR_PTR(-ENOMEM);

	state->crtcs[index].state = crtc_state;
	state->crtcs[index].old_state = crtc->state;
	state->crtcs[index].new_state = crtc_state;
	state->crtcs[index].ptr = crtc;
	crtc_state->state = state;

	DRM_DEBUG_ATOMIC("Added [CRTC:%d:%s] %p state to %p\n",
			 crtc->base.id, crtc->name, crtc_state, state);

	return crtc_state;
}

drm_atomic_get_crtc_state--> crtc_state = crtc->funcs->atomic_duplicate_state(crtc);

2.2 drm_atomic_get_plane_state

cpp 复制代码 
struct drm_plane_state *
drm_atomic_get_plane_state(struct drm_atomic_state *state,
			  struct drm_plane *plane)
{
	int ret, index = drm_plane_index(plane);
	struct drm_plane_state *plane_state;

	WARN_ON(!state->acquire_ctx);

	plane_state = drm_atomic_get_existing_plane_state(state, plane);
	if (plane_state)
		return plane_state;

	ret = drm_modeset_lock(&plane->mutex, state->acquire_ctx);
	if (ret)
		return ERR_PTR(ret);

	plane_state = plane->funcs->atomic_duplicate_state(plane);
	if (!plane_state)
		return ERR_PTR(-ENOMEM);

	state->planes[index].state = plane_state;
	state->planes[index].ptr = plane;
	state->planes[index].old_state = plane->state;
	state->planes[index].new_state = plane_state;
	plane_state->state = state;

	DRM_DEBUG_ATOMIC("Added [PLANE:%d:%s] %p state to %p\n",
			 plane->base.id, plane->name, plane_state, state);

	if (plane_state->crtc) {
		struct drm_crtc_state *crtc_state;

		crtc_state = drm_atomic_get_crtc_state(state,
						       plane_state->crtc);
		if (IS_ERR(crtc_state))
			return ERR_CAST(crtc_state);
	}

	return plane_state;
}

drm_atomic_get_plane_state--> plane_state = plane->funcs->atomic_duplicate_state(plane);

2.3 drm_atomic_get_connector_state

cpp 复制代码 
struct drm_connector_state *
drm_atomic_get_connector_state(struct drm_atomic_state *state,
			  struct drm_connector *connector)
{
	int ret, index;
	struct drm_mode_config *config = &connector->dev->mode_config;
	struct drm_connector_state *connector_state;

	WARN_ON(!state->acquire_ctx);

	ret = drm_modeset_lock(&config->connection_mutex, state->acquire_ctx);
	if (ret)
		return ERR_PTR(ret);

	index = drm_connector_index(connector);

	if (index >= state->num_connector) {
		struct __drm_connnectors_state *c;
		int alloc = max(index + 1, config->num_connector);

		c = krealloc(state->connectors, alloc * sizeof(*state->connectors), GFP_KERNEL);
		if (!c)
			return ERR_PTR(-ENOMEM);

		state->connectors = c;
		memset(&state->connectors[state->num_connector], 0,
		       sizeof(*state->connectors) * (alloc - state->num_connector));

		state->num_connector = alloc;
	}

	if (state->connectors[index].state)
		return state->connectors[index].state;

	connector_state = connector->funcs->atomic_duplicate_state(connector);
	if (!connector_state)
		return ERR_PTR(-ENOMEM);

	drm_connector_get(connector);
	state->connectors[index].state = connector_state;
	state->connectors[index].old_state = connector->state;
	state->connectors[index].new_state = connector_state;
	state->connectors[index].ptr = connector;
	connector_state->state = state;

	DRM_DEBUG_ATOMIC("Added [CONNECTOR:%d:%s] %p state to %p\n",
			 connector->base.id, connector->name,
			 connector_state, state);

	if (connector_state->crtc) {
		struct drm_crtc_state *crtc_state;

		crtc_state = drm_atomic_get_crtc_state(state,
						       connector_state->crtc);
		if (IS_ERR(crtc_state))
			return ERR_CAST(crtc_state);
	}

	return connector_state;
}

drm_atomic_get_connector_state--> connector_state = connector->funcs->atomic_duplicate_state(connector);

2.4 struct _drm{object}_state

该函数触发复制state操作后，还会将复制后的state及原本的state填入drm_atomic_state中对应的__drm_{object}_state中。

cpp 复制代码 
struct __drm_{object}_state {
        struct drm_{object} *ptr;
        struct drm_{object}_state *state, *old_state, *new_state;
    
        /* extra fields may exist */
};
cpp 复制代码 
	state->connectors[index].state = connector_state;
	state->connectors[index].old_state = connector->state;
	state->connectors[index].new_state = connector_state;
	state->connectors[index].ptr = connector;
	connector_state->state = state;

这里的old_state保存drm_{object}现有的state，而state及new_state就保存我们复制后的state。

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