Re: [PATCH 04/18] [media] media-device: supress backlinks at G_TOPOLOGY ioctl

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Mauro,

Thank you for the patch.

On Sunday 06 September 2015 14:30:47 Mauro Carvalho Chehab wrote:
> Due to the graph traversal algorithm currently in usage, we
> need a copy of all data links. Those backlinks should not be
> send to userspace, as otherwise, all links there will be
> duplicated.
> 
> Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
> 
> diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
> index 0238885fcc74..97eb97d9b662 100644
> --- a/drivers/media/media-device.c
> +++ b/drivers/media/media-device.c
> @@ -333,6 +333,9 @@ static long __media_device_get_topology(struct
> media_device *mdev, /* Get links and number of links */
>  	i = 0;
>  	media_device_for_each_link(link, mdev) {
> +		if (link->is_backlink)
> +			continue;
> +
>  		i++;
> 
>  		if (ret || !topo->links)
> diff --git a/drivers/media/media-entity.c b/drivers/media/media-entity.c
> index cd4d767644df..4868b8269204 100644
> --- a/drivers/media/media-entity.c
> +++ b/drivers/media/media-entity.c
> @@ -648,6 +648,7 @@ media_create_pad_link(struct media_entity *source, u16
> source_pad, backlink->source = &source->pads[source_pad];
>  	backlink->sink = &sink->pads[sink_pad];
>  	backlink->flags = flags;
> +	backlink->is_backlink = true;
> 
>  	/* Initialize graph object embedded at the new link */
>  	media_gobj_init(sink->graph_obj.mdev, MEDIA_GRAPH_LINK,
> diff --git a/include/media/media-entity.h b/include/media/media-entity.h
> index e1a89899deef..3d389f142a1d 100644
> --- a/include/media/media-entity.h
> +++ b/include/media/media-entity.h
> @@ -96,6 +96,7 @@ struct media_pipeline {
>   * @reverse:	Pointer to the link for the reverse direction of a pad to 
pad
>   *		link.
>   * @flags:	Link flags, as defined at uapi/media.h (MEDIA_LNK_FL_*)
> + * @is_backlink: Indicate if the link is a backlink.
>   */
>  struct media_link {
>  	struct media_gobj graph_obj;
> @@ -112,6 +113,7 @@ struct media_link {
>  	};
>  	struct media_link *reverse;
>  	unsigned long flags;
> +	bool is_backlink;

I agree with the purpose of this patch (and as stated for other patches in the 
series I believe you should squash it with the patch that introduces the 
G_TOPOLOGY ioctl) but I won't whether you couldn't do with the additional 
variable by adding a flag for backlinks (flag that wouldn't be shown to 
userspace).

Now that I think about it an even better implementation could be to avoid 
creating backlinks at all. As links are now dynamically allocated you could 
have two struct list_head in the link structure, one for the source and one 
for the sink. It sounds too easy to be true, I wonder if I'm overlooking 
something.

>  };
> 
>  /**

-- 
Regards,

Laurent Pinchart