Messages in this thread | | | Date | Mon, 15 Mar 1999 08:24:18 +1100 | From | Richard Gooch <> | Subject | Re: MTRR & NVTNT |
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Alan Cox writes: > > I think (2) *is* going to be more complex than (1). More code and more > > data. And for what benefit? Not that many people have multiple video > > cards, and those who do probably have CPUs with >2 MTRRs. Even the el > > Why are you obsessed with MTRR's and video cards. The Centaur C6 > wants some of the MTRR's mapping main memory for performance. There > are plenty of other memory mapped cards. Almost any device with a > block shared memory buffer will benefit.
Hm. "Obsessed" is an unkind word. I just haven't seen many other cards where MTRRs would be an advantage. For example, I don't see a use for MTRRs with frame grabbers because you read from these cards. Making frame-grabber memory cachable is probably unwise. It seems to me that most cards with mappable memory provide it for reading.
Cards which come to mind that have writable memory are: - framebuffers - network cards - ???
Would write-combining help for NICs? When I timed the data-transfer function for a network driver (admittedly a cheap PCI NE2000), it took a millisecond to transfer a full packet. That's roughly a megabyte per second. Write-combining won't help for such a shite interface. Is that the exception or the rule?
And then there is the main RAM case. Here the BIOS is supposed to set up the MTRRs. Is it wise to (automatically) fiddle with that? If I disable caching in the BIOS, I don't want the kernel enabling automatically either.
Anyway, if you don't want me to "obsess", then why not list the cards where MTRRs would be useful and why? It would help if I had a picture of the things you have in mind.
> > cheapo Centaurs have 8. Time for AMD to pull their finger out... > > AMD are quite obviously aiming at a standard desktop PC mentality, > one MTRR for the video ram, one for the texture ram, and a seperate > MSRR for the RAM area.
What would they do then for a linear FB with the last page filled with control registers? Looks like you often need two MTRRs for the basic card before you even start thinking about texture RAM.
Regards,
Richard....
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