Quoting Colin D Bennett, who wrote the following on Mon, 28 Jun 2010:
On Fri, 25 Jun 2010 09:58:34 +0100 Colin Watson <cjwat...@ubuntu.com> wrote:I recently posted ("Subject: [PATCH] Optimise memset on i386" - sorry, I don't seem to have a route to lists.gnu.org at the moment so I can't post an archive link) about optimising GRUB's video initialisation, and hinted that it might be possible to do better by implementing MTRRs as well in order to allow the system to combine writes to video memory rather than taking a cache stall for every single write. I can report that, at least on the hardware I was using, it does make a significant difference: filling the screen with solid colour now takes 10 milliseconds rather than 160! This ended up shaving about a second off the boot time of the project I'm working on.In addition to the improved startup speed, I see the potential for a huge increase in graphical menu responsiveness if caching is enabled. Would framebuffer draw and image blitting performance be improved by using write-combining with MTRRs?
Yes, I think it would. I was thinking about this over the weekend and I think if we examine the PCI BARs and ensure that the memory we're adding to the variable-length MTRRs is marked prefetchable, that should exclude those cards that have registers mapped in that area (besides, I seriously doubt there would be registers in the framebuffer section of memory -- that would just be counterproductive for any graphics adapter), and should be the safety check vladimir's asking for as well.
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