[Mesa-dev] [Bug 47375] Blender crash on startup after upgrade to mesa 8.0.1

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=47375

--- Comment #35 from Barto <mister.freeman at laposte.net> 2012-07-26 20:33:26 PDT ---
(In reply to comment #31)
> (In reply to comment #29)
> Unfortunately I think this is expected. Previously swrast didn't map whole
> buffers, only parts of them. For much simpler and more robust code though
> swrast fallbacks now maps the full buffers, an operation which is hideously
> expensive at least with radeons (as you've got to blit back the buffers from
> the card memory to system memory). I believe blender draws some little things
> on a large buffer, so the performance of the swrast fallback is fully dominated
> by copying the buffers around, the actual cost of the drawing operation in the
> swrast fallback is completely irrelevant. This is also why it's much faster to
> just use swrast to begin with as then you don't need to copy the buffers around 
> (you should see this if you some some profiling tool like sysprof, oprofile,
> perf).
> I'm not sure how this could be solved really, I think the best option would be
> to modify blender so it doesn't rely on these legacy features leading to
> fallbacks. Or similarly try to accelerate these in metaops so it doesn't need
> to fallback.
> A third option would be to try to figure out the area accessed and only map
> those parts (to swrast it would still look like the whole buffer would be
> mapped to keep things simple, but the driver would only blit the needed parts).
> The infrastructure for that isn't there and I'm not sure it's desired in any
> case, and it would probably only work for the draw buffers and not for textures
> neither.


very interesting, thanks,

I'm not sure to understand all the facts ( I'm a newbie in opengl/multimedia
programming ), maybe, as you said the best solution would be to modify Blender,

but there is one thing I don't understand: why blender is very fast if I use
the software mode for opengl with 8.0.4 mesa lib ( by using the command "export
LIBGL_ALWAYS_SOFTWARE=1" before running blender ) ?

is it because only the CPU power works here for opengl instructions and not the
graphic card ? ( in this case swrast works differently ? )

it's weird to see the graphic performances decreasing when I use the normal
mode ( graphic card acceleration ) with Blender and see the exact opposite when
I use the software mode for opengl operations in mesa 8.0.4

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