[Intel-gfx] [PATCH v6] drm/i915/vlv: Added a rendering specific Hw WA 'WaTlbInvalidateStoreDataBefore'

[Gupta, Sourab]

On Tue, 2014-03-25 at 10:59 +0000, Chris Wilson wrote:
> On Tue, Mar 25, 2014 at 03:23:34PM +0530, sourab.gupta at intel.com wrote:
> > From: Akash Goel <akash.goel at intel.com>
> > 
> > Added a new rendering specific Workaround 'WaTlbInvalidateStoreDataBefore'.
> > This workaround has to be applied before doing TLB Invalidation on render ring.
> > In this WA, before pipecontrol with TLB invalidate set, need to add 2 MI
> > Store data commands.
> > Without this, hardware cannot guarantee the command after the PIPE_CONTROL
> > with TLB inv will not use the old TLB values.
> 
> Note, that our command programming sequence already has multiple dword
> writes between the flush of the last batch and the invalidation of the
> next.
> 
> Is this w/a still required? Why?
> -Chris
> 
Hi Chris,
Are you referring to the MI_STORE_DWORD_INDEX commands emitted as a part
of add_request functions?
We couldn't find any other place where we are storing dwords between
flush of last batch and invalidation of next.
In that case, we agree that as a part of command programming sequence,
we'll have one set of store dwords emitted.

But, as per spec, it is required to emit 2 sets of store dwords before
the tlb invalidation.
Also, our motive for having this w/a is just being paranoid, and not
assuming that dwords would already have been emitted before doing tlb
invalidation. So, we try to explicitly ensure the same through our w/a.

Regards,
Sourab