[RFC] dpms handling on atomic drivers

[Rob Clark]

On Thu, Nov 6, 2014 at 6:29 PM, Daniel Vetter <daniel at ffwll.ch> wrote:
>>> Proposal. Add a new boolean ->active to the crtc state, which will track
>>> the dpms state of the crtc. Add a helper to the atomic core functions
>>> which will compute ->active from the state update according to the
>>> proposals for issue 1&2. Require that all callers of
>>> ->atomich_check/commit update ->active properly and require
>>> implementations to obey it. That means the atomic helpers will switch from
>>> looking at ->enable to looking at ->active to decided whether a crtc an
>>> all its outputs should be enabled or not.
>>
>> hmm.. then I'm starting to get a bit fuzzy on the distinction between
>> enabled and active..  I guess I should go back and look more closely
>> at the differentiation between 'enabled' and 'active' in current i915
>> helpers..
>>
>> Seems like we should be able to just repurpose enabled in the new
>> helpers, to not automatically be true when a mode is set, but instead
>> be true iff:
>>   1) >=1 attached connector is ON
>>   2) valid mode is set
>>   3) >=1 attached plane has an fb
>>
>> seems like the existing problem w/ enabled is just to do with how
>> current helpers use it.. but maybe I'm overlooking something..
>
> Well 3) is imo wrong, since black screen with no fbs is imo a valid
> state (hopefully only transitional). And the core/helper will ensure
> that 1&2 never get out of sync (you won't get an -EINVAL though since
> existing userspace does stupid stuff that violates this, the core
> setCrtc ioctl cleans that up though).

(hmm, I wonder whether that is even possible on all hw..)

>
> That aside I guess I need to elaborate on what makes dpms special in
> i915, and why there's a real difference between crtc->enable == true
> && ->active == false and crtc->enable == false in i915. For complex
> configs we do resource checking (shared dplls) and that's done in the
> modeset. For a pipe which has been disabled just with dpms we then
> guarantee that we'll keep these resources reserve and so will always
> be able to enable the pipe again. If you disable the pipe completely
> (i.e. set crtc->enable to false) we'll release these resources. E.g.
> in the i915 share dpll code we have both an active refcount (does the
> ppl need to be running) and a reference mask (which crtc is
> referencing this pll, even when the crtc is disabled with dpms).

ahh, ok, "reserved but not enabled" makes a lot more sense.. that was
the distinction that I was missing.  That probably deserves to be in
headerdoc somewhere..

BR,
-R