Re: [PATCH xf86-video-amdgpu 3/5] Keep CRTC properties consistent

[Leo Li <sunpeng.li-5C7GfCeVMHo@public.gmane.org>]

On 2018-04-12 06:30 AM, Michel Dänzer wrote:
> 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
> https://cgit.freedesktop.org/xorg/xserver/commit/?id=b4e46c0444bb09f4af59d9d13acc939a0fbbc6d6
> 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)
> 

Fair enough, it's better to have the frequent redshift+monitor adjust
use-case working now instead of pushing an entirely new API.

> 
>> 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.
> 
> 

I'll take another look at RRChangeOutputProperty. Seems I missed the
'length' argument when I first went through it.

Once the blobs are gone, the other issues should be much simpler to
solve. I'll see if I can come up with some v2's.

A side question: How does xrandr display lengthy properties, like LUTs?
Can users set it via --set still?

Leo


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