dma/swiotlb: introduce Kconfig option for compile-time default pool size

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.

Jagadeesh Pagadala committed 4 months ago

Commit 3cf67bb1 parent 01886163 files · +33 −4Browse files

Unified · Split

3 files

Documentation/core-api/swiotlb.rst +5 −2
include/linux/swiotlb.h +6 −2
kernel/dma/Kconfig +22 −0

Documentation/core-api/swiotlb.rst+5−2

@@ -140,8 +140,11 @@ Data structures concepts · 139 unmodified lines
140	------------------------	140	------------------------
141	Memory used for swiotlb bounce buffers is allocated from overall system memory	141	Memory used for swiotlb bounce buffers is allocated from overall system memory
142	as one or more "pools". The default pool is allocated during system boot with a	142	as one or more "pools". The default pool is allocated during system boot with a
143	default size of 64 MiB. The default pool size may be modified with the	143	default size of 64 MiB, which can be changed at compile time via
144	"swiotlb=" kernel boot line parameter. The default size may also be adjusted	144	CONFIG_SWIOTLB_DEFAULT_SIZE_MB. The default pool size may also be
		145	modified at runtime with the "swiotlb=" kernel boot line parameter,
		146	which takes precedence over the compile-time default. The default size
		147	may also be adjusted
145	due to other conditions, such as running in a CoCo VM, as described above. If	148	due to other conditions, such as running in a CoCo VM, as described above. If
146	CONFIG_SWIOTLB_DYNAMIC is enabled, additional pools may be allocated later in	149	CONFIG_SWIOTLB_DYNAMIC is enabled, additional pools may be allocated later in
147	the life of the system. Each pool must be a contiguous range of physical	150	the life of the system. Each pool must be a contiguous range of physical

include/linux/swiotlb.h+6−2

 #define IO_TLB_SHIFT 11
 #define IO_TLB_SIZE (1 << IO_TLB_SHIFT)
-/* default to 64MB */
+/* compile-time default; overridable via CONFIG_SWIOTLB_DEFAULT_SIZE_MB */
-#define IO_TLB_DEFAULT_SIZE (64UL<<20)
+#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,

kernel/dma/Kconfig+22−0

@@ -86,6 +86,28 @@ config SWIOTLB · 85 unmodified lines
86	bool	86	bool
87	select NEED_DMA_MAP_STATE	87	select NEED_DMA_MAP_STATE
88		88
		89	config SWIOTLB_DEFAULT_SIZE_MB
		90	int "Default SWIOTLB bounce buffer size in MB"
		91	depends on SWIOTLB
		92	range 1 64
		93	default 64
		94	help
		95	Sets the default size of the software IO TLB (SWIOTLB) bounce buffer
		96	pool allocated at boot time. The default is 64 MB.
		97
		98	On memory-constrained embedded or mobile platforms (e.g., those with
		99	a hardware IOMMU such as ARM SMMU covering most DMA-capable devices),
		100	a smaller value such as 4 or 8 MB may be sufficient. The SWIOTLB is
		101	then only needed for devices that bypass the IOMMU or have restricted
		102	DMA address ranges.
		103
		104	The minimum allowed value is 1 MB. This compile-time default can be
		105	overridden at runtime using the "swiotlb=<nslabs>" kernel command line
		106	parameter. Refer to Documentation/admin-guide/kernel-parameters.txt
		107	for details.
		108
		109	If unsure, leave at the default value of 64.
		110
89	config SWIOTLB_DYNAMIC	111	config SWIOTLB_DYNAMIC
90	bool "Dynamic allocation of DMA bounce buffers"	112	bool "Dynamic allocation of DMA bounce buffers"
91	default n	113	default n

dma/swiotlb: introduce Kconfig option for compile-time default pool size

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