[LinuxBIOS] SPD as debug channel

Peter Stuge stuge-linuxbios at cdy.org
Fri Dec 1 19:46:22 CET 2006

On Fri, Dec 01, 2006 at 07:28:53PM +0100, Stefan Reinauer wrote:
> * Peter Stuge <stuge-linuxbios at cdy.org> [061201 19:16]:
> > The assumption was that not much needs to happen before the SPD I2C
> > bus is accessible by the CPU - is that valid?
> Since you need I2C, you need to get parts of the south bridge
> working.

Even though it's on the DIMM? Ugh. :(

> So, its basically the same amount of work as with the USB debug
> port.

Yeah. Not much point to it then.

> Question is: Will we see systems without USB (debug port) but with
> DDR2 sockets?

See wiki page with list of EHCI chips that lack the debug port. On
the other hand maybe all boards using them will have a serial port?

> > If not, what IS easily accessible besides the boot ROM? (Which we
> > don't want to rely on since we don't know exactly what it will speak
> > when in the future.) We just need one bit that can do kHz signalling.
> > SMI#?
> Boot rom is a good start I bet. That will always be there (as long
> as we all do firmware development at least).
> It might be parallel yesterday, LPC today and SPI tomorrow, but that
> is only the interface it connects to. Different connector, different
> VHDL source and we should be fine, no?

True. TSOPs are a bit unfriendly but still doable with a steady hand.

> > > LPC is going away on many boards, I understand.
> > 
> > But DDR(2) SDRAM will probably stay a while longer. Or not?
> If LPC goes away, we need to do SPI. Ok. If we get DDR3, we would
> have to do that. Technical standards come and go,.. I'd prefer a
> solution that is so cheap that I dont mind throwing the whole kit
> away after doing a port or two, rather than trying to create
> something that is good forever. 

Me too. But the kit only gets that cheap if it can be produced in at
least a run of 100. And while those are too small quantities for even
receiving a quote from some suppliers it's already too big for us. :\

The testing infrastructure is the dealbreaker right now.

> > > I think that we are going into a world where we have to figure
> > > out usb debug port.
> > 
> > It's certainly one good debugging option but maybe not the only
> > good one.
> Especially it does not help for reflashing. Finding something nifty
> here would be nice.

I hear you.


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