Thanks a lot! I would agree with your explanations. However, I have another confusion. In mixed pixmap mode, when the arguments 'w' and 'h' passed to exaCreatePixmap_mixed is either zero, this pixmap will become a driver pixmap through exaCreateDriverPixmap_mixed right away. Usually, in this situation, the exaModifyPixmapHeader_mixed should be called with non-null pPixData. But, exaModifyPixmapHeader_mixed will delete pixmap driver private when pPixData is non-null and set this pixmap pinned. Therefore, I think this(create pixmap driver private, then destroy it without using it) is meaningless. Do I understand it wrong? Thanks! Best wishes <div class="gmail_quote">2010/10/12 Adam Jackson <<a href="mailto:ajax@nwnk.net">ajax@nwnk.net</a>> <blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"> <div class="im">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 ! </div>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 </blockquote></div>