On Sun, 14 Oct 2001, Keith Whitwell wrote:
>
> > Yep. What I was trying to understand is that, since I already setup my own
> > region, what can I do to it ? For example, how do I get to "AGP
> > space" addresses (as oppposed to physical or virtual).
>
> Userspace programs always deal in virtual addresses. You need to map the
> section of agp space you're interested in to your process' virtual address
> space - this is the function of drmAddMap/drmMap.
>
> When you call drmAddMap you tell it which bit of agp space you're interested
> in making available to the 3d client (this is done in the 2d driver).
>
> When you call drmMap in the 3d driver, that returns a pointer to the virtual
> mapping of that portion of agp space.
>
> You can't use that pointer to talk to hardware, of course, but you can use it
> to put stuff into the agp memory from your userspace program. The hardware
> will need to know either the offset from the beginning of the agp aperture or
> the physical address. When I say physical address I mean
> (beginning-of-agp-aperture + offset-in-agp-aperture). The agp aperture is
> just a special region of the physical address space.
>
Ok, that's what I wanted to know - thanks.
>
> > Well, for once, the size of AGP aperture is limited. 1 meg of system RAM
> > on 512 meg machine is much less than 1 meg off 32 meg AGP aperture.
>
> Usually they're 64 meg. And there's usually plenty left over - don't worry
> about it.
Are you sure ? I am asking because there are newer cards with 64megs of
RAM and they way AGP was marketed is to allow to extend the amount of
offscreen memory..
>
> > Secondly, if 3d only touches AGP stuff and capture only uses pci-gart
> > tables I can have a separate driver for video capture - and it won't
> > interfere with 3d stuff..
>
> I can't say you're wrong, but if I was doing it I'd work with agpgart first
> simply because all the infrastructure is there and debugged.
>
> > > > * Do we actually need to set AGP registers to translate _all_
> > > > addresses ?
> > >
> > > This sounds like a misconception - the agp aperture is only 32 or 64 meg,
> > > and it's only addresses in that range which get translated. Addresses
> > > outside the aperture are unaffected by agp translations. Otherwise the
> > > whole computer would break???
> >
> > Well, that's what I thought as well, but something confused me.. Hmmm, I
> > don't seem to find the relevant place in the driver - I'll assume it does
> > not exist and move on.
>
> I hope I've explained a little better above. You map the agp region into the
> process virtual address space useing drmAddMap/drmMap, then just get a
> regular old pointer that you can write to and read from (but reads are very
> slow because agp is uncached)
Wait a moment - I thought that was regular RAM, wasn't it ? Is the
slowdown because CPU does not cache it or something else ? If reads are
really slow, then there is no way I can use AGP memory as that's what I
want to do...
thanks
Vladimir Dergachev
>
> Keith
>
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