How to use the structure of 'pScreen->pScratchPixmap' ? ( in Xserver code )

[Cui, Hunk]

Hi, guys,
	I want to ask some question, by part of Xserver code,
http://cgit.freedesktop.org/xorg/xserver/tree/dix/pixmap.c#n58 -> Line
59 pScreen->pScratchPixmap = NULL; why do this operation? Any body
knows?

	In our xf86-video-geode driver, I just go to inverted mode
rotation and back to normal, and afterwards various icons are gone, so I
want to know what destination about
http://cgit.freedesktop.org/xorg/xserver/tree/dix/pixmap.c#n58 -> Line
59 pScreen->pScratchPixmap = NULL; because it is main point.

	When go to inverted mode rotation, pScreen->pScratchPixmap =
NULL in GetScratchPixmapHeader function (
http://cgit.freedesktop.org/xorg/xserver/tree/dix/pixmap.c#n53 ), after
come back to normal, pScreen->pScratchPixmap = pPixmap in
FreeScratchPixmapHeader function
(http://cgit.freedesktop.org/xorg/xserver/tree/dix/pixmap.c#n76 ), I
want to know the root reason.

	I wish someone would help.

Thanks,
Hunk Cui