[Mesa-dev] [PATCH v3] i915: fixing driver crashes if too few vertices are submitted

[Predut, Marius]

Ok ,
So I propose this code to be included into the patch:

count -= (count - start) % 4;
if (cont == 0) 
   return;

What do you think?



> -----Original Message-----
> From: Ian Romanick [mailto:idr at freedesktop.org]
> Sent: Saturday, September 12, 2015 4:46 AM
> To: Ilia Mirkin; Predut, Marius
> Cc: mesa-dev at lists.freedesktop.org; Romanick, Ian D
> Subject: Re: [Mesa-dev] [PATCH v3] i915: fixing driver crashes if too few
> vertices are submitted
> 
> On 09/11/2015 10:55 AM, Ilia Mirkin wrote:
> > On Fri, Sep 11, 2015 at 12:36 PM, Marius Predut <marius.predut at intel.com>
> wrote:
> >> Comparison with a signed expression and unsigned value is converted
> >> to unsigned value, reason for minus value is interpreted as a big
> >> unsigned value. For this case the "for" loop is going into unexpected
> >> behavior.
> >>
> >> v1: Brian Paul: code style fix.
> >> v2: Ian Romanick: glDrawArrays(GL_QUADS, 0, (n * 4) + k) fail , k < 4.
> >>
> >> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38109
> >> Signed-off-by: Marius Predut <marius.predut at intel.com>
> >> ---
> >>  src/mesa/tnl_dd/t_dd_dmatmp.h | 7 +++++++
> >>  1 file changed, 7 insertions(+)
> >>
> >> diff --git a/src/mesa/tnl_dd/t_dd_dmatmp.h
> >> b/src/mesa/tnl_dd/t_dd_dmatmp.h index 7be3954..f99d977 100644
> >> --- a/src/mesa/tnl_dd/t_dd_dmatmp.h
> >> +++ b/src/mesa/tnl_dd/t_dd_dmatmp.h
> >> @@ -627,6 +627,13 @@ static void TAG(render_quads_verts)( struct gl_context
> *ctx,
> >>        LOCAL_VARS;
> >>        GLuint j;
> >>
> >> +      /* Page 18 (page 32 of the PDF) of the OpenGL 2.1 spec says:
> >> +       * The total number of vertices between Begin and End is 4n + k,
> >> +       * where 0 ≤ k ≤ 3; if k is not zero, the final k vertices are
> ignored.
> >> +       */
> >> +      count = (count / 4) * 4;
> >
> > Might be just me, but I'd find
> >
> > count &= ~0x3
> >
> > to be a lot clearer. Don't know if the compiler can make such an
> > optimization.
> 
> I think it can if count is unsigned.  Of course, GLsizei is not unsigned.  It
> is already invalid for count < 0, so your optimization is safe.
> 
> > However this seems wrong... you're supposed to draw start..count, so
> > that's the value that has to be div-by-4. Further up, when there's
> > native quad support, the logic does:
> 
> I don't think that's right.  Count is the number of vertices, not the index of
> the last vertex.  Calling
> 
>     glDrawArrays(GL_QUADS, 47000, 4);
> 
> still draws one quad.
> 
> Look at the pseudocode on page 28 (page 42 of the PDF) of the OpenGL 2.1
> spec:
> 
>     "The command
> 
>         void DrawArrays(enum mode, int first, sizei count);
> 
>     constructs a sequence of geometric primitives using elements first
>     through first + count − 1 of each enabled array. mode specifies
>     what kind of primitives are constructed; it accepts the same token
>     values as the mode parameter of the Begin command. The effect of
> 
>         DrawArrays(mode, first, count);
> 
>     is the same as the effect of the command sequence
> 
>         if (mode or count is invalid)
>             generate appropriate error
>         else {
>             Begin(mode);
>             for (int i = 0; i < count; i++)
>                 ArrayElement(first + i);
>             End();
>         }"
> 
> Combining that with the language previously quoted, I think this change is
> right.
> 
> >       /* Emit whole number of quads in total.  dmasz is already a multiple
> >        * of 4.
> >        */
> >       count -= (count-start)%4;
> >
> > Which seems more accurate.
> >
> >> +      if(count == 0) return;
> >
> > if (count == 0)
> >
> > note the space. That's the style used in all of mesa.
> >
> > $ git grep '\sif(' | wc -l
> > 1076
> > $ git grep '\sif (' | wc -l
> > 58071
> >
> > I guess a few 'if(' instances snuck through, mostly in src/gallium.
> > But the overwhelming majority are of the 'if (' style.
> >
> >> +
> >>        INIT(GL_TRIANGLES);
> >>
> >>        for (j = start; j < count-3; j += 4) {
> >> --
> >> 1.9.1
> >>
> >> _______________________________________________
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> >>
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