Subject: Re: [PATCH] powerpc: Avoid panic during boot due to divide by zero in init_cache_info()

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sun, 05 Mar 2017 11:25:07 +1100

On Sun, 2017-03-05 at 10:54 +1100, Anton Blanchard wrote:
> From: Anton Blanchard <[email-masked]>
> 
> I see a panic in early boot when building with a recent gcc
> toolchain.
> The issue is a divide by zero, which is undefined. Older toolchains
> let us get away with it:

Maybe we should panic though ... not having a valid cache block size is
going to be fatal in other areas...

> int foo(int a) { return a / 0; }
> 
> foo:
> 	li 9,0
> 	divw 3,3,9
> 	extsw 3,3
> 	blr
> 
> But newer ones catch it:
> 
> foo:
> 	trap
> 
> Add a check to avoid the divide by zero.
> 
> Fixes: bd067f83b084 ("powerpc/64: Fix naming of cache block vs. cache
> line")
> Signed-off-by: Anton Blanchard <[email-masked]>
> ---
>  arch/powerpc/kernel/setup_64.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/setup_64.c
> b/arch/powerpc/kernel/setup_64.c
> index adf2084..afd1c26 100644
> --- a/arch/powerpc/kernel/setup_64.c
> +++ b/arch/powerpc/kernel/setup_64.c
> @@ -408,7 +408,8 @@ static void init_cache_info(struct ppc_cache_info
> *info, u32 size, u32 lsize,
>  	info->line_size = lsize;
>  	info->block_size = bsize;
>  	info->log_block_size = __ilog2(bsize);
> -	info->blocks_per_page = PAGE_SIZE / bsize;
> +	if (bsize)
> +		info->blocks_per_page = PAGE_SIZE / bsize;
>  
>  	if (sets == 0)
>  		info->assoc = 0xffff;