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

[Daniel Vetter]

On Fri, Apr 10, 2015 at 10:00:33AM +0100, Chris Wilson wrote:
> 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.

In the above referenced commit request->tail is sampled _before_ we call
->add_request and all that stuff. That seems to have changed in

commit 6d3d8274bc45de4babb62d64562d92af984dd238
Author: Nick Hoath <nicholas.hoath at intel.com>
Date:   Thu Jan 15 13:10:39 2015 +0000

    drm/i915: Subsume intel_ctx_submit_request in to drm_i915_gem_request

by essentially doing an s/request->tail/request->postfix/ and adding a new
request->tail (real tail) to satisfy execlist.
-Daniel
>  
> > > 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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch