[Intel-gfx] [PATCH 50/70] drm/i915: The argument for postfix is redundant

[Chris Wilson]

On Fri, Apr 10, 2015 at 10:53:02AM +0200, Daniel Vetter wrote:
> On Tue, Apr 07, 2015 at 04:21:14PM +0100, Chris Wilson wrote:
> > We are conservative on the amount of free space available in the ring to
> > avoid overruning the potential MI_INTERRUPT after the seqno write.
> > Further undermining the justification for the change was that it was
> > applied incorrectly.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> 
> Hm, where are we conservative with our estimates? Could we just wait for
> req->head instead? And I don't see what's been implemented wrongly with
> postfix.

It took more patches for it to get fixed, when it wasn't actually broken as
the calculation of space remaining is conservative. req->head would be a
reasonable compromise to the addition of another variable, and the extra bit
of hysteresis here would probably be useful. Hmm.

> Looking at
> 
> commit a71d8d94525e8fd855c0466fb586ae1cb008f3a2
> Author: Chris Wilson <chris at chris-wilson.co.uk>
> Date:   Wed Feb 15 11:25:36 2012 +0000
> 
>     drm/i915: Record the tail at each request and use it to estimate the head
> 
> ->postfix does get updated where ->tail was, Nick just renamed it from
> ->tail to ->postfix since execlist used ->tail with a different meaning.
> 
> Or do I miss something?

tail was always the position of the end of the request.
 
> > diff --git a/drivers/gpu/drm/i915/intel_dvo.c b/drivers/gpu/drm/i915/intel_dvo.c
> > index 9a27ec7100ef..f45caa6af7d2 100644
> > --- a/drivers/gpu/drm/i915/intel_dvo.c
> > +++ b/drivers/gpu/drm/i915/intel_dvo.c
> > @@ -496,7 +496,7 @@ void intel_dvo_init(struct drm_device *dev)
> >  		int gpio;
> >  		bool dvoinit;
> >  		enum pipe pipe;
> > -		uint32_t dpll[2];
> > +		uint32_t dpll[I915_MAX_PIPES];
> 
> Unrelated change and there's indeed only 2 dvo plls ever on gen2.

Accidental squashing for a compiler warning.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre