bug in lm32 port

[Jukka Pietarinen]

Hi Michael,

Michael Walle wrote:
> there is a bug in the lm32 port in the startup routines. There are
> immediate branches to the isr handler and main function.
> 
> On lm32 the opcode can encode 26bits adresses in an immediate branch,
> which permits relative jumps of 64 MiB.
> 
> The startup code is divided into a .boot and a .text section. The use of
> immediate branches implies, that the 'distance' between these two sections
> (and the section that holds the isr handler) must not be greater than
> 64mb.
> 
> So if, for example, one defines the .boot section in the embedded block
> ram to be at 0x00000000 and the .text section in flash/sram (whatever) at
> 0x40000000, he gets the following error:
> (.boot+0xc0): relocation truncated to fit: R_LM32_CALL against symbol
> `_ISR_Handler' defined in .text section ...
> 
> Proposed fix:
>  - move crt0 in libbsp/lm32/shared/start/start.S to .boot
>  - use indirect branches
> 
> But there is a problem with the ISR handler call: when the cpu jumps to
> the interrupt vector, no registers have been saved yet. So we cannot use
> indirect jumps. We can't even use register r0 (which is supposed to be
> zero) because the "mvhi rX, imm16", which we need for indirect branches,
> is just a pseudo operation for "orhi rX, r0, imm16".
> 
> Some Thoughts:
>  - we could change EBA (which is the exception base address) in the
> bootcode and branch to it right after power up/reset. so our 'real'
> exception vectors are no longer in the .boot section (well .. that
> doesn't resolve the problem, but relocate it)
>  - some support code in the .boot section that saves some registers onto
> the stack
> 
> Any other ideas?

We could move both crt0 and _ISR_Handler to the .boot section. Wouldn't 
that work?

I just noticed crt0 does use immediate call to boot_card...

Thanks,
Jukka