[PATCH V7 11/12] Documentation: bridge: Add documentation for ps8622 DT properties

[Laurent Pinchart]

On Tuesday 23 September 2014 13:18:30 Andrzej Hajda wrote:
> On 09/23/2014 01:10 PM, Laurent Pinchart wrote:
> > On Tuesday 23 September 2014 12:02:45 Andrzej Hajda wrote:
> >> On 09/23/2014 11:30 AM, Tomi Valkeinen wrote:
> >>> On 23/09/14 09:21, Thierry Reding wrote:
> >>>>> Well, I can write almost any kind of bindings, and then evidently my
> >>>>> device would work. For me, on my board.
> >>>> 
> >>>> Well, that's the whole problem with DT. For many devices we only have a
> >>>> single setup to test against. And even when we have several they often
> >>>> are derived from each other. But the alternative would be to defer
> >>>> (possibly indefinitely) merging support for a device until a second,
> >>>> wildly different setup shows up. That's completely unreasonable and we
> >>>> need to start somewhere.
> >>> 
> >>> Yes, but in this case we know of existing boards that have complex
> >>> setups. It's not theoretical.
> >>> 
> >>> I'm not saying we should stop everything until we have a 100% solution
> >>> for the rare complex cases. But we should keep them in mind and, when
> >>> possible, solve problems in a way that will work for the complex cases
> >>> also.
> >>> 
> >>>>> I guess non-video devices haven't had need for those. I have had lots
> >>>>> of boards with video setup that cannot be represented with simple
> >>>>> phandles. I'm not sure if I have just been unlucky or what, but my
> >>>>> understand is that other people have encountered such boards also.
> >>>>> Usually the problems encountered there have been circumvented with
> >>>>> some hacky video driver for that specific board, or maybe a static
> >>>>> configuration handled by the boot loader.
> >>>> 
> >>>> I have yet to encounter such a setup. Can you point me at a DTS for one
> >>>> such setup? I do remember a couple of hypothetical cases being
> >>>> discussed at one time or another, but I haven't seen any actual DTS
> >>>> content where this was needed.
> >>> 
> >>> No, I can't point to them as they are not in the mainline (at least the
> >>> ones I've been working on), for obvious reasons.
> >>> 
> >>> With a quick glance, I have the following devices in my cabinet that
> >>> have more complex setups: OMAP 4430 SDP, BeagleBoneBlack + LCD, AM43xx
> >>> EVM. Many Nokia devices used to have such setups, usually so that the
> >>> LCD and tv-out were connected to the same video source.
> >>> 
> >>>>> Do we have a standard way of representing the video pipeline with
> >>>>> simple phandles? Or does everyone just do their own version? If
> >>>>> there's no standard way, it sounds it'll be a mess to support in the
> >>>>> future.
> >>>> 
> >>>> It doesn't matter all that much whether the representation is standard.
> >>> 
> >>> Again, I disagree.
> >>> 
> >>>> phandles should simply point to the next element in the pipeline and
> >>>> the OS abstractions should be good enough to handle the details about
> >>>> how to chain the elements.
> >>> 
> >>> I, on the other hand, would rather see the links the other way around.
> >>> Panel having a link to the video source, etc.
> >>> 
> >>> The video graphs have two-way links, which of course is the safest
> >>> options, but also more verbose and redundant.
> >>> 
> >>> When this was discussed earlier, it was unclear which way the links
> >>> should be. It's true that only links to one direction are strictly
> >>> needed, but the question raised was that if in the drivers we end up
> >>> always going the links the other way, the performance penalty may be
> >>> somewhat big. (If I recall right).
> >> 
> >> I do not see why performance may drop significantly?
> >> If the link is one-way it should probably work as below:
> >> - the destination registers itself in some framework,
> >> - the source looks for the destination in this framework using phandle,
> >> - the source starts to communicate with the destination - since now full
> >> two way link can be established dynamically.
> >> 
> >> Where do you see here big performance penalty?
> > 
> > The performance-related problems arise when you need to locate the remote
> > device in the direction opposite to the phandle link direction. Traversing
> > a link forward just involves a phandle lookup, but traversing it
> > backwards isn't possible the same way.
> 
> But you do not need to traverse backwards. You just wait when the source
> start to communicate with the destination, at this moment destination can
> build back-link dynamically.

Your driver might not need it today for your use cases, but can you be certain 
that no driver on any OS would need to ?

This becomes an issue even on Linux when considering video-related devices 
that can be part of either a capture pipeline or a display pipeline. If the 
link always goes in the data flow direction, then it will be easy to locate 
the downstream device (bridge or panel) from the display controller driver, 
but it would be much more difficult to locate the same device from a camera 
driver as all of a sudden the device would become an upstream device.

-- 
Regards,

Laurent Pinchart