[PATCH v2 3/4] ARM: mm: truncate memory banks to fit in 4GB space for classic MMU

[Nicolas Pitre]

On Wed, 11 Apr 2012, Will Deacon wrote:

> If a bank of memory spanning the 4GB boundary is added on a !CONFIG_LPAE
> kernel then we will hang early during boot since the memory bank will
> have wrapped around to zero.
> 
> This patch truncates memory banks for !LPAE configurations when the end
> address is not representable in 32 bits.
> 
> Cc: Nicolas Pitre <nico at fluxnic.net>
> Signed-off-by: Will Deacon <will.deacon at arm.com>

Acked-by: Nicolas Pitre <nico at linaro.org>

Now what if start = 1G and size = 5G? The size variable is an unsigned 
long, meaning that right now the size might be truncated to 1G.

> ---
>  arch/arm/kernel/setup.c |   16 +++++++++++++++-
>  1 files changed, 15 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> index b914113..ebfac78 100644
> --- a/arch/arm/kernel/setup.c
> +++ b/arch/arm/kernel/setup.c
> @@ -523,7 +523,21 @@ int __init arm_add_memory(phys_addr_t start, unsigned long size)
>  	 */
>  	size -= start & ~PAGE_MASK;
>  	bank->start = PAGE_ALIGN(start);
> -	bank->size  = size & PAGE_MASK;
> +
> +#ifndef CONFIG_LPAE
> +	if (bank->start + size < bank->start) {
> +		printk(KERN_CRIT "Truncating memory at 0x%08llx to fit in "
> +			"32-bit physical address space\n", (long long)start);
> +		/*
> +		 * To ensure bank->start + bank->size is representable in
> +		 * 32 bits, we use ULONG_MAX as the upper limit rather than 4GB.
> +		 * This means we lose a page after masking.
> +		 */
> +		size = ULONG_MAX - bank->start;
> +	}
> +#endif
> +
> +	bank->size = size & PAGE_MASK;
>  
>  	/*
>  	 * Check whether this memory region has non-zero size or
> -- 
> 1.7.4.1
>