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

[Andrzej Hajda <a.hajda@samsung.com>]

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?

Regards
Andrzej


> 
>  Tomi
> 
>