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

[Mauro Carvalho Chehab]

Em Mon, 23 Nov 2015 21:56:50 +0200
Laurent Pinchart  escreveu:

> 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 
> > 
> > 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 = >pads[source_pad];
> > backlink->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.

I tried that. This was actually one of my pans. However, it is not as
trivial as it seems. I explored several different alternatives:

1) we could have two lists at struct media_entity, one for links and another
one for backlinks:

struct media_link {
...
struct list_head list;
...
};

struct media_entity {
...
struct list_head links;
struct list_head backlinks;
...
};

And add the link to either one of the list. However, in such case,
container_of() would not work, because the offset of the data at the 
struct media_entity will be different, if the link is at the links
or backlinks list.

2) to have 2 lists also at struct media_link, one for the links and
another one for the backlinks, but this will require non trivial
changes at the graph traversal logic.

I can foresee some other alternatives that I didn't try yet:

- to use a separate data struct for the backlinks, like:
struct media_obj_group {
struct *media_object;
struct list_head list;
}
  and use this to store backlinks. Again, would require nontrivial
  changes at the graph traversal logic and would end by spending
  some extra memory for the backlinks;

- using the same structs as already defined, patching all routines
  that use links to add the capability for them to identify if the
  link is actually a backlink. This would be non-trivial and may
  be messy.

So, while I would love to get rid of backlinks, fixing it is not
trivial at all.

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

[Laurent Pinchart]

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 
> 
> 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 = >pads[source_pad];
>   backlink->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

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

[Hans Verkuil]

On 09/06/2015 07:30 PM, 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 

Acked-by: Hans Verkuil 

> 
> 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 = >pads[source_pad];
>   backlink->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;
>  };
>  
>  /**
> 

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

[Mauro Carvalho Chehab]

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 

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 = >pads[source_pad];
backlink->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;
 };
 
 /**
-- 
2.4.3


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