[Mesa-dev] [PATCH V2 10/11] genX/cmd_buffer: Enable fast depth clears

[Nanley Chery]

On Mon, Oct 03, 2016 at 06:21:30PM -0700, Jason Ekstrand wrote:
> On Mon, Oct 3, 2016 at 6:11 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> 
> > On Tue, Sep 27, 2016 at 3:23 PM, Nanley Chery <nanleychery at gmail.com>
> > wrote:
> >
> >> On Tue, Sep 27, 2016 at 03:12:17PM -0700, Chad Versace wrote:
> >> > On Tue 27 Sep 2016, Nanley Chery wrote:
> >> > > On Tue, Sep 27, 2016 at 11:00:21AM -0700, Chad Versace wrote:
> >> >
> >> > > > As a consequence of that reasoning, we should set
> >> 3DSTATE_CLEAR_PARAMS.DepthClearValueValid = 1
> >> > > > whenever hiz is enabled, even if we don't care about the actual
> >> clear value.
> >>
> >
> > The logic seems to imply that we can't trust the context to save/restore
> > our depth clear value so we have to set it every time.  At the very least,
> > once per batch?  In any case, I doubt there's all that much cost involved
> > in emitting 3DSTATE_CLEAR_PARAMS so I don't think re-emitting it is that
> > big of a deal.
> >
> 
> Thinking about it a bit more...
> 
> We only set up dept/stencil packets once per subpass and we only do clears
> once per subpass so... I don't think we're actually saving anything by
> emitting it at clear time rather than at depth/stencil setup time.  It is a
> bit more convenient because the clear values may be more accessible at
> clear time.
> 
> As far as "should we emit 3DSTATE_CLEAR_PARAMS all the time?"  Let's not go
> to any heroics to try and avoid re-emitting it.  Once per subpass is not a
> big deal at all.
> 

I wouldn't say the code to implement it was complex, but I'm fine with
trading off efficiency for simplicity here. I'll add a comment describing the
situation.

-Nanley

> --Jason
> 
> 
> > > >
> >> > > In the V3, I plan to emit that packet once at device initialization
> >> time
> >> > > HSW+, and to always emit it (in the expected location) for IVB/BYT.
> >> Only
> >> > > the latter platforms have the restriction that it must always be
> >> > > programmed with the other depth/stencil commands.
> >> >
> >> > Is there any benefit to emitting it multiple times on ivb/byt? Does
> >> > emitting once during initialization, as for hsw, also work for ivb/byt?
> >> > If so, the code is cleaner if the two gens share the same workaround
> >> > code.
> >>
> >> The benefit for emitting it multiple times on IVB/BYT is that we're
> >> (possibly) following the oddly-worded programming note for the packet:
> >>
> >>    From the IVB PRM Vol2P1, 11.5.5.4 3DSTATE_CLEAR_PARAMS:
> >>
> >>       3DSTATE_CLEAR_PARAMS must always be programmed in the along with
> >>       the other Depth/Stencil state commands(i.e.  3DSTATE_DEPTH_BUFFER,
> >>       3DSTATE_STENCIL_BUFFER, or 3DSTATE_HIER_DEPTH_BUFFER)
> >>
> >> HSW+ doesn't have this restriction, so we're free to only do it once.
> >>
> >
> >