[Intel-gfx] [PATCH 1/6] drm: Create Color Management DRM properties

Daniel Stone daniel at fooishbar.org
Mon Jan 11 12:37:09 PST 2016


On 5 January 2016 at 10:23, Daniel Vetter <daniel at ffwll.ch> wrote:
> On Wed, Dec 23, 2015 at 09:47:00AM +0000, Daniel Stone wrote:
>> It's not even a legacy vs. atomic thing, this can happen in
>> pure-atomic as well. Same as the render-compression plane property
>> that I just replied to here as well.
>> - Weston starts and sets up palette/CTM properties
>> - VT switch to Mutter, which isn't aware of new properties
>> - Mutter atomically sets new plane state, containing framebuffer which
>> is already colour-corrected for the target
>> - result is now double-corrected as Mutter didn't know to unset the
>> old properties
>> IOW, in the current model, any user of CM has to explicitly unset it
>> before handover (not always actually possible), or any generic
>> userspace must unset every property it sees and doesn't know about,
>> hoping that setting to 0 will do the right thing.
>> Or we could add another client cap, which would prevent the CM
>> properties from ever being exposed to userspace which doesn't know how
>> to work with it. Passing the client caps through to
>> plane_duplicate_state also means that we can unset the CM properties
>> at this early point for unaware clients. This would mean that we
>> wouldn't have to have code to explicitly unset it in, e.g., every
>> legacy hook.
> We don't have any support for unsetting anything really. Same argument you
> make for CM here applies to rotation too. The only generic solution (if
> you really care about this) would be for logind to once sample all atomic
> state on boot-up, and force-restore that state when switching. Restoring
> atomic state doesn't require userspace to understanding the semantics
> really.

Sure, but on the other hand, rotation has been around ~forever, and is
implemented in multiple places. Colour management is a lot less

> This kind of force-restoring is probably something we should implement in
> the fbdev emulation, which would cover about 99% of all cases where
> developers/users want to run different compositors. Or fbdev should simply
> reset CM state, like it does for rotation already (that part is easy to
> add, but indeed seems to be missing).
> Trying to create an ad-hoc solution (using opt-in flags) to this problem
> for every single feature seems pointless imo.

Fair enough, I guess it could be more difficult, so how about a new
flag to the atomic ioctl which requests state be _reset_ to scratch
rather than duplicated? That way, clients could be really sure they
weren't going to get screwed by rotation / colour management / render
compression / whatever.

>> Yeah, there is a single 'length' property per table, but given the
>> dizzying array of options available, I'm not really sure if that's
>> sufficient.
> The idea is to just give a hint to generic userspace. Fancy userspace that
> wants to exploit all the other options needs to have some baked-in
> knowledge of the hardware it runs on. This is kinda similar to how you can
> do dumb buffers, but for fancy ones userspace (at least currently) needs
> to just know how to allocate stuff. Imo there's just plain limits to how
> generic KMS can be. And it's not worth pushing beyond those.

I get the point, but am really struggling to make sense of the options
presented, and how a halfway-sensible generic userspace would work.
Exposing length/depth tuples might not be the worst idea; it also
sounds like there's more to dig out in terms of index interpolation as
well. But these are much smaller objections than having unwitting
clients inherit the CM state ...


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