[alsa-devel] [PATCH 1/1] media: Entities with sink pads must have at least one enabled link

Laurent Pinchart laurent.pinchart at ideasonboard.com
Thu Nov 15 01:46:23 CET 2012

Hi Sakari,

On Wednesday 14 November 2012 23:13:45 Sakari Ailus wrote:
> On Wed, Nov 14, 2012 at 10:23:19AM +0100, Sylwester Nawrocki wrote:
> > On 11/13/2012 03:24 PM, Sakari Ailus wrote:
> > > Hi all,
> > > 
> > > Comments would be appreciated, either positive or negative. The omap3isp
> > > driver does the same check itself currently, but I think this is more
> > > generic than that.
> > > 
> > > Thanks.
> > > 
> > > On Fri, Oct 26, 2012 at 10:46:17PM +0300, Sakari Ailus wrote:
> > >> If an entity has sink pads, at least one of them must be connected to
> > >> another pad with an enabled link. If a driver with multiple sink pads
> > >> has more strict requirements the check should be done in the driver
> > >> itself.
> > >> 
> > >> Just requiring one sink pad is connected with an enabled link is enough
> > >> API-wise: entities with sink pads with only disabled links should not
> > >> be allowed to stream in the first place, but also in a different
> > >> operation mode a device might require only one of its pads connected
> > >> with an active link.
> > >> 
> > >> If an entity has an ability to function as a source entity another
> > >> logical entity connected to the aforementioned one should be used for
> > >> the purpose.
> > 
> > Why not leave it to individual drivers ? I'm not sure if it is a good idea
> > not to allow an entity with sink pads to be used as a source only. It
> > might be appropriate for most of the cases but likely not all. I'm
> > inclined not to add this requirement in the API. Just my opinion though.
> I'm just wondering what would be the use case for that.
> What comes closest is generating a test pattern, but even that should be a
> separate subdev: the test pattern can be enabled by enabling the link from
> the test pattern generator subdev.

That would force creating a separate subdev just to support test pattern 
generation, I'm not sure if I want to push for that. There might also be other 
use cases not related to V4L (thus the cross-posting).

> As it seems not everyone is outright happy about the idea of making this
> mandatory, then how about making it optional?
> I'd hate having a link validate function for each subdev e.g. in the OMAP 3
> ISP driver that just checks that its sink pad is actually connected with an
> enabled link. That'd be lots of mostly useless code.


> If this is done in the framework, the drivers will be spared from copying
> this code in a number of places. Which was why I originally wrote this
> patch. The alternative is to re-parse the whole graph in the driver which
> I'd also like to avoid.

I'd also prefer to avoid that *if*possible*, as we already walk the graph 
during link validation.

> One opion I can think of is to call link_validate op of struct
> media_entity_operations also for disabled links on entities that are
> connected through active links (on V4L2 to a video node right before
> streaming, for example).
> That'd make it easy to perform the check in the drivers.
> What do you think?

I think that would be a bit too complex. Drivers (or the V4L core) would need 
to gather data from multiple links in some state object to find out if the 
complete pipeline is valid or not.

Another option would be to set a flag somewhere to indicate whether the check 
should be performed by the media core or left to drivers. As different types 
of drivers might need different types of checks, I think I would prefer for 
now to walk the graph one more time in the OMAP3 ISP driver, as currently 
done, and revisit this issue when we will have a couple of drivers 
implementing pipeline validity checks. I'm just a bit uncomfortable adding 
core code for a feature that has a single user at the moment without a clear 
view regarding how it would scale.


Laurent Pinchart

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