Re: [PATCH v5 3/6] dma: swiotlb: Centralize minimal pool sizing
From: Robin Murphy <robin.murphy@arm.com>
Date: 2026-09-21 16:50:14
Also in:
linux-iommu, linux-mips, linux-riscv, linux-s390, linuxppc-dev, lkml, loongarch
On 21/09/2026 7:36 am, Aneesh Kumar K.V (Arm) wrote:
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A default SWIOTLB pool used only for unaligned kmalloc bouncing can be smaller than one required for limited DMA addressing. Arm64 and RISC-V currently implement this sizing independently, using 1 MiB of SWIOTLB space per GiB of RAM. The SWIOTLB core now identifies this case with SWIOTLB_POOL_MINIMAL. Size that policy in swiotlb_adjust_pool_size() and remove the architecture-specific adjustments. Explicit swiotlb= sizing remains unchanged because swiotlb_adjust_size() preserves a user-configured size. Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org> --- arch/arm64/mm/init.c | 11 ----------- arch/riscv/mm/init.c | 4 ---- kernel/dma/swiotlb.c | 11 ++++++++++- 3 files changed, 10 insertions(+), 16 deletions(-)diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c index 7d51fb5f5a71..95075b5c8207 100644 --- a/arch/arm64/mm/init.c +++ b/arch/arm64/mm/init.c@@ -340,17 +340,6 @@ void __init arch_mm_preinit(void) { unsigned int flags = SWIOTLB_VERBOSE; - if (max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) { - /* - * If no bouncing needed for ZONE_DMA, reduce the swiotlb - * buffer for kmalloc() bouncing to 1MB per 1GB of RAM. - */ - unsigned long size = - DIV_ROUND_UP(memblock_phys_mem_size(), 1024); - - swiotlb_adjust_size(min(swiotlb_size_or_default(), size)); - } - if (max_pfn > PFN_DOWN(arm64_dma_phys_limit)) flags |= SWIOTLB_INIT_ADDRESSING_LIMIT;diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c index e0e4c103d603..96856114b387 100644 --- a/arch/riscv/mm/init.c +++ b/arch/riscv/mm/init.c@@ -183,10 +183,6 @@ void __init arch_mm_preinit(void) * memory when DRAM starts above dma32_phys_limit. Allocate * ~1 MB per 1 GB of RAM. */ - unsigned long size = - DIV_ROUND_UP(memblock_phys_mem_size(), 1024); - swiotlb_adjust_size(min(swiotlb_size_or_default(), size)); - swiotlb = true; swiotlb_flags |= SWIOTLB_ANY; }diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c index 8f86deb25be2..f368a73f4ed0 100644 --- a/kernel/dma/swiotlb.c +++ b/kernel/dma/swiotlb.c@@ -483,9 +483,18 @@ static bool __init swiotlb_kmalloc_needs_bounce(void) static void __init swiotlb_adjust_pool_size(enum swiotlb_pool_policy policy) { + if (swiotlb_default_size_changed()) + return; +
This appears to be entirely redundant, as ultimately the point of this function is to call swiotlb_adjust_size() (if it does anything at all), and the first thing that does is this same exact check. We hardly need to micro-optimise short-circuiting a handful of arithmetic in a one-off setup path, and it's convoluted enough as it is, so please try to avoid redundancy that makes it even more confusing to follow.
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switch (policy) { - case SWIOTLB_POOL_MINIMAL: + case SWIOTLB_POOL_MINIMAL: { + unsigned long size; + + /* Use 1MB per 1GB of RAM for kmalloc() bouncing. */ + size = DIV_ROUND_UP(memblock_phys_mem_size(), 1024); + swiotlb_adjust_size(min(swiotlb_size_or_default(), size)); break;
Similarly I think it would be clearer if we had a common swiotlb_adjust_size() call at the end of the function, and then either calculate a size or return early in each switch case as appropriate. Furthermore, swiotlb_size_or_default() is awful IMO - and in fact after this series we could perhaps clean it up entirely by making the size implicit in swiotlb_init_late() - not to mention misleadingly redundant. I'd say just open-code "default_nslabs << IO_TLB_SHIFT" like elsewhere in the file, but in fact it may as well just be IO_TLB_DEFAULT_SIZE (think about it...) Thanks, Robin.
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+ } case SWIOTLB_POOL_CC_GUEST: break; case SWIOTLB_POOL_NONE: