DRM全解析 —— CREATE_DUMB（4）

本文参考以下博文：

特此致谢！

上一回讲解了Intel i915、AMD Raedon和AMDGPU三类显卡驱动中的dumb_create函数指针所指向的实际函数，最终聚焦到drm_gem_handle_create函数。本回就来讲解一下这个函数。

drm_gem_handle_create函数在drivers/gpu/drm/drm_gem.c中，代码如下：

cpp 复制代码

/**
 * drm_gem_handle_create - create a gem handle for an object
 * @file_priv: drm file-private structure to register the handle for
 * @obj: object to register
 * @handlep: pointer to return the created handle to the caller
 *
 * Create a handle for this object. This adds a handle reference to the object,
 * which includes a regular reference count. Callers will likely want to
 * dereference the object afterwards.
 *
 * Since this publishes @obj to userspace it must be fully set up by this point,
 * drivers must call this last in their buffer object creation callbacks.
 */
int drm_gem_handle_create(struct drm_file *file_priv,
			  struct drm_gem_object *obj,
			  u32 *handlep)
{
	mutex_lock(&obj->dev->object_name_lock);

	return drm_gem_handle_create_tail(file_priv, obj, handlep);
}
EXPORT_SYMBOL(drm_gem_handle_create);

此函数比较简单，在加互斥锁后，把实际的工作交给了drm_gem_handle_create_tail函数。drm_gem_handle_create_tail函数就在它的上边，代码如下：

cpp 复制代码

/**
 * drm_gem_handle_create_tail - internal functions to create a handle
 * @file_priv: drm file-private structure to register the handle for
 * @obj: object to register
 * @handlep: pointer to return the created handle to the caller
 *
 * This expects the &drm_device.object_name_lock to be held already and will
 * drop it before returning. Used to avoid races in establishing new handles
 * when importing an object from either an flink name or a dma-buf.
 *
 * Handles must be release again through drm_gem_handle_delete(). This is done
 * when userspace closes @file_priv for all attached handles, or through the
 * GEM_CLOSE ioctl for individual handles.
 */
int
drm_gem_handle_create_tail(struct drm_file *file_priv,
			   struct drm_gem_object *obj,
			   u32 *handlep)
{
	struct drm_device *dev = obj->dev;
	u32 handle;
	int ret;

	WARN_ON(!mutex_is_locked(&dev->object_name_lock));
	if (obj->handle_count++ == 0)
		drm_gem_object_get(obj);

	/*
	 * Get the user-visible handle using idr.  Preload and perform
	 * allocation under our spinlock.
	 */
	idr_preload(GFP_KERNEL);
	spin_lock(&file_priv->table_lock);

	ret = idr_alloc(&file_priv->object_idr, obj, 1, 0, GFP_NOWAIT);

	spin_unlock(&file_priv->table_lock);
	idr_preload_end();

	mutex_unlock(&dev->object_name_lock);
	if (ret < 0)
		goto err_unref;

	handle = ret;

	ret = drm_vma_node_allow(&obj->vma_node, file_priv);
	if (ret)
		goto err_remove;

	if (obj->funcs->open) {
		ret = obj->funcs->open(obj, file_priv);
		if (ret)
			goto err_revoke;
	}

	*handlep = handle;
	return 0;

err_revoke:
	drm_vma_node_revoke(&obj->vma_node, file_priv);
err_remove:
	spin_lock(&file_priv->table_lock);
	idr_remove(&file_priv->object_idr, handle);
	spin_unlock(&file_priv->table_lock);
err_unref:
	drm_gem_object_handle_put_unlocked(obj);
	return ret;
}

drm_gem_handle_create_tail主要是使用idr_alloc将drm_gem_object对象添加到file_priv->object_idr，并返回handle。

这里顺带提一下idr_alloc函数，其在lib/idr.c中，代码如下：

cpp 复制代码

/**
 * idr_alloc() - Allocate an ID.
 * @idr: IDR handle.
 * @ptr: Pointer to be associated with the new ID.
 * @start: The minimum ID (inclusive).
 * @end: The maximum ID (exclusive).
 * @gfp: Memory allocation flags.
 *
 * Allocates an unused ID in the range specified by @start and @end.  If
 * @end is <= 0, it is treated as one larger than %INT_MAX.  This allows
 * callers to use @start + N as @end as long as N is within integer range.
 *
 * The caller should provide their own locking to ensure that two
 * concurrent modifications to the IDR are not possible.  Read-only
 * accesses to the IDR may be done under the RCU read lock or may
 * exclude simultaneous writers.
 *
 * Return: The newly allocated ID, -ENOMEM if memory allocation failed,
 * or -ENOSPC if no free IDs could be found.
 */
int idr_alloc(struct idr *idr, void *ptr, int start, int end, gfp_t gfp)
{
	u32 id = start;
	int ret;

	if (WARN_ON_ONCE(start < 0))
		return -EINVAL;

	ret = idr_alloc_u32(idr, ptr, &id, end > 0 ? end - 1 : INT_MAX, gfp);
	if (ret)
		return ret;

	return id;
}
EXPORT_SYMBOL_GPL(idr_alloc);

为了便于理解，再看一下调用代码片段：

cpp 复制代码

	ret = idr_alloc(&file_priv->object_idr, obj, 1, 0, GFP_NOWAIT);

idr_alloc函数的作用是使用一个id与一个obj绑定，这样就可以通过id找到对应obj。这里将handle与分配的gem_object进行绑定，后面通过handle就可以找到gem_object进而找到对应的物理地址或者虚拟地址了。

至此，整个CREATE_DUMB流程就基本分析完了。