[Mesa-dev] [Bug 103672] _mesa_meta_GenerateMipmap sub-optimal code and possible bug

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=103672

            Bug ID: 103672
           Summary: _mesa_meta_GenerateMipmap sub-optimal code and
                    possible bug
           Product: Mesa
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Mesa core
          Assignee: mesa-dev at lists.freedesktop.org
          Reporter: weilercdale at gmail.com
        QA Contact: mesa-dev at lists.freedesktop.org

Was just reviewing this code since I hit a pathological edge case where it was
severly impacting the performance of generating mip levels and was surprised to
see the approach taken.

Couple of alarming issues:

1. It uploads vertex data for every mip level to draw a quad, when this data
only needs to be uploaded once, lifting that out of the for loop is a trivial
one line change but will need further changes because of the
_mesa_meta_setup_texture_coords stuff, probably best to create one large vertex
buffer for all of this upfront instead.

2. It allocates storage for every mip level one iteration at a time when it
couple upfront allocate all the storage needed to represent all mip levels in
one shot.

3. When setting minification to the source level, I'm sure this is a copy-paste
bug (since it's called above to set max level but then called again), but
appears to use GL_TEXTURE_MAX_LEVEL instead of GL_TEXTURE_MIN_LEVEL.

4. The comment says it does not support 3D textures yet but it appears the code
does handle 3D textures.

5. The approach itself seems like it could better be done via glCopyTexImage*
family of functions which already take from the read buffer, thus avoiding ugly
vbo/vao state changes (as well as viewport changes) and issuing a draw call for
each mip level.

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