Re: [PATCH v3] drm/i915: Added write-enable pte bit support

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, May 13, 2014 at 03:43:12PM -0700, Jesse Barnes wrote:
> On Wed, 14 May 2014 00:30:34 +0200
> Daniel Vetter <daniel@ffwll.ch> wrote:
> 
> > On Tue, May 13, 2014 at 03:05:24PM -0700, Jesse Barnes wrote:
> > > On Tue, 11 Feb 2014 14:19:03 +0530
> > > akash.goel@intel.com wrote:
> > > 
> > > > @@ -810,6 +815,7 @@ static void gen6_ppgtt_insert_entries(struct i915_address_space *vm,
> > > >  		pt_vaddr[act_pte] =
> > > >  			vm->pte_encode(sg_page_iter_dma_address(&sg_iter),
> > > >  				       cache_level, true);
> > > > +
> > > >  		if (++act_pte == I915_PPGTT_PT_ENTRIES) {
> > > >  			kunmap_atomic(pt_vaddr);
> > > >  			pt_vaddr = NULL;
> > > 
> > > Some extra whitespace here.
> > > 
> > > Otherwise:
> > > Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
> > 
> > Hm, looking at the patch again encoding this into the cache_level enum is
> > fraught with fun. And due to IS_VLV aliasing chv this will blow up on chv
> > very likely. My old idea was to eventually add a pte_flags param all over
> > for this stuff with additional bits.
> 
> That works too; and yeah CHV is all different here isn't it.  But won't
> it go through the gen8 paths anyway?

Yes, but we have a switch on the cache_level in the gen8 pte encode
function. Since the bit31 gets or'ed in for VLV, it'll hit also chv and
wreak havoc in that specific switch - we'll hit the default case when we
don't expect to.

cache_level = functional behaviour relevant for the kernel's clflushing
code

> Ack on the cache_level abuse though; it's an implementation detail that
> should be changed if/when this is properly exposed to userspace (or
> maybe we shouldn't bother and just use CHV/BDW stuff going forward,
> supporting a proper mprotect ioctl).

Not talking about exposing to userspace, but adding pte_flags to the
low-level platform pte encode functions to prevent the above bugs. I'm ok
with keeping obj->gt_ro.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch