Re: kernel course no. 1

Hi again,
On Thu, Aug 30, 2001 at 05:29:04PM +0200, Benjamin Herrenschmidt wrote:

quoted

I am checking kernel source from penguinppc.org (development tree), and try to
boot it... but, when I checked code in head_4xx.h I saw that code from
'_start' doesn't jump to 'start_here' because of 'strange coding' (sorry, but
it really seems strange to me ;-):

li	r4,MSR_KERNEL
lis	r3,start_kernel@h
ori	r3,r3,start_kernel@l
mtspr	SRR0,r3
mtspr	SRR1,r4
rfi /* enable MMU and jump to start_kernel */

In this part it stops... why don't we just use 'bl start_here'? I tried and it
works, but seems strange why would it be so complicated without a reason. Is
there a reason? :-)

Yes, and a big one ! Normally, at that point, you are running with MMU off,
you need to re-enable it before entering the kernel, that's what the "rfi"
does for you when the proper value is set into SRR1.

OK, thanks for information (I presumed that it is 'catch' with rfi :-), but
I've got problem with booting my kernel anyway. I was wondering why would
booting stop when this 'rfi' occurs... I was thinking it might be problem with
the stack, but I'm not be sure. I don't know how to tell kernel where the
stack 'lies' in memory (I don't use boot that comes with kernel).
Could problem be somewhere else?

[snip]

quoted

OK, just one more thing... is there any 'roadmap' or some other kind of
documentation that would describe this early stage of kernel booting? (like
at first it is '_start', which do this and this and then jumps to
'start_here' which does initialization of... etc etc).

?

Thanks for any answers...

best wishes, Jure


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Jure Menart writes:

OK, thanks for information (I presumed that it is 'catch' with rfi :-), but
I've got problem with booting my kernel anyway. I was wondering why would
booting stop when this 'rfi' occurs...

How do you know that it stops there exactly?

Paul.

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