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

[Michael Kelley]

From: bibek.patro@xxxxxxxxxxxxxxxx <bibek.patro@xxxxxxxxxxxxxxxx> Sent: Tuesday, June 16, 2026 2:36 PM
> 
> 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.
> 
> 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)

This message sequence is surprising to me -- there should not be any
roundup if the default size is 2 MiB and there are 8 CPUs. I ran the patch
with 8 CPUs and set CONFIG_SWIOTLB_DEFAULT_SIZE_MB=2, and
no roundup was reported. I'm curious as to what configuration produced
the "roundup" message.

Also, there's some text in Documentation/core-api/swiotlb.rst that
describes the default size of the swiotlb, and how it can be overridden
with the swiotlb= kernel boot option. This patch should include an update
to that text to mention the new CONFIG option.

Michael 

> ---
>  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>
>