DRM Format Modifiers in v4l2

[Laurent Pinchart]

Hi Nicolas,

On Thursday, 31 August 2017 19:12:58 EEST Nicolas Dufresne wrote:
> Le jeudi 31 août 2017 à 17:28 +0300, Laurent Pinchart a écrit :
> >> e.g. if I have two devices which support MODIFIER_FOO, I could attempt
> >> to share a buffer between them which uses MODIFIER_FOO without
> >> necessarily knowing exactly what it is/does.
> > 
> > Userspace could certainly set modifiers blindly, but the point of
> > modifiers is to generate side effects benefitial to the use case at hand
> > (for instance by optimizing the memory access pattern). To use them
> > meaningfully userspace would need to have at least an idea of the side
> > effects they generate.
> 
> Generic userspace will basically pick some random combination.

In that case userspace could set no modifier at all by default (except in the 
case where unmodified formats are not supported by the hardware, but I don't 
expect that to be the most common case).

> To allow generically picking the optimal configuration we could indeed rely
> on the application knowledge, but we could also enhance the spec so that
> the order in the enumeration becomes meaningful.

I'm not sure how far we should go. I could imagine a system where the API 
would report capabilities for modifiers (e.g. this modifier lowers the 
bandwidth, this one enhances the quality, ...), but going in that direction, 
where do we stop ? In practice I expect userspace to know some information 
about the hardware, so I'd rather avoid over-engineering the API.

-- 
Regards,

Laurent Pinchart