[PATCH v2] swiotlb: introduce Kconfig option for compile-time default pool size
From: bibek . patro
Date: Thu Jul 02 2026 - 10:15:22 EST
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=1, 8 CPUs):
[ 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)
After (CONFIG_SWIOTLB_DEFAULT_SIZE_MB=2, 8 CPUs):
[ 0.000000] software IO TLB: area num 8.
[ 0.000000] software IO TLB: mapped [mem 0x00000000ffdff000-0x00000000fffff000] (2MB)
---
Changes in v2:
- Fix incorrect boot log label in cover letter: the roundup message is
produced by CONFIG_SWIOTLB_DEFAULT_SIZE_MB=1 (not =2) with 8 CPUs.
- Update Documentation/core-api/swiotlb.rst to mention
CONFIG_SWIOTLB_DEFAULT_SIZE_MB as per Michael's suggestion.
- Link to v1: https://patch.msgid.link/20260617-swiotlb-v1-1-abfee3faf4ea@xxxxxxxxxxxxxxxx
To: Jonathan Corbet <corbet@xxxxxxx>
To: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>
To: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
To: Robin Murphy <robin.murphy@xxxxxxx>
Cc: linux-doc@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: iommu@xxxxxxxxxxxxxxx
---
Documentation/core-api/swiotlb.rst | 7 +++++--
include/linux/swiotlb.h | 8 ++++++--
kernel/dma/Kconfig | 22 ++++++++++++++++++++++
3 files changed, 33 insertions(+), 4 deletions(-)
diff --git a/Documentation/core-api/swiotlb.rst b/Documentation/core-api/swiotlb.rst
index 9e0fe027dd3b..71b4e4c27eb5 100644
--- a/Documentation/core-api/swiotlb.rst
+++ b/Documentation/core-api/swiotlb.rst
@@ -140,8 +140,11 @@ Data structures concepts
------------------------
Memory used for swiotlb bounce buffers is allocated from overall system memory
as one or more "pools". The default pool is allocated during system boot with a
-default size of 64 MiB. The default pool size may be modified with the
-"swiotlb=" kernel boot line parameter. The default size may also be adjusted
+default size of 64 MiB, which can be changed at compile time via
+CONFIG_SWIOTLB_DEFAULT_SIZE_MB. The default pool size may also be
+modified at runtime with the "swiotlb=" kernel boot line parameter,
+which takes precedence over the compile-time default. The default size
+may also be adjusted
due to other conditions, such as running in a CoCo VM, as described above. If
CONFIG_SWIOTLB_DYNAMIC is enabled, additional pools may be allocated later in
the life of the system. Each pool must be a contiguous range of physical
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>