[KLUG Members] Re: XFree86 and AGP4 -- AGP "speeds" != AGP GART

[Bryan J. Smith]

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On Tue, 2003-01-07 at 23:58, Adam Williams wrote:
> Someone asked at tonights newbie meeting (man was that place jumpin') if
> XFree86 could work with AGP4.
> Answer: Yes.  Assuming of course that the card is supported.

Er, remember, speed-wise AGP itself is just a "faster" 32-bit PCI bus.=20
Since it is a single slot per bus (unlike PCI), it can be "throttled" to
different speeds x1 (32-bit x 66MHz =3D 266MBps), 2x (133MHz or 66MHz DDR?
=3D 533MBps), 4x (133MHz DDR =3D 1GBps), 8x (266MHz DDR =3D 2GBps).  This i=
s
done in the kernel, with various chipset support.

No big deal.  In fact, your BIOS usually is the "first contact" for
setup and configuration of this _regardless_ of OS.

Now if you mean the "CPU-like" functionality of AGP that PCI doesn't
have, the GART like sideband addressing, direct memory execution, etc...
that's different.  But that too is controlled by the kernel as well,
with various CPU, chipset and controller GART fucntionality.

Both can be tweaked via the kernel /proc interfaces, or possibly the
XF86Config file (especially for 3rd party drivers, like nVidia's).

History ...

AGP was solely invented (largely by Digital, who also created the first
working PCI controllers) because Intel didn't want to build proper I/O
(i.e. multiple PCI busses) into its chipsets.  It was an engineering
ideal that bombed, because AGP is still a heck of a lot slower than
local, on-controller memory.  In fact, Intergraph regularly slammed even
the best AGPx2 performance with 64-bit, 66MHz PCI video cards
(especially when used in tandem on different PCI busses).

Today AGP video cards act more like CPUs than I/O.  Which, IMHO, is why
they should be on the damn CPU interconnect, and not an I/O one with
"dangerous tricks" (which cause 90% of PC instability).  AMD's
HyperTransport is one such technology that can accommodate this, and I
await some big announcement from AMD-nVidia in 2003.


--=20
Bryan J. Smith, E.I. (BSECE)       Contact Info:  http://thebs.org
[ http://thebs.org/files/resume/BryanJonSmith_certifications.pdf ]
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