[Mesa-dev] [PATCH 4/9] r200: Enable extensions by just setting the flags

[Roland Scheidegger]

Am 08.09.2011 19:53, schrieb Ian Romanick:
> On 09/06/2011 03:21 PM, Roland Scheidegger wrote:
>> Am 06.09.2011 22:13, schrieb Ian Romanick:
>>> From: Ian Romanick <ian.d.romanick at intel.com>
>>>
>>> Core Mesa already does the dispatch offset remapping for every
>>> function that could possibly ever be supported.  There's no need to
>>> continue using that cruft in the driver.
>>>
>>> Since the call to _mesa_enable_imaging_extensions (via
>>> driInitExtensions) is removed, EXT_blend_logic_op is explicitly added
>>> to the list.  EXT_blend_color is also added, but it depends on the
>>> drmSupportsBlendColor flag.
> 
>> Hmm, I don't think EXT_blend_logic_op was advertized before. The reason
>> for this is that EXT_blend_logic_op together with
> 
> EXT_blend_logic_op *was* previously enabled.  r200CreateContext called
> driInitExtensions( ctx, card_extensions, GL_TRUE );.  The GL_TRUE
> parameter tells driInitExtensions to call
> _mesa_enable_imaging_extensions.  _mesa_enable_imaging_extensions in
> turn enables:
> 
>    GL_EXT_blend_color
>    GL_EXT_blend_logic_op
>    GL_EXT_blend_minmax
>    GL_EXT_blend_subtract
That's right. _mesa_enable_imaging_extensions however did not always
enable EXT_blend_logic_op.
I was curious (as I was sure it was correct at one point for r200) when
this got broken for r200 and that's the answer:
6775c1e8ccc2c543d97eb273a342140a62d99aee - that is OLD (interestingly
the commit mentions some discussion apparently however I did not realize
it in fact made r200 advertize EXT_blend_logic_op which I knew would be
incorrect).

> 
> I didn't see anything in r200_state.c to handle blend equation being set
> to GL_LOGIC_OP.
Yes - there was code initially handling this (trivial as long as it's
the same on all RGBA channels) but I removed that a decade or so ago
when adding support for EXT_blend_equation_separate (and removing
support for EXT_blend_logic_op at the same time).

> 
> Of course, we have *zero* piglit tests for this extension.
> 
>> EXT_blend_equation_separate allows some unholy combinations which the
>> r200 (possibly other hw too) can't handle correctly. Namely this
>> combination makes it possible to have logic ops on rgb or alpha channels
>> and color blending on the other channels.
>> I know that at least sometime in the past this driver did not advertize
>> EXT_blend_logic_op, since OpenGL 1.1 style logic ops do not have that
>> problem and EXT_blend_logic_op wasn't really all that important. I guess
>> though it's not exactly a severe problem since surely apps old enough to
>> use EXT_blend_logic_op wouldn't try to use EXT_blend_equation_separate
>> (though in theory some app could be clever and really want to do that...).
> 
> That's a good point.  I suspect that no hardware actually handles this
> case correctly.  I seem to recall that this is the reason NVIDIA doesn't
> support GL_EXT_blend_logic_op in their drivers.  I know the non-Quadro
> cards don't support it, anyway.
> 
> Does this work on later chips in the Radeon family?
> 
> I don't think anyone will miss GL_EXT_blend_logic_op if we just remove
> it altogether.

I don't think it works at least for r300. FWIW there's a mesa helper
function (_mesa_rgba_logicop_enabled) which also only makes sense if the
logicop blend equation is set for either both of none of RGB/A.

I certainly wouldn't mourn the loss of EXT_blend_logic_op.

Roland