Re: [PATCH v3 26/31] adv748x: csi2: add internal routing configuration

[Luca Ceresoli <luca@lucaceresoli.net>]

Hi,

thanks for the follow-up.

On 20/03/19 18:14, Jacopo Mondi wrote:
>>>> This is probably the wrong patch to use an example, as this one is for
>>>> a multiplexed interface, where there is no need to go through an
>>>> s_stream() for the two CSI-2 endpoints, but as you pointed out in our
>>>> brief offline chat, the AFE->TX routing example for this very device
>>>> is a good one: if we change the analogue source that is internally
>>>> routed to the CSI-2 output of the adv748x, do we need to s_stream(1)
>>>> the now routed entity and s_stream(0) on the not not-anymore-routed
>>>> one?
>>>>
>>>> My gut feeling is that this is up to userspace, as it should know
>>>> what are the requirements of the devices in the system, but this mean
>>>> going through an s_stream(0)/s_stream(1) sequence on the video device,
>>>> and that would interrupt the streaming for sure.
>>>>
>>>> At the same time, I don't feel too much at ease with the idea of
>>>> s_routing calling s_stream on the entity' remote subdevices, as this
>>>> would skip the link format validation that media_pipeline_start()
>>>> performs.
>>>
>>> The link validation must be done in this case as well, it may not be
>>> simply skipped.
>>
>> Agreed.
>>
>> The routing VS pipeline validation point is a very important one. The
>> current proposed workflow is:
>>
>>  1. the pipeline is validated as a whole, having knowledge of all the
>>     entities
> 
> let me specify this to avoid confusions:
>      "all the entities -with an active route in the pipeline-"
> 
>>  2. streaming is started
>>  3. s_routing is called on an entity (not on the pipeline!)
>>
>> Now the s_routing function in the entity driver is not in a good
>> position to validate the candidate future pipeline as a whole.
>>
>> Naively I'd say there are two possible solutions:
>>
>>  1. the s_routing reaches the pipeline first, then the new pipeline is
>>     computed and verified, and if verification succeeds it is applied
>>  2. a partial pipeline verification mechanism is added, so the entity
>>     receiving a s_routing request to e.g. change the sink pad can invoke
>>     a verification on the part of pipeline that is about to be
>>     activated, and if verification succeeds it is applied
>>
>> Somehow I suspect neither is trivial...
> 
> I would say it is not, but if you have such a device that does not
> require going through a s_stream(0)/s_stream(1) cycle and all the
> associated re-negotiation and validations, it seems to me nothing
> prevents you from handling this in the driver implementation. Maybe it
> won't look that great, but this seems to be quite a custom design that
> requires all input sources to be linked to your sink pads, their
> format validated all at the same time, power, stream activation and
> internal mux configuration controlled by s_routing. Am I wrong or
> nothing in this series would prevent your from doing this?

You're right, nothing prevents me from doing a custom hack for my custom
design. It's what I'm doing right now. My concern is whether the
framework will evolve to allow modifying a running pipeline without
custom hacks.

> tl;dr: I would not make this something the framework should be
> concerned about, as there's nothing preventing you from
> implementing support for such a use case. But again, without a real
> example we can only guess, and I might be overlooking the issue or
> missing some relevant detail for sure.

I'm a bit surprised in observing that my use case looks so strange,
perhaps yours is so different that we don't quite understand each other.
So below is an example of what I have in mind. Can you explain your use
case too?


Here's a use case. Consider a product that takes 3 camera inputs,
selects one of them and produces a continuous video stream with the
camera image and an OSD on top of it. The selected camera can be changed
at any time (e.g. upon user selection).

                  OSD FB ---.
                            |
            .--------.      V
Camera 0 -->|        |   .-----.
Camera 1 -->|  MUX   |-->| OSD |--> DMA --> /dev/video0
Camera 2 -->|        |   `-----'
            `--------'

A prerequisite is obviously that each piece of hardware and software
involved is able to cope with a sudden stream change. Perhaps not that
common, but no rocket science.

It looks to me that each of these pieces can be modeled as an entity and
the s_routing API is a perfect fit for the mux block. Am I wrong?

Thanks,
-- 
Luca