Re: [PATCH] x86: enable swiotlb for > 4GiG ram on 32-bit kernels

[Thomas Gleixner]

On Sun, 14 Oct 2018, Christoph Hellwig wrote:

> We already build the swiotlb code for 32b-t kernels with PAE support,
> but the code to actually use swiotlb has only been enabled for 64-bit
> kernel for an unknown reason.
> 
> Before Linux 4.18 we papers over this fact because the networking code,
> the scsi layer and some random block drivers implenented their own
> bounce buffering scheme.
> 
> Fixes: 21e07dba ("scsi: reduce use of block bounce buffers")
> Fixes: ab74cfeb ("net: remove the PCI_DMA_BUS_IS_PHYS check in illegal_highdma")
> Reported-by: tedheadster <tedheadster@xxxxxxxxx>
> Tested-by: tedheadster <tedheadster@xxxxxxxxx>

I'll add your SOB when picking this up :)

> ---
>  arch/x86/kernel/pci-swiotlb.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/pci-swiotlb.c b/arch/x86/kernel/pci-swiotlb.c
> index 661583662430..71c0b01d93b1 100644
> --- a/arch/x86/kernel/pci-swiotlb.c
> +++ b/arch/x86/kernel/pci-swiotlb.c
> @@ -42,10 +42,8 @@ IOMMU_INIT_FINISH(pci_swiotlb_detect_override,
>  int __init pci_swiotlb_detect_4gb(void)
>  {
>  	/* don't initialize swiotlb if iommu=off (no_iommu=1) */
> -#ifdef CONFIG_X86_64
>  	if (!no_iommu && max_possible_pfn > MAX_DMA32_PFN)
>  		swiotlb = 1;
> -#endif
>  
>  	/*
>  	 * If SME is active then swiotlb will be set to 1 so that bounce
> -- 
> 2.19.1
> 
>