[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 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.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre