Re: [PATCH] swiotlb: introduce Kconfig option for compile-time default pool size
From: Bibek Kumar Patro
Date: Thu Jul 02 2026 - 09:44:50 EST
On 6/22/2026 8:15 PM, Bibek Kumar Patro wrote:
On 6/18/2026 9:34 PM, Michael Kelley wrote:
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.
I might have misunderstood your query Michael, you're correct.
I rechecked my system configuration with 8 CPUs and
CONFIG_SWIOTLB_DEFAULT_SIZE_MB=2, and I did not get the "roundup"
message. I did a few experiments while developing the patch initially
with CONFIG_SWIOTLB_DEFAULT_SIZE_MB across a range of values.
CONFIG_SWIOTLB_DEFAULT_SIZE_MB=1 gives the roundup message:
[ 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)
With CONFIG_SWIOTLB_DEFAULT_SIZE_MB=1:
nslabs = CONFIG_SWIOTLB_DEFAULT_SIZE_MB << 9 = 1 << 9 = 512
With 8 CPUs, default_nareas = 8, so the minimum required nslabs is:
IO_TLB_SEGSIZE * default_nareas = 128 * 8 = 1024
Since 512 < 1024, nslabs is rounded up to 1024 (2MB), which explains
the roundup message.
I have incorrectly labelled this log in cover letter by posting the logs for CONFIG_SWIOTLB_DEFAULT_SIZE_MB=1 instead of
CONFIG_SWIOTLB_DEFAULT_SIZE_MB=2
I will correct this info in the cover letter while sending the next
revision along with the documentation update as you suggested.
Thanks & regards,
Bibek
I see inside swiotlb.c, we have a print for rounding off based on the
number of slabs, inside the check for "round_up_default_nslabs()"
pr_info("SWIOTLB bounce buffer size roundup to %luMB",
Probability of getting the print based on the number of slabs.
<I am assuming your configuration is having nslabs in power of 2>
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.
Ack, thanks for this suggestion. I will incorporate in next revision.
It would be a good information for someone trying a similar approach for
setting the default option to custom value.
Thanks & regards,
Bibek
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>