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

On Tuesday, June 22nd, 2021 at 09:15, Pekka Paalanen <ppaalanen@gmail.com> 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.

My mind crossed another point that must be considered: When plugin in a Monitor a list of possible Resolutions+Framerate combinations is created for xrandr and other userspace (I guess by atomic checks? but I don't know). During this drm properties are already considered, which is no problem atm because as far as i can tell there is currently no drm property that would make a certain Resolutions+Framerate combination unreachable that would be possible with everything on default.

However for example forcing YCbCr420 encoding would limit the available resolutions (my screen for example only supports YCbCr420 on 4k@60 and @50Hz and on no other resolution or frequency (native is 2560x1440@144Hz).

So would a "force color format" that does not get resetted on repluging/reenabling a monitor break the output, for example, of an not updated xrandr, unaware of this new property?

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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