[Intel-gfx] [PATCH] drm/i915/gt: Reset execlists registers before HWSP

[Chris Wilson]

Quoting Mika Kuoppala (2020-05-13 10:32:37)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > Upon gt resume, we first poison then sanitize the engine. However, our
> > testing shows that gen9 will very rarely retain the poisoned value from
> > the HWSP mappings of the execlists status registers. This suggests that
> > it is reading back from the HWSP, so rejig the register reset.
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> 
> Acked-by: Mika Kuoppala <mika.kuoppala at linux.intel.com>

It failed in exactly the same way, got past the
	GEM_BUG_ON(*csb_write != reset_value)
and still ended up with
	*csb_write == 0x5a [90]
in process_csb.

How it's able to see 0x5a at all is a mystery.

We poison, we sanitize, we reset the GPU. The value comes back from out
of nowhere.
-Chris