On 2018-04-11 11:26 PM, Leo Li wrote:
> On 2018-04-11 04:39 AM, Michel Dänzer wrote:
>> Hmm. So either legacy or non-legacy clients won't work at all, or 
>> they'll step on each other's toes, clobbering the HW gamma LUT from
>> each other.
>> I'm afraid neither of those alternatives sound like a good user 
>> experience to me.
>> Consider on the one hand something like Night Light / redshift,
>> using legacy APIs to adjust colour temperature to the time of day.
>> On the other hand, another client using the non-legacy API for say
>> fine-tuning of a display's advanced gamma capabilities.
>> Ideally, in this case the gamma LUTs from the legacy and non-legacy
>> APIs should be combined, such that the hardware LUT reflects both
>> the colour temperature set by Night Light / refshift and the
>> fine-tuning set by the non-legacy client. Is that feasible? If not,
>> can you explain a little why?
> Hmm, combining LUTs could be feasible, but I don't think that's the 
> right approach.
> LUTs can be combined through composition h(x) = f(g(x)), with some 
> interpolation involved. Once combined, it can be set using the 
> non-legacy API, since that'll likely have a larger LUT size.
> But I think what you've mentioned raises a bigger issue of color 
> management conflicts in general. It doesn't have to be redshift vs 
> monitor correction, what if there more than 2 applications contending
> to set gamma? xrandr's brightness already conflicts with redshift,
> and so does some apps running on WINE.

What you're describing is an X11 design flaw, which we cannot do
anything about in the DDX driver.

What I want to avoid is repeating a similar situation as we had before
xserver 1.19, where one could have either working per-window colormaps
and global gamma, or per-CRTC gamma via RandR, not all at the same
time. (Before xserver 1.7, they would clobber the HW LUT from each
other) I fixed this in
by composing the LUTs.

> For the small example at hand, the ideal solution is to have
> redshift use the color transformation matrix (CTM), which will be
> exposed as part of the non-legacy color API. It'll need modification
> of redshift, but at least it won't conflict with anything gamma
> related.

From past experience, it can take a long time (up to forever) for some
clients to be updated like this. E.g. it's unlikely at this point that
libraries such as SDL1 will ever be updated for the new API, so I'm
afraid users would run into things like
https://bugs.freedesktop.org/show_bug.cgi?id=27222 again.

(Besides, it would conflict with anything else using the same API, as
you described above, so it's not even obviously the ideal solution)

> Jumping back on some patch 1 topics:
> Are there ways to get arbitrarily (no more than 4096 elements) sized 
> arrays from a client, to the DDX driver?

Seems like the RRChangeOutputProperty request could work.

> I agree, it would definitely be nicer for clients to not worry about
> DRM blobs at all.

Indeed. E.g. as a bonus, it would allow this to work even with a remote
display connection.

I'm holding off on the more detailed discussion about the other patches
until there is a plan for this fundamental issue.

Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer
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