Yummy new CPU/GPU combo

Tom Cooksey thomas.cooksey at trolltech.com
Thu Jun 5 12:56:00 CEST 2008

On Thursday 05 June 2008 11:29:10 Carsten Haitzler wrote:
> On Thu, 5 Jun 2008 09:33:58 +0200 Tom Cooksey <thomas.cooksey at trolltech.com>
> babbled:
> > On Wednesday 04 June 2008 21:56:56 Andy Green wrote:
> > > | IMO, OpenMoko's choice of using the the glamo was a big mistake
> > > (Connecting it to a
> > > | shared, 4-bit bus was probably the _biggest_ mistake).
> > > 
> > > Huh what?  It's a 16-bit memory bus, maybe you mean 2^4 ;-)  When I
> > > actually use the thing I don't notice much sluggishness.  Your best bet
> > > is to eyeball one, I think you'll find it isn't the issue you think it is.
> > 
> > I thought it was connected to the (4-wire) SDIO bus?
> it's on the memory bus, not sdio. a gfx chip on sdio would indeed be just about
> the most stupid idea i have ever heard of and anyone doing that should be
> summarily executed with a blunt spoon! :)

Well... yeah, that's kindof the reason for the initial post! I must have misunderstood
previous posts as I was convinced the glamo was itself attached to the CPU's SDIO 
interface. You can understand where I was coming from now!

> > If it's connected to the address/data bus, why is it so slow and why can't
> > DMA be used to copy data to it?
> dma can. it just is even slower than using the cpu. also the entire memory bus
> is locked up when doing a copy with cpu OR dma, so even if dma were the same
> speed as doing a copy with the cpu, we'd still have the cpu locked out of the
> memory bus while waiting for dma - so no gain anyway (as in all practice you
> will need the memory bus :)). actual tests found dma to be LESS than 1/2 the
> speed of doing a copy with the cpu. this is just a limitation of the SOC, and
> the glamo is just well... not fast at accepting read/write requests - the
> bandwidth is limited to about 7mb/sec for writes ( can't remember what reads
> are - but i think its symmetric).

Right. Shame the cpu doesn't have more cache so it has a better change of getting on
with something else while the DMA occurs.

Thanks for clearing things up. It's good to hear that the problem isn't because of
any particularly bad decision on OpenMoko's part. Renews my faith a little. :-)

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