[haiku-bugs] Re: [Haiku] #5383: MTRR regression: AGP transfer inconsistencies
- From: "bonefish" <trac@xxxxxxxxxxxx>
- Date: Tue, 16 Feb 2010 22:10:50 -0000
#5383: MTRR regression: AGP transfer inconsistencies
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Reporter: rudolfc | Owner: nobody
Type: bug | Status: new
Priority: normal | Milestone: R1
Component: - General | Version: R1/Development
Keywords: | Blockedby: 5353
Platform: All | Blocking:
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Changes (by bonefish):
* version: R1/alpha1 => R1/Development
Comment:
The Architecture Compatibility chapter reveals that the PCD and PWT flags
have been introduced in the 486.
Regarding the how to get your driver's RAM buffer working correctly: If WT
is not acceptable for that, then on pre-P6 processors (i.e. the Pentium)
the kernel will have to fall back to UC. No way around that. For Pentium
III and later PAT+PCD+PWT can be used in combination with a single WB MTR.
That leaves Pentium II and Pentium Pro (and supposedly the competition's
equivalents). Either those are sacrificed for sake of simplicity, also
using a single WB MTR with PCD+PWT and therefore having to resort to UC
when WC is requested (which would also hold for the frame buffer!), or
they continue to use MTRRs as best as possible, thus needing to be
special-cased.
--
Ticket URL: <http://dev.haiku-os.org/ticket/5383#comment:5>
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