[Open-graphics] AGP, PCI or PCI-express

[Nicolas Boulay]

2006/8/21, Jon Smirl <jonsmirl at gmail.com>:
> On 8/21/06, Timothy Miller <theosib at gmail.com> wrote:
> > On 8/20/06, Jon Smirl <jonsmirl at gmail.com> wrote:
> > > What bus interface is the card going to have? PCI is hopelessly slow
> > > for graphics.
> >
> > No, it's not.  Yes, it's bloody slow, but we've already done the
> > calculations, and it's just barely fast enough.  But only if you use
> > DMA.  (DMA makes the transfer a few % faster, but mostly, it just
> > frees up the rest of the system to do useful work.)
>
> You might want to poll your consumers and see if they are going to buy
> a PCI board. I only use PCI versions of cards for test and
> development. They are visibly too slow for normal use.
>

The poll was done many month ago. For developpement and for the first
run, PCI is more than enough.

> There is also the problem with bus saturation. If I have a PCI
> graphics card running at full speed it eats all of the bus bandwidth
> from other PCI devices. Internal IDE controllers are often on the same
> PCI bus even though they are on the motherboard.
>

IDE controller are now inside the southbridge and independent of the
main PCI bus.

> This is why we are disconnecting on discussions about frequency of DMA
> use. With AGP around DMA is less relevant.

It's same problem of masking transfert latency for parrallel operation
between the GPU and the CPU.

>
> > I remember when PCI was the rage.
> >
> > Anyhow, I've been writing graphics drivers for PCI cards for a LONG
> > time, even on systems with severe limitations like Sparc, and have
> > been able to get excellent performance.  If your bus is slow, you just
> > have to make efficient use of it.
> >
> > > If you do AGP you also need AGP port support.
> > >
> > > PCI-express needs the PCI-express way of mapping VRAM into the CPU/GPU
> > > address spaces.
> >
> > What is the "PCI-express way"?
>
> The GPU operates in it's own internal 32b address space. There are
> still address translation tables on the graphics card mapping from
> internal addresses to system addresses.
>
> My suggestion would be to go PCI express and make the card adapt from
> 1/4/8 lanes. That way I can plug it into an extra 1 lane socket for
> development and an 8/16 lane socket for general use. There is no real
> gain going from 8 to 16 lanes for any existing apps.
>

Maybe for the asic... If the first asic is plained to be sell a long
time in the mass market.