Re: [PATCH 04/12] drm/i915: Don't unconditionally try to deref aliasing ppgtt

[Ben Widawsky <ben@bwidawsk.net>]

On Mon, Nov 25, 2013 at 06:06:18PM +0000, Chris Wilson wrote:
> On Mon, Nov 25, 2013 at 09:54:35AM -0800, Ben Widawsky wrote:
> > Since the beginning, the functions which try to properly reference the
> > aliasing PPGTT have deferences a potentially null aliasing_ppgtt member.
> > Since the accessors are meant to be global, this will not do.
> > 
> > Introduced originally in:
> > commit a70a3148b0c61cb7c588ea650db785b261b378a3
> > Author: Ben Widawsky <ben@bwidawsk.net>
> > Date:   Wed Jul 31 16:59:56 2013 -0700
> > 
> >     drm/i915: Make proper functions for VMs
> > 
> > Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
> > ---
> >  drivers/gpu/drm/i915/i915_gem.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index 40d9dcf..bc5c865 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -4971,7 +4971,8 @@ unsigned long i915_gem_obj_offset(struct drm_i915_gem_object *o,
> >  	struct drm_i915_private *dev_priv = o->base.dev->dev_private;
> >  	struct i915_vma *vma;
> >  
> > -	if (vm == &dev_priv->mm.aliasing_ppgtt->base)
> > +	if (!dev_priv->mm.aliasing_ppgtt ||
> > +	    vm == &dev_priv->mm.aliasing_ppgtt->base)
> 
> Where's the dereference? gcc is smarter than your average bear.
> -Chris

I had assumed: dev_priv->mm.aliasing_ppgtt->base in cases when
aliasing_ppgtt was NULL. Given that I never actually hit this, I agree
GCC must be doing something.

Is such behavior documented somewhere? (forgive the lazy)

> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre

-- 
Ben Widawsky, Intel Open Source Technology Center