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.
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| @@ -86,6 +86,28 @@ config SWIOTLB · 85 unmodified lines | ||
| 86 | 86 | bool |
| 87 | 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 | 111 | config SWIOTLB_DYNAMIC |
| 90 | 112 | bool "Dynamic allocation of DMA bounce buffers" |
| 91 | 113 | default n |