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

[Akash Goel]

On Mon, 2014-05-19 at 08:56 +0200, Daniel Vetter wrote:
> On Sun, May 18, 2014 at 11:27:00AM +0530, Akash Goel wrote:
> > On Wed, 2014-05-14 at 10:14 +0200, Daniel Vetter wrote:
> > > 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 at 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 at 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.
> > 
> > Thanks, will remove this.
> > 
> > > > > > 
> > > > > > Otherwise:
> > > > > > Reviewed-by: Jesse Barnes <jbarnes at 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
> > > 
> > 
> > As the 'IS_VALLEYVIEW' check will alias with CHV also, can I just update
> > the condition here, to include 'GEN7' also (IS_GEN7) in the check.
> 
> Adding new random bits to an enum which is used all over the place (and
> not just in the pte encoding functions) makes the code much harder to
> read. Also the code that deals with enum cache_level is all about cache
> coherency, which has rather tricky logic.
> 
> Hence I expect this choice to cause further bugs down the road, which is
> why I don't really like it. My apologies for not spotting this earlier.
> -Daniel

Hi Daniel,

I understand your concern, but actually (as of now) this bit is only VLV
specific. As per an earlier suggestion from Chris, I decided to
overload the cache_level enum itself, in lieu of adding a new parameter
to all the respective 'xxx_insert_entries' & 'xxx_pte_encode' functions.

And this is being done just before calling the 'xxx_insert_entries'
function, which simply passes the flag as it is to 'xxx_pte_encode'
function. So there may not be any risk here, if we use the appropriate
VLV check.

Best Regards
Akash