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

Anshuman Khandual
2026-01-20T09:25:11Z

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@kernel.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
> *