so ... AGP { 1x, 2x } = PCI (o'really?)

[DiTBho]

I've always wondered - was/is AGP only ever used for graphics cards? - (never seen ... say AGP-SCSI-HBA? AGP-optical-ethernet? ... )

Found this interesting answer 

[DiTBho]

I will buy an AGP(1)-PCI adapter to make good use of an - otherwise - unused AGP slot.


(1) AGP 2x

[DavidAlfa]

Not same as PCI at all.
The main reason these graphics buses existed (vesa, agp..) was to provide a dedicated bus, PCI shares its bandwidth with all devices, these don't.
Then AGP 1.0 was already 2x faster than PCI.

[DiTBho]

say,

AGP = exclusive channel
PCI = bus

timing and signal protocol are similar... and =< AGP-2x electrical signals also seem compatible

bandwidth

but this comment is exactly what I have always wondered

so it is just about bandwidth (and maybe latency) of AGP, or is it exactly about separate, non-coherent channel to system RAM?

"Non-coherent" means for me that it bypasses routing (in PCI, DMA might be targeted to another device and not necessarily system RAM) and bypasses cache coherence (DMA writes must at least invalidate cache lines that contain data the DMA writes to, and DMA reads must suck data from caches in writeback mode).

(my evil plan is to make good use of a 2x AGP slot with a PCI-fpga card  )

[DavidAlfa]

PCI works at 33MHz, AGP starts at 66, so no way.
I guess you're making some kind of vintage adapter...?
Otherwise, why using something so old?

[DiTBho]

PCI works at 33MHz, AGP starts at 66, so no way.

isn't the AGP clock "adaptable" to the board clock?

66Mhz means ... PCI-X, good, but definitively it greatly reduces the number of devices I can use, and my pci-fpga card is clocked at 33Mhz.

I guess you're making some kind of vintage adapter...?
Otherwise, why using something so old?

Supporting GNU/Linux on 2000-2008 RISC-boards.

[DavidAlfa]

There's the opposite, AGP to PCI converters (Run AGP card in PCI slot):
https://www.vogons.org/viewtopic.php?t=76665

You might be lucky, apparently agp runs at 33MHz if not configured properly:
https://wiki.osdev.org/AGP

Still, it seems a long and painful journey!

[ejeffrey]

Not same as PCI at all.
The main reason these graphics buses existed (vesa, agp..) was to provide a dedicated bus, PCI shares its bandwidth with all devices, these don't.
Then AGP 1.0 was already 2x faster than PCI.

Well AGP was based on PCI, appeared to the host largely as a second PCI bus that happens to have only one device, and PCI existed with 66 MHz and 3.3V variants which were widely used in high performance systems, just not in mainstream desktops.  AGP did have AGP specific transactions, including split read transactions (which were also added to PCI as part of PCI-X) and to access the AGP memory window which let the card use system memory for texture storage avoiding the need for lots of onboard memory.  But AFAIK, only a few cards made heavy use of that, as memory costs dropped and the performance advantages of having on board texture memory were too great to ignore.

Later revisions of AGP increased to speeds that PCI never supported, but it remained fundamentally based on PCI both at the physical and transaction model levels.