Quoting Oscar Mateo (2018-04-13 17:46:42)
> 
> 
> On 4/12/2018 8:21 AM, Chris Wilson wrote:
> > Add a selftest to ensure that we restore the whitelisted registers after
> > rewrite the registers everytime they might be scrubbed, e.g. module
> > load, reset and resume. For the other volatile workaround registers, we
> > export their presence via debugfs and check in igt/gem_workarounds.
> > However, we don't export the whitelist and rather than do so, let's test
> > them directly in the kernel.
> 
> I guess my question is... why? what was the problem with exporting the 
> list of whitelist registers in debugfs?

We don't... (There's no RING_NONPRIV checking currently) I wasn't fond
of the igt for it is checking kernel implementation rather than behaviour.
The kernel gives it a checklist which it dutifully follows... Now that
we have selftests, we don't need to write what I think should be unit
tests in igt anymore.

> > The test we use is to read the registers back from the CS (this helps us
> > be sure that the registers will be valid for MI_LRI etc). In order to
> > generate the expected list, we split intel_whitelist_workarounds_emit
> > into two phases, the first to build the list and the second to apply.
> > Inside the test, we only build the list and then check that list against
> > the hw.
> >
> > v2: Filter out pre-gen8 as they do not have RING_NONPRIV.
> >
> > Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> > Cc: Oscar Mateo <oscar.ma...@intel.com>
> > Cc: Mika Kuoppala <mika.kuopp...@linux.intel.com>
> > Cc: Joonas Lahtinen <joonas.lahti...@linux.intel.com>
> > ---
> > +static struct whitelist *whitelist_build(struct intel_engine_cs *engine,
> > +                                      struct whitelist *w)
> > +{
> > +     struct drm_i915_private *i915 = engine->i915;
> > +
> > +     GEM_BUG_ON(engine->id != RCS);
> > +
> > +     w->count = 0;
> > +     w->nopid = i915_mmio_reg_offset(RING_NOPID(engine->mmio_base));
> 
> :)
> 
> > +
> > +     if (INTEL_GEN(i915) < 8)
> > +             return NULL;
> > +     else if (IS_BROADWELL(i915))
> > +             bdw_whitelist_build(engine, w);
> 
> Is it worth passing the engine around? Even of we end up with 
> whitelisted register in engines != RCS, we will need more changes than this.

No idea, it was easy enough to pass around, so I did just in case it was
useful in future (pulling out i915 or whatever).
-Chris
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