[Mesa-dev] i965 GL_ARB_buffer_storage

[Kenneth Graunke]

On 02/27/2014 02:52 PM, Eric Anholt wrote:
> One thing I noticed while working on this was that we only reallocate buffer
> storage for INVALIDATE_BUFFER_BIT when UNSYNCHRONIZED_BIT is unset.  The
> ARB_mbr spec says that the contents "may be discarded", not "must be
> discarded".  However, while writing the glamor code I happened to type this
> for the wraparound case:
> 
>             glamor_priv->vb = glMapBufferRange(GL_ARRAY_BUFFER,
>                                                0, size,
>                                                GL_MAP_WRITE_BIT |
>                                                GL_MAP_INVALIDATE_BUFFER_BIT |
>                                                GL_MAP_UNSYNCHRONIZED_BIT |
>                                                GL_MAP_PERSISTENT_BIT |
>                                                GL_MAP_COHERENT_BIT);
> 
> intending that the buffer storage get reallocated, and that we not worry about
> any synchronization after that.  My code would have been broken on the i965
> driver.  I'm wondering if this is the intended behavior of the spec, or if we
> want to treat the "may" as a "must".

I don't understand.  Your code here sets GL_MAP_INVALIDATE_BUFFER_BIT
and GL_MAP_UNSYNCHRONIZED_BIT, which would have reallocated the storage,
as you intended.  Was this a mispaste - you originally lacked the
GL_MAP_UNSYNCHRONIZED_BIT, were surprised, and eventually added it?

I've had several branches to throw out the storage when
GL_MAP_INVALIDATE_BUFFER_BIT was present, regardless of whether
UNSYNCHRONIZED_BIT was set.  I think it's a great idea.  I just couldn't
find any applications that hit that case, so I couldn't demonstrate an
improvement.

Of course, as Jose pointed out, there's no point in reallocating the
storage if it isn't busy.

--Ken




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