The motivation of adding fallback_counter to exa

[Adam Jackson]

On Tue, 2010-10-12 at 11:34 +0800, fly fancy wrote:
> Hello, all
>             In newer XServer edition, the fallback_counter is
> introduced to EXA. However, I almost can not understand the motivation
> of the fallback_counter mechanism. Anyone understand it? I'll be very
> appreciated for your explanation. Thanks !

Near as I can tell (the commit message isn't great), it's like this:

Sometimes, you're doing a software fallback, and to do that fallback you
need to draw into a scratch pixmap and then scrape the bits out of it
and put them into the real destination.  Any scratch pixmap so created
should itself be rendered entirely in software in host memory, since
otherwise you'll be reading bits back out of the framebuffer and that's
_super_ slow.

So once the fallback count is non-zero, force everything to the host
memory path; and then allow it to be an integer so recursive fallbacks
work (which is pathological, but I guess it could happen).

- ajax
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