本系列文章基于linux 5.15

DRM驱动的显存由GEM（Graphics execution management）管理。

一、创建流程

创建buf时，user层提供需要buf的width,height以及bpp(bite per pixel)，然后调用drmIoctl(fd, DRM_IOCTL_MODE_CREATE_DUMB, &create)接口进入kernel层，kernel层根据这些信息分配出handle和pitch提供给user层使用。

二、kernel层调用流程

kernel层会调用drmIoctl(fd, DRM_IOCTL_MODE_CREATE_DUMB, &create)，然后通过DRM_IOCTL_DEF 宏进行映射到drm_mode_create_dumb_ioctl中。

/*DRM_IOCTL_DEF 宏用于简化 DRM 子系统中 IOCTL 命令的定义和映射。
它通过将 IOCTL 命令、处理函数和标志打包到一个 drm_ioctl_desc 结构体中，
方便内核开发者管理和扩展 DRM 的 IOCTL 接口。*/
DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_DUMB, drm_mode_create_dumb_ioctl, 0),

int drm_mode_create_dumb_ioctl(struct drm_device *dev,
			       void *data, struct drm_file *file_priv)
{
	return drm_mode_create_dumb(dev, data, file_priv);
}

2.drm_mode_create_dumb

drm_mode_create_dumb首先user层传来的参数width/height/bpp是否符合条件，然后调用dev->driver->dumb_create这一回调获取handle和pitch。

int drm_mode_create_dumb(struct drm_device *dev,
			 struct drm_mode_create_dumb *args,
			 struct drm_file *file_priv)
{
	u32 cpp, stride, size;
/*判别user层提供的width/height/bpp是否符合条件*/
	if (!dev->driver->dumb_create)
		return -ENOSYS;
	if (!args->width || !args->height || !args->bpp)
		return -EINVAL;

	/* overflow checks for 32bit size calculations */
	if (args->bpp > U32_MAX - 8)
		return -EINVAL;
	cpp = DIV_ROUND_UP(args->bpp, 8);
	if (cpp > U32_MAX / args->width)
		return -EINVAL;
	stride = cpp * args->width;
	if (args->height > U32_MAX / stride)
		return -EINVAL;

	/* test for wrap-around */
	size = args->height * stride;
	if (PAGE_ALIGN(size) == 0)
		return -EINVAL;

	args->handle = 0;
	args->pitch = 0;
	args->size = 0;

	return dev->driver->dumb_create(file_priv, dev, args);
}

dumb_create这一回调是各大厂商自己实现的，以msm厂商为例，会调用msm_gem_dumb_create这一函数.

static const struct drm_driver msm_driver = {
	.driver_features    = DRIVER_GEM |
				DRIVER_RENDER |
				DRIVER_ATOMIC |
				DRIVER_MODESET |
				DRIVER_SYNCOBJ,
	.open               = msm_open,
	.postclose           = msm_postclose,
	.lastclose          = drm_fb_helper_lastclose,
	
	.dumb_create        = msm_gem_dumb_create,
	
	.dumb_map_offset    = msm_gem_dumb_map_offset,
	.prime_handle_to_fd = drm_gem_prime_handle_to_fd,
	.prime_fd_to_handle = drm_gem_prime_fd_to_handle,
	.gem_prime_import_sg_table = msm_gem_prime_import_sg_table,
	.gem_prime_mmap     = drm_gem_prime_mmap,
#ifdef CONFIG_DEBUG_FS
	.debugfs_init       = msm_debugfs_init,
#endif
	.ioctls             = msm_ioctls,
	.num_ioctls         = ARRAY_SIZE(msm_ioctls),
	.fops               = &fops,
	.name               = "msm",
	.desc               = "MSM Snapdragon DRM",
	.date               = "20130625",
	.major              = MSM_VERSION_MAJOR,
	.minor              = MSM_VERSION_MINOR,
	.patchlevel         = MSM_VERSION_PATCHLEVEL,
};

int msm_gem_dumb_create(struct drm_file *file, struct drm_device *dev,
		struct drm_mode_create_dumb *args)
{
	args->pitch = align_pitch(args->width, args->bpp);
	args->size  = PAGE_ALIGN(args->pitch * args->height);
	return msm_gem_new_handle(dev, file, args->size,
			MSM_BO_SCANOUT | MSM_BO_WC, &args->handle, "dumb");
}

int msm_gem_new_handle(struct drm_device *dev, struct drm_file *file,
		uint32_t size, uint32_t flags, uint32_t *handle,
		char *name)
{
	struct drm_gem_object *obj;
	int ret;
	/*创建一个新的 GEM 对象*/
	obj = msm_gem_new(dev, size, flags);

	if (IS_ERR(obj))
		return PTR_ERR(obj);

	if (name)
		msm_gem_object_set_name(obj, "%s", name);
	/*为 GEM 对象创建句柄*/
	ret = drm_gem_handle_create(file, obj, handle);

	/* 减少对象的引用计数（句柄已经持有引用） */
	drm_gem_object_put(obj);

	return ret;
}

2.1drm_gem_handle_create

drm_gem_handle_create主要使用idr_alloc将drm_gem_object对象添加到file_priv->object_idr，并返回handle。

idr_alloc:是为了使用一个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可以找到gem_object。

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);
}

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;
}