Re: [PATCH v5 1/6] dma: swiotlb: Centralize default pool policy selection

[Robin Murphy <robin.murphy@arm.com>]

On 21/09/2026 7:36 am, Aneesh Kumar K.V (Arm) wrote:
[...]
> diff --git a/arch/powerpc/kernel/dma-swiotlb.c b/arch/powerpc/kernel/dma-swiotlb.c
> index ba256c37bcc0..97fffa46f05a 100644
> --- a/arch/powerpc/kernel/dma-swiotlb.c
> +++ b/arch/powerpc/kernel/dma-swiotlb.c
> @@ -14,8 +14,10 @@ unsigned int ppc_swiotlb_flags;
>   
>   void __init swiotlb_detect_4g(void)
>   {
> -	if ((memblock_end_of_DRAM() - 1) > 0xffffffff)
> +	if ((memblock_end_of_DRAM() - 1) > 0xffffffff) {
>   		ppc_swiotlb_enable = 1;

Nit: Perhaps it's beyond the functional scope of this patch, but I do 
wonder if these <arch>_swiotlb_enable conditions become redundant and 
could just be replaced with "if (<arch>_swiotlb_flags != 0)" too.

> +		ppc_swiotlb_flags |= SWIOTLB_INIT_ADDRESSING_LIMIT;
> +	}
>   }
>   
>   static int __init check_swiotlb_enabled(void)
[...]
> diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
> index 75cf8f6ae8cd..0cff255827ba 100644
> --- a/arch/x86/kernel/pci-dma.c
> +++ b/arch/x86/kernel/pci-dma.c
> @@ -44,8 +44,10 @@ static unsigned int x86_swiotlb_flags;
>   static void __init pci_swiotlb_detect(void)
>   {
>   	/* don't initialize swiotlb if iommu=off (no_iommu=1) */
> -	if (!no_iommu && max_possible_pfn > MAX_DMA32_PFN)
> +	if (!no_iommu && max_possible_pfn > MAX_DMA32_PFN) {
>   		x86_swiotlb_enable = true;
> +		x86_swiotlb_flags |= SWIOTLB_INIT_ADDRESSING_LIMIT;
> +	}
>   
>   	/*
>   	 * Set swiotlb to 1 so that bounce buffers are allocated and used for
> @@ -81,8 +83,10 @@ static void __init pci_xen_swiotlb_init(void)
>   	if (!xen_swiotlb_enabled())
>   		return;
>   	x86_swiotlb_enable = true;
> -	x86_swiotlb_flags |= SWIOTLB_ANY;
> -	swiotlb_init_remap(true, x86_swiotlb_flags, xen_swiotlb_fixup);
> +	/* Xen can use a SWIOTLB pool anywhere in directly mapped memory. */
> +	x86_swiotlb_flags &= ~SWIOTLB_INIT_ADDRESSING_LIMIT;
> +	x86_swiotlb_flags |= SWIOTLB_INIT_REMAP | SWIOTLB_ANY;

Similarly, I see how having a SWIOTLB_INIT_REMAP flag keeps 
swiotlb_select_pool_policy() neat, but couldn't we technically already 
infer that from the fact that a non-NULL remap function was passed?

Nits aside, the overall change seems reasonable to me;

Reviewed-by: Robin Murphy <robin.murphy@arm.com>