Loading kernel on MPC86x

[Martin Hinner]

On Mon, Aug 26, 2013 at 7:14 PM, Scott Wood <scottwood at freescale.com> wrote:
>> that kernel must be at location 0. Another problem was that interrupts
>> got re-enabled during execution of my bootloader (I am doing some
>> syscalls -> goes  to Cisco rom),
> Do you mean you're calling into the rom after Linux has already started
> executing?  That's not normal for 8xx.

No, in bootloader. I have disabled interrupts and then later did a
syscall which probably enabled them again. As I have overwritten some
of Cisco ROM data it crashed (at random place).

>> I am also curious why CONFIG_PPC_EARLY_DEBUG_CPM uses
>> CONFIG_PPC_EARLY_DEBUG_CPM_ADDR as pointer to transmit SMC buffer and
>> not address of CPM/SCM parameter RAM ? TX buffer address can be read
>> from SMC parameter RAM. Wouldn't this solution be more portable? At
>> least this way I do it when I take over console from Cisco
>> startup/rommon.
>
> The point was to keep things as simple as possible (e.g. for use in
> temporary handcoded asm as needed).  This is a hacky debugging feature
> that assumes you know what you're doing and can set the address to match
> what the loader does (and that the loader's choice of address is
> static).  If you have an improvement that keeps it simple, feel free to
> send a patch.

How about making CONFIG_PPC_EARLY_DEBUG_CPM_PARRAM that woud carry
address of SMCx parameter RAM (IMMR+0x04180 on MPC866) and this value
would be used in case CONFIG_PPC_EARLY_DEBUG_CPM_ADDR is zero ? This
would allow kernel hackers to still use
CONFIG_PPC_EARLY_DEBUG_CPM_ADDR for assembly debugging (+legacy use)
and everyone else can use it as a more reliable option that does not
rely on particular bootloader behavior. Early debug is good even for
end-users so as they can send debug output if anything goes wrong at
early stage.

Anyway, difference between _PARRAM and _ADDR is only one lwz
instruction, so I guess it is possible to completely discard _ADDR if
there is no legacy use for it. I am also not sure if this works with
SCC UART ports or only CPM SMC UART.

Martin