[Mesa-dev] [PATCH 2/2] intel: make sure hardware choose the right filter mode

[Eric Anholt]

On Thu, 3 Nov 2011 10:16:06 +0800, Yuanhan Liu <yuanhan.liu at linux.intel.com> wrote:
> On Wed, Nov 02, 2011 at 02:18:46PM -0700, Eric Anholt wrote:
> > On Wed, 2 Nov 2011 11:12:07 +0800, Yuanhan Liu <yuanhan.liu at linux.intel.com> wrote:
> > > On Tue, Nov 01, 2011 at 05:57:36PM +0800, Yuanhan Liu wrote:
> > > > According to bspec, MIPCnt(was set to intelObj->_MaxLevel) was used for
> > > > min/mag filter mode determination. For a normal case with no mipmap like
> > > > this:
> > > > 
> > > >   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
> > > >   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
> > > > 
> > > > hardware would always choose mag filter(GL_LINEAR) here since MIPCnt was
> > > > set to 0 zero, then would make LOD be zero. Then according the formula:
> > > >   MagMode = (LOD - Base <= 0)
> > > 
> > > Here are some more comments about this: LOD is a pre-computed value. if
> > > preClamp is enabled, then LOD would be:
> > >   LOD = min(min(MIPCnt, MAX_LOD), LOD);
> > > 
> > > So, if MIPCnt was set to 0, and preClamp was enabled, then the _final_
> > > LOD would be 0, and hardware choose mag filter.
> > > 
> > > Thoughts?
> > 
> > It took me a long time to understand what you were trying to fix here.
> 
> Sorry for that :(
> 
> > The story I've worked out is:
> > 
> > "We can't optimize out uploading the other mipmap levels for
> > non-mipmapped filtering modes if the min filter is not the same as the
> > mag filter.  This is because the min/mag decision is based on the
> > computed LOD of the sample, and if we clamp the LOD to BaseLevel by not
> > including the other levels, it will always choose the mag filter even if
> > minification should have occurred. 
> 
> Yes.
> 
> > See page FINISHME: citation of PRM."
> 
> Sorry, what does this mean?

If you're going to make a reference to the spec, if possible please
quote the exact section from the public PRM, so the next person to find
it has a nice reference to read up on.

> Yeah, I saw that. While thinking about this issue, I thought another
> patch to fix this issue. It's somehow against with the Bspec: preclamp
> should be enabled for OpenGL driver.
> 
> Anyway, here is the patch, and I'd like to know what's your thoughts.

I think I liked the previous one more.  While I couldn't come up with a
testcase for how this one would break, I still think just disabling the
minor optimization in this weird case is more obviously correct than
this change.
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