Subject: Re: [PATCH] Stop pci_set_dma_mask() from failing when RAM doesn't exceed the mask anyway

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sat, 01 Aug 2009 08:25:05 +1000

On Fri, 2009-07-31 at 20:41 +0100, David Woodhouse wrote:
> On an iMac G5, the b43 driver is failing to initialise because trying to
> set the dma mask to 30-bit fails. Even though there's only 512MiB of RAM
> in the machine anyway:
> 	https://bugzilla.redhat.com/show_bug.cgi?id=514787
> 
> We should probably let it succeed if the available RAM in the system
> doesn't exceed the requested limit.
> 
> Signed-off-by: David Woodhouse <[email-masked]>

Also, isn't our iommu code smart enough to clamp allocations to the DMA
mask nowadays ? In that case, we could probably just force iommu on
always...

Cheers,
Ben.

> diff --git a/arch/powerpc/kernel/dma.c b/arch/powerpc/kernel/dma.c
> index 20a60d6..1769a8e 100644
> --- a/arch/powerpc/kernel/dma.c
> +++ b/arch/powerpc/kernel/dma.c
> @@ -90,11 +90,11 @@ static void dma_direct_unmap_sg(struct device *dev,
> struct scatterlist *sg,
>  static int dma_direct_dma_supported(struct device *dev, u64 mask)
>  {
>  #ifdef CONFIG_PPC64
> -	/* Could be improved to check for memory though it better be
> -	 * done via some global so platforms can set the limit in case
> +	extern unsigned long highest_memmap_pfn;
> +	/* Could be improved so platforms can set the limit in case
>  	 * they have limited DMA windows
>  	 */
> -	return mask >= DMA_BIT_MASK(32);
> +	return (mask >> PAGE_SHIFT) >= highest_memmap_pfn;
>  #else
>  	return 1;
>  #endif
>