Hello all,
I've discussed adding Page Attribute Table (PAT) support to the kernel w/ a few developers offline. They were very supportive and suggested I bring the discussion to lkml so others could get involved.
PAT support allows setting cache attributes via the virtual page table entries that are traditionally set via the MTRRs. The specific cache attribute graphics companies such as ourselves (nvidia), ATI, Matrox, and others are becoming interested in is Write-Combining (WC), both for the AGP and framebuffer apertures. Traditionally, these apertures are marked WC by setting the physical memory ranges to WC in the MTRRs. This has traditionally worked very well, but is becoming a problem with workstation systems with 1+ Gigs of memory.
The problem here is that the system bios typically covers physical ram with Write-Back (WB) MTRRs. On systems with large amounts of physical ram, especially when physical memory ranges can intersperse with ram, the bioses are using multiple MTRRs with strange results. In some cases, enough MTRRs are used to cover physical ram, such that MTRRs are not left over for the AGP or framebuffer apertures. In other cases, 1 MTRR is used to mark non-physical ram as Uncached (which covers both apertures). When trying to mark the appropriate apertures as WC, the kernel refuses to overlap the MTRRs.
Windows works around this MTRR issue by using the PATs.
An example of such a report recently sent to lkml is here:
http://www.ussg.iu.edu/hypermail/linux/kernel/0303.1/0606.html
I discussed this some with Jeff Hartmann, who had some initial development code that was integrated into agpgart, for marking agp pages WC as they were allocated. I think it would be preferable to have pat support seperate from agpgart. In that way, other drivers could make use of PAT support for other means (such as mapping the framebuffer). Jeff Hartmann sent us a pass at adding PAT support to agpgart. We've modified his code slightly to be more generic (standalone from agpgart) and usable via the traditional __pgprot() macros (and therefore with the change_page_attr() function).
Please cc me on any responses, as I'm not on the list.
Thanks,
Terence
This archive was generated by hypermail 2b29 : Fri May 23 2003 - 22:00:41 EST