Re: Fwd: [PATCH] drm/i915: Avoid vblank counter for gen9+

[Imre Deak <imre.deak@intel.com>]

On Fri, 2016-02-26 at 10:02 -0800, Rodrigo Vivi wrote:
> [...]
> Well, I have this tree:
> https://cgit.freedesktop.org/~vivijim/drm-intel/log/?h=rpm-domains-psr-vblank-counter-full
> with mainly:
> 1 - vblank domain on pre-enable post-disable vblanks hooks as Ville
> had suggested
> 2 - psr domain so we just enable dc state when screen is really in
> idle.
> 3 - restore counter on vblank enable.
> 
> From what I understood so far of this problem, only the patch 1
> should
> be enough, but with only this one I don't get the screen frozen but
> the typying is so slow that is visible that we have something
> wrong.... Maybe dc state transition with mutexes there are slow?

I'm not aware of any big latencies caused by toggling DC states alone.

> Patch 2 by iitself also doesn't solve this and I still have frozen
> screens, but when combined to  patch 1 everything works really
> well...
> In the point that I believe we really don't need patch 3.

I think something like 1 and 3 is a good idea (and both are needed).
About 2, it's strange that you have to disable DC states when enabling
the panel. Since the pipe is active it should prevent DC5 (and hence
DC6). We wouldn't waste any power with your changes, since you re-
enable DC states before entering PSR, but imo we should find out why
exactly this is needed.

Some notes/ideas about the patches:
- intel_display_power_get() is called from page_flip_completed(), which
 is bad since we can be in interrupt context.
- There is a drm_crtc_vblank_get() in intel_crtc_page_flip(), but there
is no corresponding intel_display_power_get() for it.
- The same goes for the FBC, CRC code, couldn't you just call the new
vblank hooks from drm_vblank_get/put()?
- PIPE_FLIPCOUNT_G4X is also read-only, so it could get corrupted the
same way as the frame counter register. page_flip_finished() depends on
PIPE_FLIPCOUNT_G4X, so isn't this a problem?

I haven't checked this in detail, but it could be that we need to exit
PSR explicitly when waiting for a vblank or doing a flip. In PSR mode
the pipe may not be running, so I'm not sure how the vblank and flip
interrupts would be delivered.

--Imre
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