linux-kernel - Re: [PATCH] arm64: swiotlb: Don’t shrink default buffer when bounce is forced

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Message-ID: <79a50bba-2805-4901-a258-817c584fff96@arm.com>
Date: Tue, 20 Jan 2026 14:55:05 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: "Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 iommu@...ts.linux.dev
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Marek Szyprowski <m.szyprowski@...sung.com>,
 Robin Murphy <robin.murphy@....com>, suzuki.poulose@....com,
 steven.price@....com
Subject: Re: [PATCH] arm64: swiotlb: Don’t shrink default buffer when bounce is forced

On 20/01/26 12:31 PM, Aneesh Kumar K.V (Arm) wrote:
> arm64 reduces the default swiotlb size (for unaligned kmalloc()
> bouncing) when it detects that no swiotlb bouncing is needed.
> 
> If swiotlb bouncing is explicitly forced via the command line
> (swiotlb=force), this heuristic must not apply. Add a swiotlb helper to
> query the forced-bounce state and use it to skip the resize when
> bouncing is forced.

Makes sense not to reduce the SWIOTLB buffer size when being
forced by the administrator.

> 
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@...nel.org>
> ---
>  arch/arm64/mm/init.c    | 3 ++-
>  include/linux/swiotlb.h | 7 +++++++
>  kernel/dma/swiotlb.c    | 5 +++++
>  3 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 524d34a0e921..7046241b47b8 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -345,7 +345,8 @@ void __init arch_mm_preinit(void)
>  		flags |= SWIOTLB_FORCE;
>  	}
>  
> -	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && !swiotlb) {
> +	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) &&
> +	    !(swiotlb || force_swiotlb_bounce())) {
>  		/*
>  		 * If no bouncing needed for ZONE_DMA, reduce the swiotlb
>  		 * buffer for kmalloc() bouncing to 1MB per 1GB of RAM.

Should the comment here be updated as well ?

> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index 3dae0f592063..513a93dcbdbc 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -185,6 +185,7 @@ bool is_swiotlb_active(struct device *dev);
>  void __init swiotlb_adjust_size(unsigned long size);
>  phys_addr_t default_swiotlb_base(void);
>  phys_addr_t default_swiotlb_limit(void);
> +bool force_swiotlb_bounce(void);
>  #else
>  static inline void swiotlb_init(bool addressing_limited, unsigned int flags)
>  {
> @@ -234,6 +235,12 @@ static inline phys_addr_t default_swiotlb_limit(void)
>  {
>  	return 0;
>  }
> +
> +static inline bool force_swiotlb_bounce(void)
> +{
> +	return false;
> +}
> +
>  #endif /* CONFIG_SWIOTLB */
>  
>  phys_addr_t swiotlb_tbl_map_single(struct device *hwdev, phys_addr_t phys,
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 0d37da3d95b6..85e31f228cc9 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1646,6 +1646,11 @@ phys_addr_t default_swiotlb_base(void)
>  	return io_tlb_default_mem.defpool.start;
>  }
>  
> +bool force_swiotlb_bounce(void)
> +{
> +	return swiotlb_force_bounce;
> +}
> +
>  /**
>   * default_swiotlb_limit() - get the address limit of the default SWIOTLB
>   *