Re: [PATCH v4 12/17] drm/uAPI: Add "preferred color format" drm property as setting for userspace

[Pekka Paalanen]

On Tue, 29 Jun 2021 08:12:54 +0000
Simon Ser <contact@xxxxxxxxxxx> wrote:

> On Tuesday, June 22nd, 2021 at 09:15, Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> 
> > yes, I think this makes sense, even if it is a property that one can't
> > tell for sure what it does before hand.
> >
> > Using a pair of properties, preference and active, to ask for something
> > and then check what actually worked is good for reducing the
> > combinatorial explosion caused by needing to "atomic TEST_ONLY commit"
> > test different KMS configurations. Userspace has a better chance of
> > finding a configuration that is possible.
> >
> > OTOH, this has the problem than in UI one cannot tell the user in
> > advance which options are truly possible. Given that KMS properties are
> > rarely completely independent, and in this case known to depend on
> > several other KMS properties, I think it is good enough to know after
> > the fact.
> >
> > If a driver does not use what userspace prefers, there is no way to
> > understand why, or what else to change to make it happen. That problem
> > exists anyway, because TEST_ONLY commits do not give useful feedback
> > but only a yes/no.  
> 
> By submitting incremental atomic reqs with TEST_ONLY (i.e. only changing one
> property at a time), user-space can discover which property makes the atomic
> commit fail.

That works if the properties are independent of each other. Color
range, color format, bpc and more may all be interconnected,
allowing only certain combinations to work.

If all these properties have "auto" setting too, then it would be
possible to probe each property individually, but that still does not
tell which combinations are valid.

If you probe towards a certain configuration by setting the properties
one by one, then depending on the order you pick the properties, you
may come to a different conclusion on which property breaks the
configuration.

> I'm not a fan of this "preference" property approach. The only way to find out
> whether it's possible to change the color format is to perform a user-visible
> change (with a regular atomic commit) and check whether it worked
> after-the-fact. This is unlike all other existing KMS properties.

I agree. FWIW, "max bpc" exists already.

> I'd much rather see a more general approach to fix this combinatorial explosion
> than to add special-cases like this.

What would you suggest?

Maybe all properties should have an "auto" value in addition to the
explicit no-negotiation values where at all possible?

That might help probing each property individually with TEST_ONLY
commits, but it says nothing about combinations.

A feedback list perhaps? TEST_ONLY commit somehow returning a list of
property/value tuples indicating what value the "auto" valued
properties actually get?

What should a kernel driver optimize for when determining "auto" values?


Thanks,
pq

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