--- Felix Kühling <[EMAIL PROTECTED]> wrote:
> Hi Marcin,
>
> Also CCing to Alex, he probably knows more about this.
>
> Currently the driver looks like it's not supposed to render to the
> front
> buffer at all. When single buffering is used, the driver draws to the
> back buffer and copies it over to the front buffer after processing a
> Mesa vertex buffer. I have no idea if this is GL compliant. Anyway,
> it
> seems that frequent copying from the back buffer to the front buffer
> is
> a big performance problem.
>
> Anyway, the reason is that accelerated 3D rendering only works with
> tiled memory but the front buffer is linear.
>
> Software fallbacks draw to tiled surfaces, which map linear or tiled
> memory. If I understand it correctly it shouldn't make any difference
> whether the underlying memory is linear or tiled (as long as the
> tiled
> surface registers are setup correctly).
That's my understanding as well, but I could be wrong. Also, tiling
the front buffer results in lower performance. I'm not sure why. It
might have just been due to the way the code was initially architected.
your changes to the 3d driver may have fixed that.
>
> It seems that the front buffer surface uses a different line pitch
> than
> the back buffer. I don't know why that is. Alex, can this be changed
> easily or would it require big changes in the 2D driver?
The pitch is different because of alignment requirements. I think I
tried changing the front pitch to match the others and it didn't seem
to break anything as I recall, so perhaps we can get away with it.
Unfortunately, tiling of the front buffer broke when I ported S3's
changes to Tim's code. I'm not sure what the problem is as the set up
is pretty much identical. I never really dug into it too much since
linear seemed to work fine for most things. However, back when we were
using S3's driver and tiling worked, drawing to the front buffer was
still broken (looked the same whether the front buffer was tiled or
not).
Alex
>
> Regards,
> Felix
>
> On Tue, 23 Mar 2004 11:18:46 +0100
> Marcin Krotkiewski <[EMAIL PROTECTED]> wrote:
>
> > Hi again,
> >
> > > I think this is a problem with software fallbacks drawn to the
> front
> > > buffer. It's definitely a driver bug. As a workaround try drawing
> to the
> > > back buffer and call glXSwapBuffers when you're done with a
> frame. The
> > > worst thing that would happen here is that the window remains
> black (or
> > > whatever your clear color is). ;-)
> > >
> > > If you want to try to fix the bug take a look at savagespan.c in
> the 3D
> > > driver in the Mesa source code
> (src/mesa/drivers/dri/savage/savagespan.c).
> >
> > I modified savagespan.c a bit. This line helped:
> >
> > static void savageDDSetBuffer(GLcontext *ctx, GLframebuffer
> *buffer,
> > GLuint bufferBit)
> > {
> > savageContextPtr imesa = SAVAGE_CONTEXT(ctx);
> > char *map;
> >
> > assert( (bufferBit == FRONT_LEFT_BIT) || (bufferBit ==
> BACK_LEFT_BIT) );
> >
> > map = (bufferBit == FRONT_LEFT_BIT)
> > ? (char*)imesa->apertureBase[TARGET_FRONT]
> > : (char*)imesa->apertureBase[TARGET_BACK];
> > + map = (char*)imesa->apertureBase[TARGET_BACK];
> >
> > Though I do not think this is a solution, it might help you track
> > down the bug...
> > I'll let you know if I find anything else ;)
> >
> > Cheers
> >
> > Marcin
> >
> > >
> > > I'm not yet ready to start fixing bugs in the driver. There is at
> least
> > > one big chunk of work that I'm going to tackle in the next couple
> of
> > > weeks. Later on you could help improve the driver by sending bug
> reports
> > > with short OpenGL example programmes that demonstrate the
> problem.
> > > Usually this helps a lot in tracking down 3D driver bugs.
> > >
> > >>
> > >> Cheers,
> > >>
> > >> Marcin Krotkiewski
> > >
> > > Regards,
> > > Felix
> > >
> > >
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