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

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

[bibek.patro]

From: Jagadeesh Pagadala <jpagadal@qti.qualcomm.com>

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@qti.qualcomm.com>
Signed-off-by: Bibek Kumar Patro <bibek.patro@oss.qualcomm.com>
---
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@oss.qualcomm.com>

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@oss.qualcomm.com> wrote:
>
> From: Jagadeesh Pagadala <jpagadal@qti.qualcomm.com>
>
> 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@qti.qualcomm.com>
> Signed-off-by: Bibek Kumar Patro <bibek.patro@oss.qualcomm.com>
> ---
> 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@oss.qualcomm.com>
>
>

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

[Bibek Kumar Patro]

On 6/17/2026 2:17 PM, Mostafa Saleh wrote:
> On Tue, Jun 16, 2026 at 10:36 PM <bibek.patro@oss.qualcomm.com> wrote:
>>
>> From: Jagadeesh Pagadala <jpagadal@qti.qualcomm.com>
>>
>> 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?
> 

We are looking for an approach where we can set a default value of
IO_TLB_DEFAULT_SIZE to maintain and sustain it for a
specific platform or a kernel configuration.

So introducing a Kconfig tunable seemed to be a maintainable approach
compared to "swiotlb=" runtime variable.
(since defconfig is the right mechanism for platform specific 
compile-time defaults)

Further in case of any bootloader constraints - to make default runtime 
parameter, it might require firmware changes on production devices as 
well. (since in embedded/mobile platforms the kernel command line is 
typically managed by the bootloader)

Thanks & regards,
Bibek

> Thanks,
> Mostafa
> 
>> Signed-off-by: Jagadeesh Pagadala <jpagadal@qti.qualcomm.com>
>> Signed-off-by: Bibek Kumar Patro <bibek.patro@oss.qualcomm.com>
>> ---
>> 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@oss.qualcomm.com>
>>
>>

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

[Michael Kelley]

From: bibek.patro@oss.qualcomm.com <bibek.patro@oss.qualcomm.com> Sent: Tuesday, June 16, 2026 2:36 PM
> 
> From: Jagadeesh Pagadala <jpagadal@qti.qualcomm.com>
> 
> 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@qti.qualcomm.com>
> Signed-off-by: Bibek Kumar Patro <bibek.patro@oss.qualcomm.com>
> ---
> 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@oss.qualcomm.com>
> 

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

[Bibek Kumar Patro]

On 6/18/2026 9:34 PM, Michael Kelley wrote:
> From: bibek.patro@oss.qualcomm.com <bibek.patro@oss.qualcomm.com> Sent: Tuesday, June 16, 2026 2:36 PM
>>
>> From: Jagadeesh Pagadala <jpagadal@qti.qualcomm.com>
>>
>> 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@qti.qualcomm.com>
>> Signed-off-by: Bibek Kumar Patro <bibek.patro@oss.qualcomm.com>
>> ---
>> 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 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@oss.qualcomm.com>
>>
> 

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

[Bibek Kumar Patro]

On 6/22/2026 8:15 PM, Bibek Kumar Patro wrote:
> 
> 
> On 6/18/2026 9:34 PM, Michael Kelley wrote:
>> From: bibek.patro@oss.qualcomm.com <bibek.patro@oss.qualcomm.com> 
>> Sent: Tuesday, June 16, 2026 2:36 PM
>>>
>>> From: Jagadeesh Pagadala <jpagadal@qti.qualcomm.com>
>>>
>>> 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@qti.qualcomm.com>
>>> Signed-off-by: Bibek Kumar Patro <bibek.patro@oss.qualcomm.com>
>>> ---
>>> 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@oss.qualcomm.com>
>>>
>>
> 

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

[Michael Kelley]

From: Bibek Kumar Patro <bibek.patro@oss.qualcomm.com> Sent: Thursday, July 2, 2026 6:45 AM
> 
> On 6/22/2026 8:15 PM, Bibek Kumar Patro wrote:
> >
> >
> > On 6/18/2026 9:34 PM, Michael Kelley wrote:
> >> From: bibek.patro@oss.qualcomm.com <bibek.patro@oss.qualcomm.com>
> >> Sent: Tuesday, June 16, 2026 2:36 PM
> >>>
> >>> From: Jagadeesh Pagadala <jpagadal@qti.qualcomm.com>
> >>>
> >>> 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@qti.qualcomm.com>
> >>> Signed-off-by: Bibek Kumar Patro <bibek.patro@oss.qualcomm.com>
> >>> ---
> >>> 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. That all makes sense now.

Michael