Re: [PATCH] swiotlb: introduce Kconfig option for compile-time default pool size

[Mostafa Saleh]

On Tue, Jun 16, 2026 at 10:36 PM <bibek.patro@xxxxxxxxxxxxxxxx> wrote:
>
> From: Jagadeesh Pagadala <jpagadal@xxxxxxxxxxxxxxxx>
>
> The SWIOTLB bounce buffer pool size is hardcoded at 64 MB via
> IO_TLB_DEFAULT_SIZE with no compile-time knob to adjust it. On
> memory-constrained embedded or mobile platforms equipped with a
> hardware IOMMU (e.g., ARM SMMU) covering most DMA-capable devices,
> reserving 64 MB at boot is unnecessarily wasteful — the SWIOTLB is
> only exercised for devices that bypass the IOMMU or have restricted
> DMA address ranges.
>
> Introduce CONFIG_SWIOTLB_DEFAULT_SIZE_MB, an integer Kconfig option
> (range 1–64 MB, default 64) that allows platforms to set a smaller
> compile-time default. IO_TLB_DEFAULT_SIZE is updated to derive from
> this value when CONFIG_SWIOTLB is enabled, preserving the existing
> 64 MB default when the option is not configured.
>
> The runtime "swiotlb=<nslabs>" kernel parameter override remains
> fully supported and takes precedence over the compile-time default.
>

Just curious, why is not "swiotlb=" enough in your case?

Thanks,
Mostafa

> Signed-off-by: Jagadeesh Pagadala <jpagadal@xxxxxxxxxxxxxxxx>
> Signed-off-by: Bibek Kumar Patro <bibek.patro@xxxxxxxxxxxxxxxx>
> ---
> The SWIOTLB bounce buffer pool size is hardcoded at 64 MB. On
> memory-constrained platforms with a hardware IOMMU (e.g., ARM SMMU),
> this reservation is wasteful as SWIOTLB is only needed for devices
> that bypass the IOMMU or have restricted DMA address ranges.
>
> Introduce CONFIG_SWIOTLB_DEFAULT_SIZE_MB (range 1–64 MB, default 64)
> to allow a smaller compile-time default. The runtime "swiotlb="
> parameter override remains supported and takes precedence.
>
> Before (default 64 MB):
>   [    0.000000] software IO TLB: area num 8.
>   [    0.000000] software IO TLB: mapped [mem 0x00000000fbfff000-0x00000000fffff000] (64MB)
>
> After (CONFIG_SWIOTLB_DEFAULT_SIZE_MB=2):
>   [    0.000000] software IO TLB: area num 8.
>   [    0.000000] software IO TLB: SWIOTLB bounce buffer size roundup to 2MB
>   [    0.000000] software IO TLB: mapped [mem 0x00000000ffdff000-0x00000000fffff000] (2MB)
> ---
>  include/linux/swiotlb.h |  8 ++++++--
>  kernel/dma/Kconfig      | 22 ++++++++++++++++++++++
>  2 files changed, 28 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index 3dae0f592063..1665a9ce8f94 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -32,8 +32,12 @@ struct scatterlist;
>  #define IO_TLB_SHIFT 11
>  #define IO_TLB_SIZE (1 << IO_TLB_SHIFT)
>
> -/* default to 64MB */
> -#define IO_TLB_DEFAULT_SIZE (64UL<<20)
> +/* compile-time default; overridable via CONFIG_SWIOTLB_DEFAULT_SIZE_MB */
> +#ifdef CONFIG_SWIOTLB
> +#define IO_TLB_DEFAULT_SIZE ((unsigned long)CONFIG_SWIOTLB_DEFAULT_SIZE_MB << 20)
> +#else
> +#define IO_TLB_DEFAULT_SIZE (64UL << 20)
> +#endif
>
>  unsigned long swiotlb_size_or_default(void);
>  void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
> diff --git a/kernel/dma/Kconfig b/kernel/dma/Kconfig
> index 0a4ba21a57a7..3830a63ae032 100644
> --- a/kernel/dma/Kconfig
> +++ b/kernel/dma/Kconfig
> @@ -86,6 +86,28 @@ config SWIOTLB
>         bool
>         select NEED_DMA_MAP_STATE
>
> +config SWIOTLB_DEFAULT_SIZE_MB
> +       int "Default SWIOTLB bounce buffer size in MB"
> +       depends on SWIOTLB
> +       range 1 64
> +       default 64
> +       help
> +         Sets the default size of the software IO TLB (SWIOTLB) bounce buffer
> +         pool allocated at boot time. The default is 64 MB.
> +
> +         On memory-constrained embedded or mobile platforms (e.g., those with
> +         a hardware IOMMU such as ARM SMMU covering most DMA-capable devices),
> +         a smaller value such as 4 or 8 MB may be sufficient. The SWIOTLB is
> +         then only needed for devices that bypass the IOMMU or have restricted
> +         DMA address ranges.
> +
> +         The minimum allowed value is 1 MB. This compile-time default can be
> +         overridden at runtime using the "swiotlb=<nslabs>" kernel command line
> +         parameter. Refer to Documentation/admin-guide/kernel-parameters.txt
> +         for details.
> +
> +         If unsure, leave at the default value of 64.
> +
>  config SWIOTLB_DYNAMIC
>         bool "Dynamic allocation of DMA bounce buffers"
>         default n
>
> ---
> base-commit: 4fa3f5fabb30bf00d7475d5a33459ea83d639bf9
> change-id: 20260617-swiotlb-c215ce0b23f7
>
> Best regards,
> --
> Bibek Kumar Patro <bibek.patro@xxxxxxxxxxxxxxxx>
>
>