As a quick reference I'll detail the motivation and design of the new code a
bit here (mostly stitched together from patchbomb announcements and commits
introducing the new concepts).

The crtc helper code has the fundamental assumption that encoders and crtcs can
be enabled/disabled in any order, as long as we take care of depencies (which
means that enabled encoders need an enabled crtc to feed them data,
essentially).

Our hw works differently. We already have tons of ugly cases where crtc code
enables encoder hw (or encoder->mode_set enables stuff that should only be
enabled in enocder->commit) to work around these issues. But on the disable
side we can't pull off similar tricks - there we actually need to rework the
modeset sequence that controls all this. And this is also the real motivation
why I've finally undertaken this rewrite: eDP on my shiny new Ivybridge
Ultrabook is broken, and it's broken due to the wrong disable sequence ...

The new code introduces a few interfaces and concepts:

- Add new encoder->enable/disable functions which are directly called from the
crtc->enable/disable function. This ensures that the encoder's can be
enabled/disabled at a very specific in the modeset sequence, controlled by our
platform specific code (instead of the crtc helper code calling them at a time
it deems convenient).

- Rework the dpms code - our code has mostly 1:1 connector:encoder mappings and
does support cloning on only a few encoders, so we can simplify things quite a
bit.

- Also only ever disable/enable the entire output pipeline. This ensures that
we obey the right sequence of enabling/disabling things, trying to be clever
here mostly just complicates the code and results in bugs. For cloneable
encoders this requires a bit of special handling to ensure that outputs can
still be disabled individually, but it simplifies the common case.

- Add infrastructure to read out the current hw state. No amount of careful
ordering will help us if we brick the hw on the initial modeset setup. Which
could happen if we just randomly disable things, oblivious to the state set up
by the bios. Hence we need to be able to read that out. As a benefit, we grow a
few generic functions useful to cross-check our modeset code with actual hw
state.

With all this in place, we can copy&paste the crtc helper code into the
drm/i915 driver and start to rework it:

- As detailed above, the new code only disables/enables an entire output pipe.
As a preparation for global mode-changes (e.g. reassigning shared resources) it
keeps track of which pipes need to be touched by a set of bitmasks.

- To ensure that we correctly disable the current display pipes, we need to
know the currently active connector/encoder/crtc linking. The old crtc helper
simply overwrote these links with the new setup, the new code stages the new
links in ->new_* pointers. Those get commited to the real linking pointers once
the old output configuration has been torn down, before the ->mode_set
callbacks are called.

- Finally the code adds tons of self-consistency checks by employing the new hw
state readout functions to cross-check the actual hw state with what the
datastructure think it should be. These checks are done both after every
modeset and after the hw state has been read out and sanitized at boot/resume
time. All these checks greatly helped in tracking down regressions and bugs in
the new code.

With this new basis, a lot of cleanups and improvements to the code are now
possible (besides the DP fixes that ultimately made me write this), but not yet
done:

- I think we should create struct intel_mode and use it as the adjusted mode
everywhere to store little pieces like needs_tvclock, pipe dithering values or
dp link parameters. That would still be a layering violation, but at least we
wouldn't need to recompute these kinds of things in intel_display.c. Especially
the port bpc computation needed for selecting the pipe bpc and dithering
settings in intel_display.c is rather gross.

- In a related rework we could implement ->mode_valid in terms of ->mode_fixup
in a generic way - I've hunted down too many bugs where ->mode_valid did the
right thing, but ->mode_fixup didn't. Or vice versa, resulting in funny bugs
for user-supplied modes.

- Ditch the idea to rework the hdp handling in the common crtc helper code and
just move things to i915.ko. Which would rid us of the ->detect crtc helper
dependencies.

- LVDS wire pair and pll enabling is all done in the crtc->mode_set function
currently. We should be able to move this to the crtc_enable callbacks (or in
the case of the LVDS wire pair enabling, into some encoder callback).

Last, but not least, this new code should also help in enabling a few neat
features: The hw state readout code prepares (but there are still big pieces
missing) for fastboot, i.e. avoiding the inital modeset at boot-up and just
taking over the configuration left behind by the bios. We also should be able
to extend the configuration checks in the beginning of the modeset sequence and
make better decisions about shared resources (which is the entire point behind
the atomic/global modeset ioctl).
Tested-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Ben Widawsky <ben@bwidawsk.net>
Tested-by: Damien Lespiau <damien.lespiau@intel.com>
Tested-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Acked-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Acked-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>