[Intel-gfx] [PATCH 48/55] drm/i915: Add *_ring_begin() to request allocation

[Chris Wilson]

On Wed, Jun 17, 2015 at 03:31:59PM +0200, Daniel Vetter wrote:
> On Fri, May 29, 2015 at 05:44:09PM +0100, John.C.Harrison at Intel.com wrote:
> > From: John Harrison <John.C.Harrison at Intel.com>
> > 
> > Now that the *_ring_begin() functions no longer call the request allocation
> > code, it is finally safe for the request allocation code to call *_ring_begin().
> > This is important to guarantee that the space reserved for the subsequent
> > i915_add_request() call does actually get reserved.
> > 
> > v2: Renamed functions according to review feedback (Tomas Elf).
> > 
> > For: VIZ-5115
> > Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
> 
> Still has my question open from the previos round:
> 
> http://mid.gmane.org/20150323091030.GL1349@phenom.ffwll.local
> 
> Note that this isn't all that unlikely with GuC mode since there the
> ringbuffer is substantially smaller (due to firmware limitations) than
> what we allocate ourselves right now.

Looking at this patch, I am still fundamentally opposed to reserving
space for the request. Detecting a request that wraps and cancelling
that request (after the appropriate WARN for the overlow) is trivial and
such a rare case (as it is a programming error) that it should only be
handled in the slow path.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre