[Intel-gfx] [PATCH i-g-t 4/7] tests/gem_mmap_gtt: Make the small-bo tiling tests work on old platforms

[Chris Wilson]

On Tue, Dec 15, 2015 at 11:41:44AM +0200, Ville Syrjälä wrote:
> On Mon, Dec 14, 2015 at 08:54:35PM +0000, Chris Wilson wrote:
> > On Mon, Dec 14, 2015 at 08:49:38PM +0000, Chris Wilson wrote:
> > > On Mon, Dec 14, 2015 at 10:15:53PM +0200, ville.syrjala at linux.intel.com wrote:
> > > > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > > 
> > > > Several factors conspire against us when trying to execute
> > > > the tiled small-bo tests:
> > > > - pre-gen4 require power of two fences, with natural alignment
> > > > - the entire gtt may be mappable
> > > > - we put a guard page at the end of gtt
> > > > 
> > > > What all that means is that when we try to use a tiled object half
> > > > the size of the mappable area, we can only fit it in the first half
> > > > of the gtt. That leads to a SIGBUS when we try to fault in the
> > > > object when there's already something (eg. fbdev) occupying the
> > > > first half of gtt.
> > > > 
> > > > So in order to make the tests run on old machines, let's further
> > > > halve the object size when things look too tight.
> > > 
> > > That defeats the point of the test. The idea is to have the two objects
> > > that just don't fit, but only just.
> > 
> > i.e. the test is meant to show that the kernel heuristics for using
> > partial vma do not prevent the page-fault-of-doom.
> 
> So just skip then?

The kernel has a bug that partial vma was supposed to address. In theory
it can strike at any time (although userspace has a similar approach to
try and not tempt fate). And other than a getparm "bug fixed", there
won't be an ABI change for userspace to take note of.

I have ~237 patches ahead of my "make partial-vma actually work" fix. :|
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre