Re: [RFC][PATCH 4/4] drm: i915: Atomic pageflip WIP

[Chris Wilson <chris@chris-wilson.co.uk>]

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On Fri, 14 Sep 2012 17:21:30 +0300, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> On Fri, Sep 14, 2012 at 02:57:26PM +0100, Chris Wilson wrote:
> > On Wed, 12 Sep 2012 18:47:07 +0300, ville.syrjala@linux.intel.com wrote:
> > > +static void intel_flip_finish(struct drm_flip *flip)
> > > +{
> > > +	struct intel_flip *intel_flip =
> > > +		container_of(flip, struct intel_flip, base);
> > > +	struct drm_device *dev = intel_flip->crtc->dev;
> > > +
> > > +	if (intel_flip->old_bo) {
> > > +		mutex_lock(&dev->struct_mutex);
> > > +
> > > +		intel_finish_fb(intel_flip->old_bo);
> > 
> > So if I understand correctly, this code is called after the flip is
> > already complete?
> 
> Yes.
> 
> > The intel_finish_fb() exists to flush pending batches and flips on the
> > current fb, prior to changing the scanout registers. (There is a
> > hardware dependency such that the GPU may be executing a command that
> > required the current modesetting.) In the case of flip completion, all
> > of those dependencies have already been retired and so the finish should
> > be a no-op. And so it should no be required, nor the changes to
> > intel_finish_fb (which should have included a change in the name to
> > indicate that is now taking the fb_obj).
> 
> Actually I'm not quite sure where this intel_finish_fb() call originated.
> Based on the name it didn't make sense to me, but I left it there for
> now. Hmm. OK it came from one patch from Imre while I was on vacation.
> I suppose he got it from intel_pipe_set_base() which does call
> intel_finish_fb() on the old fb. Why does it do that?

It all boils down to the modeset being asynchronous to the GPU
processing the command stream. So we may be currently processing a batch
that is waiting on the pipe to go past a particular scanline, and if the
modesetting were to disable that pipe, or to change its size, then we
risk the WAIT_FOR_EVENT never completing - leading to hangcheck
detecting the frozen display and an angry user.

The other aspect is to synchronize the modeset with any outstanding
pageflips.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

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