[Mesa-dev] [RFC PATCH] i965/msaa: Disable unsupported formats.

[Chad Versace]

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On 06/15/2012 11:12 AM, Paul Berry wrote:
> Due to hardware limitations, MSAA is unsupported on Gen6 for formats
> containing >64 bits of data per pixel.  From the Sandy Bridge PRM,
> vol4 part1, p72 ("Surface Format"):
> 
>     If Number of Multisamples is set to a value other than
>     MULTISAMPLECOUNT_1, this field cannot be set to the following
>     formats:
>     - any format with greater than 64 bits per element
>     - any compressed texture format (BC*)
>     - any YCRCB* format
> 
> Gen7 has a similar, but less stringent limitation: formats with >64
> bits of data per pixel only support 4x MSAA.
> 
> This patch causes the unsupported formats to report
> GL_FRAMEBUFFER_UNSUPPORTED.
> 
> Fixes piglit "multisample-formats" tests on Gen6.
> ---
> 
> It is not 100% clear to me whether this approach is compliant with the
> spec.  On the one hand, the spec requires formats RGBA32F, RGBA32I,
> and RGBA32UI to be color-renderable.  On the other hand, the spec
> allows for the implementation to report GL_FRAMEBUFFER_UNSUPPORTED if
> the set of attached images violates "an implementation-dependent set
> of restrictions", which seems to leave a lot of leeway.
> 
> If we decide that it's not ok to report GL_FRAMEBUFFER_UNSUPPORTED for
> MSAA buffers using these formats, then we'll have to figure out some
> other way to work around the hardware's lack of support for MSAA on
> these formats, and I haven't thought of a good way to do that.  The
> alternatives I've considered (substitute another buffer format, or
> substitute a non-multisampled renderbuffer) are definitely
> non-spec-compliant, and also fraught with implementation difficulties.
> 
> So for the moment my inclination is to go through with this patch as
> is, and if we later discover that it causes trouble for a client
> application, we can consider other options.  I'm curious to hear what
> others think.

After releasing GL3.0, but before implementing real MSAA, we lied to the app.
If the app requested an MSAA buffer, we said "ok" and just created a single-sample
buffer.

Can we do that for these formats? It may be better to do that than report
GL_FRAMEBUFFER_UNSUPPORTED. I'm unsure what's best, though.
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