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

[David Hawkins]

Hi Ame,

> Hello, I saw a reference to PCI booting in the docs, but could
> not find enough detail.  We would like to use PCI memory space
> booting on an upcoming design with the PPC460EX.  Does Uboot
> currently support that mode of booting?  Is it as easy as booting
> from a flash?  We are developing our app with the canyonlands
> board, but we won't be able to try the PCI boot until we have our
> own board finished.

U-Boot is a bootloader. What you are describing here
circumvents needing a bootloader.

I use an MPC8349EA Freescale processor, and it also
has an option to boot over PCI. However, I plan to use
U-Boot on Flash to boot the board. I'll tell you why ...
for my hardware anyway, your reasons may end up being similar.

If you want to boot your board over PCI, then the board
will most likely be a peripheral board (a host would need
to boot and setup bridges, so you would have a hard time
booting a host through a bridge that is not configured).
The host CPU would hold the image of the kernel that you were
planning to boot onto the PCI peripheral board. However,
your host would need to perform all of the tasks normally
performed by the bootloader; setup the memory map, memory
controllers, and peripherals that Linux expects to find
configured. Then even trickier, is to setup the kernel
boot command line arguments and device tree. My guess would
be that you would have to hardwire that info into your kernel
image, or add a small bootloader to the kernel image to
setup the arguments to the kernel proper.
There is also the issue that the host CPU is running an
OS with its MMU enabled, so the kernel image that it
can present to the board over the PCI bus is fragmented.
The only way to make this image linear would be to
preserve a chunk of memory on boot of the OS, and load
the kernel image there for the PCI board to boot from.

There's a lot of code in there that you would have to
maintain on your lonesome. For the price of a flash device,
I would recommend using U-Boot as the bootloader in
Flash on your board. The PCI interface can be used to transfer
a kernel image, and then that image can be booted by the
bootloader. Note that this transfer can account for the
fragmented image on the host by using scatter-gather
DMA into the linear address space of the DDR memory
on the board (since U-Boot is running with a flat
memory map).

Save yourself a lot of trouble, and no community support,
by doing things the standard way (and a good sign is, that
you're already asking questions).

Note that if your processor has the option that when
reset is deasserted, the core can remain in reset, make sure
you implement that feature (on the MPC8349EA its controlled
by the reset configuration words source pin strapping).
This allows you to enable the chip, including the PCI interface,
while the core is reset, and therefore does not try to boot.
The MPC8349EA has default windows over PCI that then allow
a host CPU to manipulate all the memory-mapped registers
visible to the PowerPC core. I've been using this feature of
the MPC8349EA processor to test my  board over PCI from an x86
host. The DDR controller can be enabled/disabled, the clocks
swept, and data patterns DMAed to/from PCI to confirm
the memory controller configuration prior to attempting
a U-Boot port.

Hope this helps.

Cheers,
Dave