[Mesa-dev] [PATCH] i965: Select pipeline and emit state base address in Gen8+ HiZ ops.

[Nanley Chery]

On Wed, Mar 08, 2017 at 02:17:59AM -0800, Kenneth Graunke wrote:
> On Thursday, March 2, 2017 4:36:08 PM PST Nanley Chery wrote:
> > On Mon, Feb 06, 2017 at 03:55:49PM -0800, Kenneth Graunke wrote:
> > > If a HiZ op is the first thing in the batch, we should make sure
> > > to select the render pipeline and emit state base address before
> > > proceeding.
> > > 
> > > I believe 3DSTATE_WM_HZ_OP creates 3DPRIMITIVEs internally, and
> > > dispatching those on the GPGPU pipeline seems a bit sketchy.  I'm
> > 
> > Yes, it does seem like we currently allow HZ_OPs within a GPGPU
> > pipeline. This patch should fix that problem.
> > 
> > > not actually sure that STATE_BASE_ADDRESS is necessary, as the
> > > depth related commands use graphics addresses, not ones relative
> > > to the base address...but we're likely to set it as part of the
> > > next operation anyway, so we should just do it right away.
> > > 
> > 
> > I agree, re-emitting STATE_BASE_ADDRESS doesn't seem necessary. I think
> > we should drop this part of the patch and add it back in later if we get
> > some data that it's necessary. Leaving it there may be distracting to
> > some readers and the BDW PRM warns that it's an expensive command:
> > 
> > 	Execution of this command causes a full pipeline flush, thus its
> > 	use should be minimized for higher performance.
> 
> I think it should be basically free, actually.  We track a boolean,
> brw->batch.state_base_address_emitted, to avoid emitting it multiple
> times per batch.
> 
> Let's say the first thing in a fresh batch is a HiZ op, followed by
> normal drawing.  Previously, we'd do:
> 
>     1. HiZ op commands
>     2. STATE_BASE_ADDRESS (triggered by normal rendering upload)
>     3. rest of normal drawing commands
> 
> Now we'd do:
> 
>     1. STATE_BASE_ADDRESS (triggered by HiZ op)
>     2. HiZ op commands
>     3. normal drawing commands (second SBA is skipped)
> 
> In other words...we're just moving it a bit earlier.  I suppose there
> could be a batch containing only HiZ ops, at which point we'd pay for
> a single STATE_BASE_ADDRESS...but that seems really unlikely.
> 

Sorry for not stating it up front, but the special case you've mentioned
is exactly what I'd like not to hurt unnecessarily.

> > > Cc: "17.0" <mesa-stable at lists.freedesktop.org>
> > > Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> > > ---
> > >  src/mesa/drivers/dri/i965/gen8_depth_state.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/src/mesa/drivers/dri/i965/gen8_depth_state.c b/src/mesa/drivers/dri/i965/gen8_depth_state.c
> > > index a7e61354fd5..620b32df8bb 100644
> > > --- a/src/mesa/drivers/dri/i965/gen8_depth_state.c
> > > +++ b/src/mesa/drivers/dri/i965/gen8_depth_state.c
> > > @@ -404,6 +404,9 @@ gen8_hiz_exec(struct brw_context *brw, struct intel_mipmap_tree *mt,
> > >     if (op == BLORP_HIZ_OP_NONE)
> > >        return;
> > >  
> > 
> > It would be helpful if you included the rationale here as a code
> > comment. Something like the first two sentences of your commit message
> > should work.
> 
> I can do that.
> 
> > > +   brw_select_pipeline(brw, BRW_RENDER_PIPELINE);
> > 
> > According to Vol07 of the BDW+ PRMs,
> > 
> > 	The previously active pipeline needs to be flushed via the
> > 	MI_FLUSH command immediately before switching to a different
> > 	pipeline via use of the PIPELINE_SELECT command.
> > 
> > However it doesn't look like MI_FLUSH is present after HSW. So there
> > shouldn't be any additional work to do here.
> 
> Flushes are definitely required when switching the pipeline, but I
> believe that brw_emit_select_pipeline() does that work.
> 
> FWIW, MI_FLUSH was replaced by PIPE_CONTROL many generations ago.
> I believe the validation team stopped testing MI_FLUSH on Sandybridge.
> 

Thanks for letting me know about the MI_FLUSH replacement. Do you know
what bits must be set to perform an equivalent operation? From the looks of
it, brw_emit_select_pipeline() actually avoids emitting PIPE_CONTROL BDW+.

-Nanley

> --Ken