Yummy new CPU/GPU combo

Andy Green andy at openmoko.com
Thu Jun 5 10:29:36 CEST 2008

Hash: SHA1

Somebody in the thread at some point said:
| 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?

We would rightly deserve to get our asses laughed at if we did that.
You're likely thinking about the actual SD Card interface is now via the
Glamo, which has an SD interface poking out of it.  WIFI is hooked to
the CPU SDIO bus and we don't otherwise use these buses.

| 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 be used, I seem to recall someone saying they did something with
it in Xglamo, but PIO isn't where the problems are coming from.

The Glamo offers normal async memory bus interface which we use, but it
has a bunch of timing constraints.  (There is a synchronous burst bus
mode that we don't use because the CPU doesn't support it and adding a
CPLD in there to translate will eat power and doesn't make sense.)

In addition, we currently slug the Glamo IO with quite heavy wait states
to get stable operation; without it the Glamo acts weird in ways we
never got to the bottom of.  A few weeks ago I experimented again with
reducing these and saw all kinds of framebuffer corruption.  The Glamo
has several processors in it sharing its internal DRAM (6 or so), I
guess there is some stuff going on from that direction despite we hold
the unused ones in reset in there.

So unless our understanding of that beastie evolves further than the
datasheets, the current speed of it is what we have to work with.
However like I said when you hold the thing and use it, IMO it's
perfectly adequate.  The "August" Software Update ;-) has some cool
alpha and moving things going on that look nice, it's just not so bad as
you got the impression I believe.

- -Andy
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