Re: Use of r31 on the powerpc/booke:mpc85xx architecture

On Sep 6, 2012, at 5:11 PM, Scislowicz, Adam wrote:

> 
> In sys/arch/powerpc/booke/booke_pmap.c:pmap_copy_page() The r31 register is used. When compiling for DDB support with "-fno-omit-frame-pointer" the compiler complains about the use of register 31. I am new to this architecture but it seems that register is used as the frame pointer.

Actually, r1 is the stack pointer and frame pointer.  There is no
reason to use -fno-omit-frame-pointer on powerpc.

> If you modify the inline assembly to use r23-r30 instead of r24-r31 it seems to solve the problem, but I wanted to ask if this is the right way to change this particular code as I am new to this architecture and don't want to introduce any unpredictable side effects when making the change locally.

It doesn't solve the problem since ldm/stwm will always write/read r31 anyway
so you are just corrupting memory.

> It also seems that DDB support may not be functioning yet on the powerpc/booke architecture. Is this the known state of things with this port? I am using a P2020RDB-like system and using the P2020RDB config file which references the board code in mpc85xx. While the system otherwise works to some extent: I can boot and use user-space tools when not using DDB. However when I compile with DDB support I get a trap fairly early in the kernel initialization process and then my watchdog reboots the system from out of under me.

DDB works fine.  Always has.  Couldn't have developed it without DDB.
What is it the trap?