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

[bibek . patro]

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)
---
 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>