[Mesa-dev] [PATCH 1/6] i965: Don't grow batch/state buffer on every emit after an overflow.

[Kenneth Graunke]

On Tuesday, November 28, 2017 6:15:59 PM PST Ian Romanick wrote:
> On 11/28/2017 04:13 PM, Kenneth Graunke wrote:
> > Once we reach the intended size of the buffer (BATCH_SZ or STATE_SZ), we
> > try and flush.  If we're not allowed to flush, we resort to growing the
> > buffer so that there's space for the data we need to emit.
> > 
> > We accidentally got the threshold wrong.  The first non-wrappable call
> > beyond (e.g.) STATE_SZ would grow the buffer to floor(1.5 * STATE_SZ),
> > The next call would see we were beyond STATE_SZ and think we needed to
> > grow a second time - when the buffer was already large enough.
> > 
> > We still want to flush when we hit STATE_SZ, but for growing, we should
> > use the actual size of the buffer as the threshold.  This way, we only
> > grow when actually necessary.
> > 
> > Fixes: 2dfc119f22f257082ab0 "i965: Grow the batch/state buffers if we need space and can't flush."
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103101
> > ---
> >  src/mesa/drivers/dri/i965/intel_batchbuffer.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/src/mesa/drivers/dri/i965/intel_batchbuffer.c b/src/mesa/drivers/dri/i965/intel_batchbuffer.c
> > index 216073129ba..1d0292b4b80 100644
> > --- a/src/mesa/drivers/dri/i965/intel_batchbuffer.c
> > +++ b/src/mesa/drivers/dri/i965/intel_batchbuffer.c
> > @@ -373,7 +373,7 @@ intel_batchbuffer_require_space(struct brw_context *brw, GLuint sz,
> >     if (batch_used + sz >= BATCH_SZ) {
> 
> Why not just change this test to >= batch->bo->size?

We want to flush when we reach BATCH_SZ (the intended flush point).
If we reach that point but are in the middle of something which makes
makes it unsafe to flush (no_wrap), then we keep going, growing the BO
as necessary.  When growing, we only want to grow if we exceed the
actual buffer size.

I'm not a huge fan of how this is structured, so if you have an idea
for how to make it more obvious, I'm open to suggestions.
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