[Mesa-dev] [PATCH] mesa glthread: allow asynchronous pixel transfer operation when a buffer is bound

[gregory hainaut]

On Fri, 17 Mar 2017 13:11:31 +0100
Markus Wick <markus at selfnet.de> wrote:

> Hi gregory,
> 
> Am 2017-03-17 10:25, schrieb Gregory Hainaut:
> > diff --git a/src/mesa/main/marshal.c b/src/mesa/main/marshal.c
> > index f8cad30..43a70d4 100644
> > --- a/src/mesa/main/marshal.c
> > +++ b/src/mesa/main/marshal.c
> > @@ -214,6 +218,12 @@ track_vbo_binding(struct gl_context *ctx, GLenum
> > target, GLuint buffer)
> >         */
> >        glthread->element_array_is_vbo = (buffer != 0);
> >        break;
> > +   case GL_PIXEL_UNPACK_BUFFER:
> > +      glthread->pixel_unpack_buffer_bound = (buffer != 0);
> > +      break;
> > +   case GL_PIXEL_PACK_BUFFER:
> > +      glthread->pixel_pack_buffer_bound = (buffer != 0);
> > +      break;
> >     }
> >  }
> > 
> 
> I wonder what shall happen if buffer is not 0, but neither a valid 
> buffer object. So the glBindBuffer call will fail, and there would still 
> be no buffer bound.
> The comment above this function explains it very well.
> 
> degasus

Hello degasus,

 
What shall happen ?
 => Honestly I don't know. Is the behavior 100% defined when you hit a gl error ?

What will happen ?
 => It depends on the gl transfer operation that will follow the bind.

Let's take some examples

glBindBuffer(invalid_buffer);

// if no buffer is bound, offset_to_pbo will be interpreted as a memory
// pointer. Crash is expected (even with gl thread off)
glTextureSubImage1D(....., offset_to_pbo);

// Now, if the application know that buffer isn't bound (hazardous dev or 
// using an is/get API to check the status). The application will use
// a real memory pointer. It will be fine without glthread. But it will
// crash with (the current) glthread.
glTextureSubImage1D(....., memory_ptr);

If the application is badly/strangely coded, glthread will make it worst.
The solution ought to be either fix the app or don't use glthread.

Best regards,
Gregory