Re: [PATCH] powerpc: Fix -mcmodel=medium breakage in prom_init.c

[Aaro Koskinen <aaro.koskinen@iki.fi>]

Hi,

On Tue, Mar 12, 2013 at 10:51:51PM +1100, Anton Blanchard wrote:
> Commit 5ac47f7a6efb (powerpc: Relocate prom_init.c on 64bit) made
> prom_init.c position independent by manually relocating its entries
> in the TOC.
> 
> We get the address of the TOC entries with the __prom_init_toc_start
> linker symbol. If __prom_init_toc_start ends up as an entry in the
> TOC then we need to add an offset to get the current address. This is
> the case for older toolchains.
> 
> On the other hand, if we have a newer toolchain that supports
> -mcmodel=medium then __prom_init_toc_start will be created by a
> relative offset from r2 (the TOC pointer). Since r2 has already been
> relocated, nothing more needs to be done.  Adding an offset in this
> case is wrong and Aaro Koskinen and Alexander Graf have noticed noticed
> G5 and OpenBIOS breakage.
> 
> Alan Modra suggested we just use r2 to get at the TOC which is simpler
> and works with both old and new toolchains.
> 
> Reported-by: Alexander Graf <agraf@suse.de>
> Signed-off-by: Anton Blanchard <anton@samba.org>
> ---
> 
> Thanks Aaro for reporting this, and Alexander for an initial
> fix. This tested ok for me with both a new and an old toolchain,
> but would appreciate if you could double check it fixes your issues
> too.

This seems to fix the issue also for me, so:

Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>

Thanks,

A.

> diff --git a/arch/powerpc/kernel/prom_init.c b/arch/powerpc/kernel/prom_init.c
> index 7f7fb7f..13f8d16 100644
> --- a/arch/powerpc/kernel/prom_init.c
> +++ b/arch/powerpc/kernel/prom_init.c
> @@ -2832,11 +2832,13 @@ static void unreloc_toc(void)
>  {
>  }
>  #else
> -static void __reloc_toc(void *tocstart, unsigned long offset,
> -			unsigned long nr_entries)
> +static void __reloc_toc(unsigned long offset, unsigned long nr_entries)
>  {
>  	unsigned long i;
> -	unsigned long *toc_entry = (unsigned long *)tocstart;
> +	unsigned long *toc_entry;
> +
> +	/* Get the start of the TOC by using r2 directly. */
> +	asm volatile("addi %0,2,-0x8000" : "=b" (toc_entry));
>  
>  	for (i = 0; i < nr_entries; i++) {
>  		*toc_entry = *toc_entry + offset;
> @@ -2850,8 +2852,7 @@ static void reloc_toc(void)
>  	unsigned long nr_entries =
>  		(__prom_init_toc_end - __prom_init_toc_start) / sizeof(long);
>  
> -	/* Need to add offset to get at __prom_init_toc_start */
> -	__reloc_toc(__prom_init_toc_start + offset, offset, nr_entries);
> +	__reloc_toc(offset, nr_entries);
>  
>  	mb();
>  }
> @@ -2864,8 +2865,7 @@ static void unreloc_toc(void)
>  
>  	mb();
>  
> -	/* __prom_init_toc_start has been relocated, no need to add offset */
> -	__reloc_toc(__prom_init_toc_start, -offset, nr_entries);
> +	__reloc_toc(-offset, nr_entries);
>  }
>  #endif
>  #endif
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