[U-Boot-Users] pci memory booting on ppc460

[Stefan Roese <sr@denx.de>]

On Wednesday 23 April 2008, David Hawkins wrote:
> Here's a few ideas then.
>
> 1) Use the main host to setup DDR on each of the targets,
>     copy a U-Boot image to each target, and then enable the
>     targets to boot from DDR.

I don't see how this could be done. The SDRAM has to be setup by the CPU 
itself and can't be setup via another PCI host.

>     The target U-Boot versions would have to skip all DDR
>     initialization, since they are running from DDR.
>
>     I believe this code would come under ...
>
> http://www.denx.de/wiki/view/DULG/CanUBootBeConfiguredSuchThatItCanBeStarte
>dInRAM
>
>     Which contains the warning:
>
>     'But it is difficult, unsupported, and fraught with peril.'
>
>     Wolfgang and Stephan could probably comment more on this.
>
> 2) Configure an inbound translation window on the host to point
>     a PCI window to its *flash*.
>
>     Configure the outbound translation windows on the targets to
>     point to the host inbound window.

This is probably only possible from the CPU itself too. And where should the 
code come from to do this?

>     Each target would then boot from the same flash as the host.
>     Assuming there was some I/O to distinguish the targets from the
>     host, the code could determine what to do at runtime.
>
>     This has the disadvantage that all boards are booting from
>     potentially slower flash memory, but that may not matter.
>
> 3) Configure the host DDR memory such that the OS does not use
>     all installed memory, eg. reserves enough memory for a U-Boot
>     image.
>
>     Setup the host inbound translation window to point to that
>     linear DDR region, and have the host copy the image there.
>
>     Setup the targets as in (2) and let them boot.
>
> The nice thing about (2) and (3) is that the target processors are
> effectively in the 'virgin' state that flash booting expects, so
> the modifications to U-Boot required to support the booting scheme
> would be minimal.

I don't think this is possible. To configure the translation windows, code is 
needed on all PPC's.

> 4) Another option would be to place SRAM on each of the other
>     processors, and copy the U-Boot image there. That way flash
>     on the targets is replaced with SRAM.
>
>     However, this would require a hardware change, and is not
>     much of an improvement.

SRAM or FLASH, no big difference here. The only advantage is that there is no 
need to update all target FLASH's.

Another idea would be to put only one NOR FLASH for all target PPC's on the 
board. Then you "release" (put out of reset) all target PPC's from the host 
PPC in a sequential order. This way there should be no problem sharing the 
NOR FLASH.

Best regards,
Stefan

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