Hi Vladimir,

On Saturday, 13 October 2018 18:10:25 EEST Vladimir Zapolskiy wrote:

On 10/12/2018 04:01 PM, Laurent Pinchart wrote:

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On Friday, 12 October 2018 14:47:52 EEST Kieran Bingham wrote:

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On 12/10/18 11:58, Vladimir Zapolskiy wrote:

[snip]

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Essentially they are multi purpose buses - which do not yet have a home.
We have used media as a home because of our use case.

The use case whether they transfer frames from a camera or to a display
are of course closely related, but ultimately covered by two separate
subsystems at the pixel level (DRM vs V4L, or other for other data)

Perhaps as they are buses - on a level with USB or I2C (except they can
of course carry I2C or Serial as well as 'bi-directional video' etc ),
they are looking for their own subsystem.

Except I don't think we don't want to add a new subsystem for just one
(or two) devices...

I'm not sure a new subsystem is needed. As you've noted there's an overlap
between drivers/media/ and drivers/gpu/drm/ in terms of supported
hardware. We even have a devices supported by two drivers, one in drivers/
media/ and one in drivers/gpu/drm/ (I'm thinking about the adv7511 in
particular). This is a well known issue, and so far nothing has been done
in mainline to try and solve it.

I agree that there's an overlap between drivers/media/ and drivers/gpu/drm/,
formally a hypothetical (sic!) DS90Ux9xx video bridge cell driver should be
added into both subsystems also, and the actual driver of two should be
selected in runtime. I call such a driver 'hypothetical', because in fact I
don't have it, and I'm not so sure that its existence is justified, but
that's only because DS90Ux9xx video bridge functionality is _transparent_,
it does not have any controls literally, but it is a pure luck eventually.

I don't think that's entirely correct, there's at least the video bus width 
(18-bit/24-bit) that needs to be selected. You currently do so through a 
pinctrl property, but that's not right.

So, as I've stated in my cover letter, I can misuse yours 'lvds-encoder'
driver only for the purpose of establishing a mediated link between
an LVDS controller and a panel over a serializer-deserializer pair.

quoted

Trying to find another home in drivers/mfd/ to escape from the problem
isn't a good solution in my opinion. The best option from a Linux point
of view would be to unify V4L2 and DRM/KMS when it comes to bridge
support, but that's a long way down the road (I won't complain if you
want to give it a go though> 
:-)).

I return you a wider smile :)

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As your use cases are display, focused, I would propose to start with
drivers/gpu/drm/bridge/, and leave the problem of camera support for first
person who will have such a use case.

Frankly speaking I would like to start from copy-pasting your 'lvds-encoder'
driver into an 'absolutely-transparent-video-bridge' driver with no LVDS or
'encoder' specifics, adding just a new compatible may suffice, if the
driver is renamed/redefined.

PS, I remember I owe you a reference OF snippet of data path to a panel.

-- 
Regards,

Laurent Pinchart